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A synthesis of radial growth patterns preceding tree mortality
Global Change Biology
Abstract
Tree mortality is a key factor influencing forest functions and dynamics, but our understanding of the mechanisms leading to mortality and the associated changes in tree growth rates are still limited. We compiled a new pan-continental tree-ring width database from sites where both dead and living trees were sampled (2,970 dead and 4,224 living trees from 190 sites, including 36 species), and compared early and recent growth rates between trees that died and those that survived a given mortality event.
Supplementary resource (1)
... We conducted the meta-regression for both meta-analyses, to see if the impact of these covariates changes between nondrought and drought conditions. Taxonomic class played an important role in explaining differences in drought responses in other studies (Cailleret et al. 2017;DeSoto et al. 2020;McDowell et al. 2008;Sohn et al. 2016;Steckel et al. 2020). Moreover, we checked the effect of thinning intensity, calculated as the ratio of SDI in thinned to SDI in control treatments, on the risk ratio in both periods. ...
... Likewise, we could not detect significant differences between tree genera and taxonomic classes. Differences in physiological drought reactions and vulnerability to pests between tree species (Cailleret et al. 2017;DeSoto et al. 2020;McDowell et al. 2008) support the idea that we should also see these differences in a meta-analysis on mortality responses to drought. ...
... Interestingly, such studies that assessed both growth response and mortality consistently exhibited higher basal area increment and higher survival in heavily thinned treatments during and after drought, indicating a higher drought resistance and drought recovery of individual trees (Calev et al. 2016;Giuggiola et al. 2013;Knapp et al. 2021) and at the stand-level (Zhang et al. 2019). Likewise, comparative investigations of annual radial growth of trees deceased during droughts and survivors revealed that the deceased trees already had displayed lower radial growth in previous droughts (Cailleret et al. 2017;Dulamsuren et al. 2022), typically with lower drought resistance values in angiosperms and lower drought recovery values in gymnosperms (DeSoto et al. 2020). In addition to such a retrospective approach using radial increment, the response to drought and thinning could be monitored through repeated assessments of crown conditions, for example with unmanned aerial vehicles (Sankey & Tatum 2022), or ongoing measurement of growth using dendrometers (Aldea et al. 2023). ...
Context
Forest scientists are challenged to identify and propose evidence-based silvicultural options to mitigate the impacts of drought events induced by climate change. For example, it has been suggested that thinning increases soil water availability for individual trees by reducing stand density and stand-level transpiration. Many studies have assessed the impact of thinning on stem growth and transpiration of individual trees during and after drought events. Often, growth increases were observed, but not consistently, and their impact on tree survival following drought has rarely been addressed.
Aims
We aimed to assess the effect of thinning on tree mortality, the ultimate indicator of tree resistance to soil water deficit induced by drought, with a focus on dominant trees.
Methods
We conducted a risk ratio meta-analysis on tree mortality before and after an extreme drought event with 32 thinning experiments from nine studies in Europe and North America.
Results
We showed that thinning reduced the overall mortality risk of trees. However, the lower mortality rate in thinned stands relative to unthinned stands in pre-drought periods was not further reduced during and after extreme drought events ( p > 0.05). This may be due to the large heterogeneity and inconsistent reporting of mortality across the studies included in our analysis. Thinning did not exacerbate mortality among dominant trees.
Conclusion
Since thinning did not increase mortality, its application can still be recommended for many other management objectives such as maintaining tree species richness or lower disturbance risks from windthrow. We propose better documentation of thinning trials to improve the data base for systematic reviews.
... In addition to this comparison, it is possible to compare tree growth before and after the understory degradation by analyzing tree-ring records. Annual ring widths and stable isotope ratios in stem wood are used to retrospectively record tree growth trends and physiological processes (Cailleret et al., 2017). These metrics enable researchers to document the timing of environmental changes and their effects on tree growth (Cailleret et al., 2017;Creber, 1977;Dobbertin, 2005). ...
... Annual ring widths and stable isotope ratios in stem wood are used to retrospectively record tree growth trends and physiological processes (Cailleret et al., 2017). These metrics enable researchers to document the timing of environmental changes and their effects on tree growth (Cailleret et al., 2017;Creber, 1977;Dobbertin, 2005). For example, drought-induced reductions in tree growth can be inferred from reductions in annual ring width after periods of reduced precipitation (Cailleret et al., 2017;Shen et al., 2020). ...
... These metrics enable researchers to document the timing of environmental changes and their effects on tree growth (Cailleret et al., 2017;Creber, 1977;Dobbertin, 2005). For example, drought-induced reductions in tree growth can be inferred from reductions in annual ring width after periods of reduced precipitation (Cailleret et al., 2017;Shen et al., 2020). In addition, the trend of intrinsic water use efficiency (iWUE) can be obtained by calculating the stable carbon isotope ratio ( 13 C/ 12 C ratio) in tree-rings. ...
... pathogen attack to extensive, synchronous death from infrequent disturbance (Peet and Christensen 1987;Franklin, Shugart, and Harmon 1987). It is often assumed that within a forest stand faster growing trees face lower mortality risks and that this can be revealed by examining how tree mortality depends on growth (e.g., Wunder et al. 2008;Macalady and Bugmann 2014;Cailleret et al. 2017;Russo et al. 2021). Recent changes (< 10 years) in tree diameter are often used to depict growth in such relationships (e.g., Wyckoff and Clark 2002;Cailleret et al. 2017). ...
... It is often assumed that within a forest stand faster growing trees face lower mortality risks and that this can be revealed by examining how tree mortality depends on growth (e.g., Wunder et al. 2008;Macalady and Bugmann 2014;Cailleret et al. 2017;Russo et al. 2021). Recent changes (< 10 years) in tree diameter are often used to depict growth in such relationships (e.g., Wyckoff and Clark 2002;Cailleret et al. 2017). Hartmann et al. (2018) proposed a logistic relationship between a risk factor and tree mortality, and this conceptual model translates into a logistic relationship between tree growth and survival. ...
... Re-measurements of the tagged trees were used to determine annual survival probability (hereafter survival) and annual diameter growth rate (hereafter growth in mm year −1 ). Recent changes (< 10 years) in tree diameter are widely used to generate individual growth metrics in ecological studies (e.g., Clark and Clark 1999;Wunder et al. 2008;Cailleret et al. 2017). Our growth metrics, based upon changes in diameter, reflect individual vitality and productivity (Dobbertin 2005;Cailleret et al. 2017). ...
Tree growth–survival relationships link two demographic processes that individually dictate the composition, structure and functioning of forest ecosystems. While these relationships vary intra‐specifically, it remains unclear how this reflects environmental variation and disturbance. We examined the influence of a 700‐m elevation gradient and an Mw = 6.7 earthquake on intra‐specific variability in growth–survival relationships. We expected that survival models that incorporated recent growth would be better supported than those only using other factors known to influence tree survival. We used a permanent plot network that representatively sampled a monodominant Nothofagus forest in New Zealand's Southern Alps in 1974 and that was remeasured seven times through to 2009. The relationships were assessed using pre‐earthquake growth and survival, pre‐earthquake growth and post‐earthquake survival (0–5 years post‐earthquake), and post‐earthquake growth and survival (5+ years post‐earthquake). Survival was related to growth of 4504 trees on 216 plots using Bayesian modelling. We hypothesised there would be a positive, logistic relationship between growth and survival. Pre‐earthquake, we found a positive, logarithmic growth–survival relationship at all elevations. At higher elevations, trees grew more slowly but had higher survival than trees at lower elevations, supporting our hypothesised demographic trade‐off with elevation. The earthquake altered growth–survival relationships from those found pre‐earthquake and 0–5 years post‐earthquake survival held little relationship with growth. A strong, logarithmic growth–survival relationship developed 5+ years post‐earthquake because of enhanced survival of fast‐growing trees yet low survival of slow‐growing trees. Synthesis. Our findings demonstrate a trend in growth–survival relationships along an elevation gradient. If we assume a gradual climate warming is the equivalent of a forest stand shifting to a lower elevation, then data from our pre‐earthquake period suggest that tree growth–survival relationships at any elevation could adjust to faster growth and lower survival. We also show how these novel growth–survival relationships could be altered by periodic disturbance.
... The large areal fraction recovery from subtle anomalies of biomass or greenness, possibly due to impaired ecophysiological capability to repair partial damage. This indicates increased vulnerability to further climate perturbations and thus increased likelihood of approaching a tipping point where abrupt loss may occur [28][29][30] . Such an early warning signal has been found useful in predicting temperate and tropical forest losses 28,31 . ...
... Observational studies have measured boreal forest resilience by comparing observed growth, density and composition after droughts and fires to those predisturbance 16,34,35 . These metrics, however, mostly measure resilience after disturbance events, with the exception of diagnosing radial growth before disturbance or forest loss using tree-ring records 30,36,37 and are limited by spatial-temporal coverage. Diagnosing resilience, including that before abrupt changes, requires a broader scope entailing the ability to recover from pulse disturbances such as acute droughts and fires but also press disturbances related to more gradual changes in climate variables, such as temperature and long-term moisture availability 38,39 . ...
... The rationale behind this approach is that high autocorrelation indicates slow recovery from anomalies due to changes in vegetation photosynthetic capacity, thereby leaving the ecosystem less capable of withstanding further perturbations. For example, slow recovery has been linked to reduced growth, hydraulic impairment and depletion of non-structural carbon storage under drought stress before abrupt losses 30,37,43 . The existing regional to global studies have suggested reduced resilience in the tropics and arid temperate forests, in contrast to mixed but generally enhanced resilience in Arctic-boreal regions in response to climate change 26,41,42,44 . ...
Rapid warming and increasing disturbances in high-latitude regions have caused extensive vegetation shifts and uncertainty in future carbon budgets. Better predictions of vegetation dynamics and functions require characterizing resilience, which indicates the capability of an ecosystem to recover from perturbations. Here, using temporal autocorrelation of remotely sensed greenness, we quantify time-varying vegetation resilience during 2000–2019 across northwestern North American Arctic-boreal ecosystems. We find that vegetation resilience significantly decreased in southern boreal forests, including forests showing greening trends, while it increased in most of the Arctic tundra. Warm and dry areas with high elevation and dense vegetation cover were among the hotspots of reduced resilience. Resilience further declined both before and after forest losses and fires, especially in southern boreal forests. These findings indicate that warming and disturbance have been altering vegetation resilience, potentially undermining the expected long-term increase of high-latitude carbon uptake under future climate.
... This pattern of A. Rubio-Cuadrado et al. Forest Ecology and Management 590 (2025) 122797 gradual decline in growth is the most common found in dendroecological studies (Amoroso et al., 2017;Cailleret et al., 2017;Pedersen, 1998), including those on P. sylvestris and A. alba , although it should be noted that these studies always use growth trends at stand level, which makes it difficult to properly detect sudden mortality processes in isolated trees. This decline in growth, which can last several decades (Gea-Izquierdo et al., 2021), can be used as a tool for early detection of mortality processes (Cailleret et al., 2019), allowing managers to design and implement appropriate silvicultural measures to improve the vigor of affected stands. ...
... Compound climate events of warm and dry years can trigger tree mortality . Low precipitation combined with elevated temperatures would lead to partial hydraulic failure and carbon starvation, which would reduce tree vitality and ultimately lead to tree death (Cailleret et al., 2017;Powers et al., 2020;Trugman et al., 2021). If drought and temperature lead to a complete collapse of the hydraulic system, sudden tree death will occur (Arend et al., 2021). ...
In recent decades, forest die-off events have increased worldwide due to warmer droughts, with Southwestern Europe emerging as a mortality hotspot. This region is notably affected by widespread decline and die-off episodes , particularly in mountain forests dominated by Abies alba (silver fir) and Pinus sylvestris (Scots pine). We study four stands (two Scots pine, two silver fir) showing ongoing die-off in the Spanish Pyrenees, near the southwestern edge of both species' distributions. Our objectives were to analyze growth patterns of declining trees in mortality hotspots, and to study the climatic (drought) and biotic (mistletoe) factors contributing to this decline. Using dendroecological methods and a recently developed change-point analysis approach, we assessed differences in growth patterns at stand and individual tree levels. Results showed that mortality events were recent, especially for Scots pine. Although mortality patterns were tree-specific, Scots pine typically exhibited a sudden growth decline before death or a sudden mortality without prior growth decline, whereas silver fir generally showed gradual and prolonged growth decline preceding mortality. Drought and elevated temperatures were the primary triggers for Scots pine and the warmer-drier silver fir site, with secondary contributors such as historical management and microsite conditions (soil, aspect, slope, moisture, density, size, age, competition). Conversely, in the colder-wetter silver fir site, mistletoe was the sole driver of decline and mortality. These findings highlight the complex interplay of primary and secondary stressors underlying forest die-off.
... Tree mortality is a crucial dynamic component of forest stand structure and net productivity (Sims et al. 2009;Maleki and Kiviste 2016;Cailleret et al. 2017;Valkonen et al. 2020). Predicting the future survival of individual trees is important in forest modeling as well as for determining the proper timing of thinning so that it precedes self-thinning. ...
... We found that the best CIs for predicting growth and mortality were not the same, which has also been observed in other studies (Sun et al. 2018). Deceleration of stem diameter growth was the most significant predictor of mortality probability in plantations, similar to what has been observed in forestland birch stands (Maleki and Kiviste 2016) and which is considered to be a universal marker of potential future mortality in many species (Cailleret et al. 2017). Growth decline preceding the 6-year period was a more reliable predictor of mortality likelihood than competition (calculated based on CIs at the start of the study period) alone, as it also reflects the impact of other biotic/abiotic stressors as well as the accumulated effect of prior competition. ...
Silver birch plantations on former agricultural lands grow faster than birch stands on forest land in Northern Europe. For optimal management decisions, there is a need to improve understanding of ecological processes such as competition in plantations where stand structure and soil conditions differ from native forests. We analyzed the effects of intraspecific competition on mortality and periodic annual increment during 15–21 years of age (PAI6) in silver birch plantations. We evaluated six individual tree competition indices (CI) and two competitor selection methods, and tested whether weighting competitor trees based on their cardinal direction from the subject tree would improve the goodness-of-fit of growth models based on CIs. Tree growth parameters were measured and tree locations were mapped in the 0.1 ha sample plots of 11 silver birch plantations. The best CI for predicting PAI6 was the sum of diameter ratios combined with the reverse search cone method for competitor selection. Reducing the weight of competitors toward north improved the performance of the best CI for below-average sized trees but not for above-average sized trees, thus size-class-wise estimation of CIs can be advantageous. Asymmetric CIs performed better than symmetric CIs suggesting that light competition was the prevailing mode of competition.
... As long-lived perennials, trees experience environmental changes at sub-hourly to millennial time scales. Tree physiological responses to drought mirror those timescales and range from rapid responses such as stomatal closure, to annual changes in wood development, to responses that can take decades to manifest in mortality (Cailleret et al., 2017), and to the eventual mismatch in long-lived trees whose genotypes and adaptive traits no longer match contemporary conditions (Ledig & Kitzmiller, 1992). Legacy effects of a current year's drought on future years' growth (Anderegg et al., 2015) is but one example of the many complexities involved in understanding drought physiology in trees without direct parallels in better studied model annual plants. ...
... Recovery from drought requires rebuilding of canopy foliage, root biomass, carbohydrate stores, and xylem conduits (Gauthey et al., 2022). The recovery rate of wood growth can last decades and trees that do not recover wood growth are more likely to die than those that fully recover (Cailleret et al., 2017). The recovery rate of carbohydrate stores after drought is short in angiosperms but long in gymnosperms (Piper & Paula, 2020) and depletion of stored carbohydrates reduce osmoregulation capacity (Sapes et al., 2021). ...
Droughts of increasing severity and frequency are a primary cause of forest mortality associated with climate change. Yet, fundamental knowledge gaps regarding the complex physiology of trees limit the development of more effective management strategies to mitigate drought effects on forests. Here, we highlight some of the basic research needed to better understand tree drought physiology and how new technologies and interdisciplinary approaches can be used to address them. Our discussion focuses on how trees change wood development to mitigate water stress, hormonal responses to drought, genetic variation underlying adaptive drought phenotypes, how trees ‘remember’ prior stress exposure, and how symbiotic soil microbes affect drought response. Next, we identify opportunities for using research findings to enhance or develop new strategies for managing drought effects on forests, ranging from matching genotypes to environments, to enhancing seedling resilience through nursery treatments, to landscape‐scale monitoring and predictions. We conclude with a discussion of the need for co‐producing research with land managers and extending research to forests in critical ecological regions beyond the temperate zone.
... Some of these variables, such as declining tree-ring growth, have been linked to tree mortality across North America and Europe (Cailleret et al., 2017), but less research exists on the influence of other variables, such as sediment particle size. Better understanding of the different influences on mortality can inform mortality forecasting and potential management interventions, since tree mortality is expected to increase globally as climate change increases the prevalence of hotter drought (Allen et al., 2010). ...
... This difference is superimposed on the decline in ring growth of all trees in the decades prior to the mortality event, likely linked to declining late summer streamflow. Low tree-ring growth has been shown to predispose trees to mortality across different species and environmental settings in North America and Europe (Cailleret et al., 2017). Lower recent growth in trees that died compared to surviving trees is likely driven by the coarser deep sediments underlying trees that died. ...
Riparian ecosystems are some of the most valuable and vulnerable on the planet. Riparian tree mortality is increasing in the western United States, where altered streamflows are combining with warming climate. Between 2011 and 2013, one third of an extensive stand of Populus deltoides var. wislizeni (Rio Grande cottonwood) died along the middle Rio Grande on the Pueblo of Santa Ana in New Mexico. Mortality coincided with a severe drought that followed a decade of decreasing streamflow, but it was heterogeneous, with adjacent patches of dead and live trees. The goal of this research was to determine the drivers of mortality to provide insights into future risks of die‐off and potential management interventions. We compared tree age, competition, tree‐ring widths, sediment particle size and climate influences between live and dead forest patches in a nested plot design. Live and dead trees had similar age, stand density and particle sizes of shallow sediments. Tree‐ring widths had the highest correlations with July–September streamflow (1932–2013). All trees had declining ring growth since 1992, coinciding with declining late summer streamflow. An accelerated decline in growth began in 2002, corresponding to recent warmer droughts. Trees that died had lower ring growth 3 years prior to death and in the mid‐1900s. Dead trees also had coarser deep sediments 2.4–3.7 m below ground, suggesting that reduced water holding capacity was an important factor for mortality. Water management to increase streamflow during the late summer, especially during times of extended drought, could reduce mortality risk in the face of projected increasingly warm droughts.
... Dendro-ecological analyses are frequently employed for analyzing and predicting drought-related tree mortality Cailleret et al., 2017;Wunder et al., 2008). A considerable number of studies have shown that mortality becomes already visible in radial stem growth patterns prior to tree death Cailleret et al., 2018;Camarero et al., 2015). ...
... Growth ratios. To test hypothesis 1, growth ratios were used to examine whether growth of dead trees deviated from that of living trees (Berdanier and Clark, 2016;Cailleret et al., 2017). Annual growth ratios (GR) were calculated by dividing annual ring-width (RW) values of each dead tree (RW dead_tree ) by the mean annual RW value of all surviving trees at each site (RW living_mean ), as ...
... And, since forests are likely to experience increased drought severity due to climate change (IPCC, 2022), it is important to assess which trees cope better under such conditions. As the mechanisms leading to mortality are not yet fully understood (McDowell et al., 2013;Cailleret et al., 2017), in this study we focus on radial growth reductions, which are easy to measure and can serve as an indicator of a tree's vitality decline towards mortality (Pedersen, 1998;Cailleret et al., 2017). Another important characteristic of the European beech is its high morphological plasticity (Roloff, 2001;Pretzsch and Schütze, 2005), which results in a wide spectrum of crown morphologies and branch architectures as a consequence of adaption to continuously changing environmental conditions (Dieler and Pretzsch, 2013). ...
... And, since forests are likely to experience increased drought severity due to climate change (IPCC, 2022), it is important to assess which trees cope better under such conditions. As the mechanisms leading to mortality are not yet fully understood (McDowell et al., 2013;Cailleret et al., 2017), in this study we focus on radial growth reductions, which are easy to measure and can serve as an indicator of a tree's vitality decline towards mortality (Pedersen, 1998;Cailleret et al., 2017). Another important characteristic of the European beech is its high morphological plasticity (Roloff, 2001;Pretzsch and Schütze, 2005), which results in a wide spectrum of crown morphologies and branch architectures as a consequence of adaption to continuously changing environmental conditions (Dieler and Pretzsch, 2013). ...
Trees are able to adapt to changing environmental conditions through modifications in their crown morphology and branch architecture. In light-limited environments, tree structures are mainly optimised to increase light interception. This might have side effects on other properties such as the hydraulic system that determines water conduction and tree reactions to drought events. Given the increasing exposure of forest ecosystems to drought
stress it is thus crucial to investigate possible correlations between the crown morphology and branch architecture and drought responses. Our study aimed to compare different crown archetypes and branch characteristics of European beech (Fagus sylvatica L.) in their reaction to drought, which was assessed by measuring tree-ring widths of increment cores sampled at breast height. Using Terrestrial Laser Scanning and Quantitative Structure
Models, we explored various crown morphological and branch architectural characteristics of 67 beech trees and identified three species-specific crown archetypes. The crown archetypes and branch variables were contrasted with growth responses using linear mixed models. While productivity levels differed, the negative impact of drought stress on radial growth was consistent across all crown archetypes. Nevertheless, certain branch architectural variables were important predictors for radial growth in drought situations. Specifically, long water conduction paths and many branching nodes along those paths were positively influencing growth. Our results indicate that trees showing these characteristics might have a competitive edge regarding drought-affected radial growth compared to others. They could be promoted through thinning measures, which would allow improving the adaption of existing beech forests in situ to climate change and drought stress.
... Tree growth can be a marker of forest response to environmental alterations, as it is affected by drought stress. Dendrochronology studies have evinced growth decline during drought stress periods, eventually leading to forest dieback (Camarero et al. 2011;Cailleret et al. 2017). Each population of plants, including trees, is characterized by specific adaptation to local soil and microclimate conditions. ...
The way of trees adaptation to environments is a vital concern. Presented research focused on wood tissue diversity in terms of the juvenile wood proportion (wood located near the pith and of structure and properties different from outer wood zone, called as mature wood), an important characteristic for wood properties, to assess the evolutionary and functional impact of genetic variations. In this paper, the material from experimental provenance plot in Poland (Rogów) was presented. The tested trees were grown at the same time, in the same soil for the same period of time, but the parental stands of tested trees were from the different Polish regions. Based on the results it was concluded that origin of parental trees has an influence on the amount of juvenile wood expressed by the number of annual growth rings, as well as the volume occupied in the trunk of the trees. The wood formation, particularly the amount of juvenile wood, is influenced by the climatic conditions of parental trees’ habitat (epigenetic indicators). The amount of precipitation is predominantly important in this respect. The new mathematical model for estimating the number of annual increments corresponding to the juvenile wood zone was proposed. The obtained results highpoint the necessity of taking epigenetic indicators into account in future breeding strategies composed with genetic markers for both wood production and quality in the context of climate change that requires adaptation.
... The climate-change induced increasing frequency and magnitude of so-called hotter droughts has resulted in widespread forest decline and tree dieback (Allen et al. 2015;Anderegg et al. 2015;Buras et al. 2020Buras et al. , 2018Cailleret et al. 2017;Orth et al. 2016;Schuldt et al. 2020;Senf et al. 2020). As a consequence, tree-species distributions will undergo vast changes in the 21st century, depending on how the climatological boundaries under which they survive, sometimes termed their climate envelope, are realized (Buras and Menzel 2019;Chakraborty et al. 2021;Dyderski et al. 2018;Koch et al. 2022;Martes et al. 2024;Walentowski et al. 2017). ...
Forest tree species are expected to experience a substantial redistribution due to climate change. While previous work has emphasized the effects of a warmer and drier climate on European tree‐species distributions, to date no study has investigated the potential impact of a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Here, we deploy climate‐envelope models to quantile mapped, high‐resolution (1km ² ) CMIP6 climate projections and compare tree‐species distributions under an active AMOC vs. an inactive AMOC scenario. Across Europe, our tree‐species projections indicate contrasting impacts of the two scenarios. In Scandinavia, many of the currently abundant tree species were projected a dramatic decline and partial disappearance due to the strong cooling under an inactive AMOC. In Central and Southern Europe, however, some of the currently abundant species suffered less under an inactive AMOC compared to an active AMOC scenario while others—such as the economically important species of Norway spruce—almost went extinct. As opposed to the classic climate‐change scenario supporting Mediterranean species in Central Europe, projected European tree‐species portfolios consisted of a higher share of boreal, cold‐tolerant species in the inactive AMOC scenario. Finally, tree‐species diversity was projected to decline even stronger under an inactive vs. an active AMOC scenario. Altogether, while an AMOC collapse may locally result in more favorable conditions for specific species in comparison to a classic climate‐change scenario, the dramatic economic and ecological consequences suggested by our projections indicate the urgent need for climate‐change mitigation to lower the likelihood of an AMOC collapse.
... The negative growth responses for lower canopy trees on dry sites are consistent with other studies showing that drought and competition can increase mortality, especially in understory trees 106 . Mortality is usually preceded by growth declines indicative of inciting stress that can span several decades [107][108][109] . Smaller trees may furthermore have limited access to deeper water layers due to their less developed root systems 110 and may thus be more prone to drought-induced stress. ...
Floodplain forests are currently undergoing substantial reorganization processes due to the combined effects of management-induced altered hydrological conditions, climate change and novel invasive pathogens. Nowadays, the ash dieback is one of the most concerning diseases affecting European floodplain forests, causing substantial tree mortality and threatening the loss of the dominant key tree species of the hardwood floodplain forest, Fraxinus excelsior. Understanding how the increased light availability caused by pathogen-driven mortality in combination with altered hydrological conditions and climate change affects growth responses in a diverse forest community is of crucial importance for conservation efforts. Thus, we examined growth of the main tree species in response to ash dieback and how it depended on altered hydrological conditions under novel climatic conditions for the lower and upper canopy in the floodplain forest of Leipzig, Germany. Our study period encompassed the consecutive drought years from 2018 to 2020. We found that tree growth responded mostly positively to increased light availability, but only on moist sites, while tree growth largely declined on dry sites, suggesting that water availability is a critical factor for tree species to be able to benefit from increased light availability due to canopy disturbances caused by ash dieback. This hydrological effect was species-specific in the lower canopy but not in the upper canopy. While, in the lower canopy, some species such as the competitive shade-tolerant but flood-intolerant Acer pseudoplatanus and Acer platanoides benefited from ash dieback on moist sites, others were less affected or suffered disproportionally, indicating that floodplain forests might turn into a novel ecosystem dominated by competitive Acer species, which may have detrimental effects on ecosystem functioning. Our results give hints on floodplain forests of the future and have important implications for conservation measures, suggesting that a substantial revitalization of natural hydrological dynamics is important to maintain a tree composition that resembles the existing one and thus sustain their conservation status.
... As global warming continues, the constraints of water availability on vegetation growth are expected to increase Zhang, Jia, and Ustin 2021). Typically, drought weakens the carbon sink function of vegetation by decreasing non-structural carbon storage and increasing the risk of hydraulic failure, thereby reducing the resilience of ecosystems to disturbances (Boulton et al. 2022;Cailleret et al. 2017;Li et al. 2023;McDowell et al. 2022). A decrease in leaf nitrogen can also amplify the effects of drought on photosynthesis, exacerbating hydraulic failure or carbon starvation (Gessler et al. 2017). ...
Drought is considered a major contributor to carbon sink fluctuations in terrestrial ecosystems and is expected to lead to more frequent carbon sink–source transitions under future climate change. The drought threshold for carbon sink–source transition reflects the critical inflection point at which the carbon sequestration capacity of vegetation is affected by water deficit. However, the spatiotemporal patterns of the global drought threshold and their underlying mechanisms remain poorly understood. Here, we use three independent datasets from vegetation dynamics models, inversion modeling, and observational data to map and explore the drought thresholds expressed by the standardized precipitation evapotranspiration index (SPEI) during the growing season over the past four decades. Sink–source transition is indicated by changes of sign for net ecosystem productivity (NEP). The drought thresholds were identified across 66.3% of global land, with an average threshold of −1.08 ± 0.68. Regions with lower thresholds are primarily located in the Northern Hemisphere at middle and high latitudes, whereas Australia, Africa, western South America, and southern North America exhibit higher thresholds. The dominant factor influencing the spatial pattern of drought thresholds is potential evapotranspiration. Our dynamic results show that 36.4% of the thresholds increased, while 55.8% decreased. We found that disproportionate decreases in photosynthesis and respiration caused by drought in South America led to decreased thresholds and increased drought resilience in this region. Under conditions of reduced soil moisture, lower radiation, increased vapor pressure deficit, and enhanced heatwave intensity, drought in North America had a greater effect on reducing photosynthesis than it did on respiration. This resulted in an increasing threshold trend, where even relatively low levels of drought can induce a carbon sink–source transition. In addition, CO 2 fertilization plays a major role in reducing thresholds and mitigating climate change. Our findings emphasize that the risk of carbon sink–source transition is more acute in regions with rising thresholds. This implies that the stability of ecosystem carbon sequestration in these regions may decrease under persistent water stress.
... Reduced ring-widths are notably associated with high drought stress experienced by the tree (e.g. Vitasse et al., 2019), as well as high mortality risk (Cailleret et al., 2017;DeSoto et al., 2020). The reduction in tree leaf area, a key integrative trait that directly controls transpiration and carbon assimilation, is also well-documented at both tree and stand scales (e.g. ...
Background and aims
With ongoing climate change, the impact of droughts of increasing intensity on forest functioning is of critical concern. While the adverse effects of drought on tree secondary growth have been largely documented both at the tree and stand scales, our understanding of how primary growth morphological traits, which control crown development, respond to drought remains limited - especially in the long term.
Methods
Based on 14 years of monitoring of four primary growth morphological traits (e.g., shoot elongation, polycyclism rate, branching and needle length) and stem secondary growth in a rainfall exclusion experiment, we investigated (i) the climatic drivers of aboveground growth and (ii) the effect of long-term exacerbated drought conditions on the growth response to drought in a mature Pinus halepensis stand.
Key results
Aboveground growth was strongly and negatively impacted by drought duration during the current year (stem secondary growth), the previous year (polycyclism) and both years (branching, shoot length), and by drought during spring (needle length). While excluding 30% of the incoming rainfall did not significantly affect the number of ramifications, polycyclism rate or stem secondary growth, it reduced needle and shoot lengths by 14.3 and 7.7% over the entire study period, respectively. However, this effect is significant only in the first years after the treatment was established. Such acclimation to exacerbated drought conditions is also reported in the drought-growth relationships which are similar among treatments, except for needles that were slightly shorter under a similar level of drought stress in the exclusion.
Conclusions
Our study highlights the key acclimation capacity in the primary and secondary growth response of P. halepensis to drought. In addition to tree structural adjustments, the relatively limited effect of the 30% rainfall exclusion may also be caused by (i) the substantial inter-annual rainfall variability typical of Mediterranean climates, which modulates the exclusion effect on drought duration, and (ii) the inherent inter-individual variability in drought sensitivity.
... Plant responses to drought are mediated by several structural and functional traits that integrate water and carbon processes, such as tree ring width, wood anatomical traits and intrinsic water use efficiency (iWUE) assessed by carbon isotope discrimination (Cailleret et al., 2017;Camarero et al., 2023;Olano et al., 2014;Pellizzari et al., 2016;Ponton et al., 2001;Puchi et al., 2021). Tree radial growth, which reflects changes in individual vitality, productivity and carbon allocation (Babst et al., 2014;Babst et al., 2018), has been demonstrated to be driven by climate and, in particular, limited by drought (Babst et al., 2019;Camarero et al., 2021). ...
Recent extreme drought events in Central Europe have caused widespread forest dieback with detrimental effects on forest functioning and carbon and water balance. This impact has been notable on European beech (Fagus sylvatica L.), particularly at the core of its distribution, causing concern among forest stakeholders and questions about the resilience capacity of beech trees. The objective of this study is to investigate the physiological processes linked to water and carbon constraints involved in the resilience of beech cambial growth to drought.
We selected 56 beech trees distributed in four plots in north‐eastern France with different soil water deficits characterized retrospectively by a water balance model. Functional traits including tree ring width, wood anatomical traits and stable isotopes (e.g. δ¹³C and δ¹⁸O) were measured to retrospectively assess the effect of recent recurrent drought in 2015, 2018–2020, and 2022.
Decreased tree growth and increased δ¹⁸O and intrinsic water use efficiency (iWUE) were observed due to soil water shortage, whereas xylem vessel size and theoretical specific xylem hydraulic conductivity (K th) did not show obvious changes. Vessel density was negatively correlated with annual ring width index and was highly sensitive to drought. δ¹³C, δ¹⁸O and iWUE were not significantly related to tree ring width index. The plot that experienced the most severe drought intensity in 2018–2020 showed a significant decrease in tree growth resistance and resilience compared to its resistance and resilience during the 2015 drought event. Surprisingly, growth resilience was not associated with tree anatomical and isotopic traits.
Synthesis. Our results demonstrate that beech xylem structure responds to drought by adjusting the radial growth of tree rings with a relatively stable vessel diameter. Our study also highlights the impact of consecutive or recurrent drought in reducing beech tree resistance and resilience, particularly at sites with higher drought intensity. Tree resilience does not seem to involve changes in traits that would promote the hydraulic functioning to better cope with future soil water shortages.
... In order to highlight the different correlations that may exist between dieback and site and dendrometric parameters, we considered it essential to interpret the data on the basis of one-way analysis of variance at the 95% significance level. (Nichane, 2015) Dieback Index "ID" Health status ID < 2. 30 healthy stand 2.30 < ID < 2. 80 stand at the beginning of decline 2.80 < ID < 3. 4 fairly serious decline ID > 3. 4 severe or severely wasting disease This choice of study is based on the fact that we can only consider the possibility of a significant correlation on the basis of a statistical study. The analysis of variance makes it possible to acquire this possibility, especially since it is the subject of numerous research works (Dagneli, 1977). ...
Assessing the severity of green Cypress (Cupressus sempervirens L.) dieback in the Western Traras Mountains (North-West Algeria)-925-APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 23(1):925-948. Abstract. In recent years, the green cypress has experienced many problems in the Western Traras Mountains, among which are the dieback which results in browing and ultimately the death of the tree. The objective of our study is to emphasize the main causes of the decline of the green Cypress in the western mountains of Traras. The experimental approach is based on installation of the units of sampling through all tasks of the Cypress decayed (abnormal leaf coloring) being of the region of Bab El Assa and Marsa Ben M'hidi. These units of sampling among 25 are of surface concentric respectively 4, 6, 8 and 10 are. The global sanitary indication is estimated at 2.6564, showing that the sampled populations are at the beginning of decay. An analysis of the variance shows that health status is better than the stems present a maximal extension of their crowns. A high density and unfavorable social positions contribute to the decline of the green Cypress in the western mountains of Traras. The problem of decay does not stop with stationary and dendrometric factors as cause, because we have other aggravating factors, are insects. Each plot was the subject of a specific technical sheet bringing together all the data collected in the field. The data to be collected are dendrometric, symptomatological of dieback and stationary factors. In order to highlight the different correlations that may exist between dieback and stationary and dendrometric parameters, we interpreted our data based on one-way analysis of variance at the 95% significance level, using SPSS software. It appears that: The health state of the stands presents a significant variability in relation to the different microreliefs studied, to the extent that we note that this state is better in situations favoring the accumulation of water and on moderately inclined terrain. The health status of stands shows significant variability in relation to altitude to the extent that the "F value" is greater than the "Critical F value". The health status of the Cypress improves as the altitude increases. The rate and index of dieback show great variability in relation to exposure. The same goes for the degree of inclination of the terrain, where we record a high rate in uneven terrain, having a slope greater than 20°. Likewise, we are recording a loss of vigor of the Cypress to the extent that the health index reaches a value of 2.71 relating to Cypresses that are weakened or beginning to die back. The health status of the stands varies significantly in relation to the average size. We observe that the dieback rate is low among small stems (less than 0.85 m), not exceeding 10%. The health status of the stands improves among the stems at maximum size (more than 1.5 m), with much better growth. The health index presents significant variability in relation to height classes. In fact, the least vigorous stands are those with low dominant heights, less than 11 m. The health status remains better in stands of 11 to 19 m, either that with average dominant heights. Nichane et al.: Assessing the severity of green Cypress (Cupressus sempervirens L.) dieback in the Western Traras Mountains (North-West Algeria)-926-APPLIED ECOLOGY AND ENVIRONMENTAL RESEARCH 23(1):925-948.
... Our findings also indicate that oak carbon assimilation under encroached conditions is likely limited by shading, not competition for water. Because reduced productivity and growth increase tree vulnerability to mortality (Cailleret et al. 2017;DeSoto et al. 2020), decreasing Douglas-fir density in woodlands via restoration thinning treatments will likely strengthen Oregon white oak resilience against drought (Bradford & Bell 2017;Beckmann et al. 2021). ...
Oregon white oak (Quercus garryana) woodlands are threatened by conifer encroachment established during decades of fire exclusion. Widespread restoration efforts are underway to remove conifers from many oak woodlands in California, Oregon, and Washington. Using water potential, stomatal conductance, stable isotopes, and three metrics of biodiversity, we investigated the effects of Douglas-fir (Pseudotsuga menziesii) encroachment and removal at the ecosystem scale across 3 years in a northern California woodland. We found that heavily encroached stands had the highest water potential and often the lowest stomatal conductance, compared to moderately encroached and open stands. Xylem water stable isotopes indicated that oaks and Douglas-firs were likely not directly competing for water, as oaks appeared to use a relatively deeper water source. Under certain climatic conditions, heavily encroached oaks were more vulnerable to water stress than oaks in open or moderately encroached stands. Following low-intensity conifer removal, thinned stands had slightly lower predawn water potential and higher gas exchange compared to unthinned stands, particularly under dry conditions. For ecosystem biodiversity, understory plant and bird diversity did not meaningfully vary with encroachment or thinning, but mammal diversity was greater in encroached stands compared to open stands. Findings from this work demonstrate that negative impacts of conifer encroachment on oaks are not due to increased competition for water, conifer removal is physiologically beneficial for shaded oaks, and that heavier thinning treatments are likely needed to yield long-term responses and influence biodiversity.
... Thirdly, depending on the species, the decline phase leading to mortality can often last several years (Cailleret et al. 2017). For instance, an increased mortality was still observed three years after the 2003 extreme drought in France (Bréda and Badeau 2008). ...
Climate change has driven forest growth modellers to develop different climate sensitivity implementations (CSIs) for their models. Among others, a model can rely on annual climate variables or average climate variables, such as 30-year normals. The novelty of this study was to develop a framework based on lifetime analysis to enable annual or average CSI in empirical models of tree mortality. Using this framework, we compared models of individual tree mortality based on an annual CSI with similar models relying on two average CSIs, one using interval-averaged climate variables, and the other, 30-year normals. We fitted these models to permanent-plot data of eight species in Ontario and tested the effects of summer and winter temperature as well as spring and summer precipitation in the models. Our results showed that the annual CSI was not superior to the average CSIs, but could be a valid alternative for some species. Warmer winter temperature was detrimental to the survival of Betula papyrifera, Picea glauca, and Pinus strobus, whereas greater spring and summer precipitation resulted in greater mortality occurrence for Picea mariana, Pinus banksiana, and Populus tremuloides. In most cases, the effects of climate variables were contrary to our initial hypotheses. We conclude that the effects of climate on tree mortality occurrence interact with other factors such as species distribution and ecophysiology.
... Declining trees do not always exhibit reduced growth rates Trees usually display a lower growth rate prior to death (Macalady and Bugmann 2014; Cailleret et al. 2017) to the extent that growth reduction is usually considered an early indicator of tree mortality (Bigler and Bugmann 2004a). Nevertheless, the reduced growth period may vary in relation to the factor driving mortality of the tree species: from decades to just a few years before tree mortality (Cailleret et al. 2016). ...
Context Floodplain forests along the Upper Mississippi River System (UMRS) are highly influenced by the human-altered river flow regime. More intense and frequent floods are inducing decline in less flood-tolerant tree species. Of special concern is the situation of northern pecan tree (Carya illinoinensis (Wangenh. K.Koch)), which shows clear symptoms of decline ( i.e. defoliation, mortality) and a lack of regeneration. Objective We attempt to investigate changes in C. illinoinensis decline and health and determine the biometric factors that lead to these differences. Methods We established 15 plots along the UMRS floodplain and within each plot, we recorded species composition, health status, diameter at breast height and tree location. Additionally, we collected increment cores to study the disturbance regime, and the effects of climate and river flow on radial growth. We applied a principal component analysis and a mixed effect regression model to determine the factors related to pecan health at site and individual level, respectively. Results Two major disturbance events occurred during the 20th century: in 1965 and 1993. Both events are related to extraordinary flow discharges in the river that led to significant tree mortality in the overstory. Pecans exhibited growth release after the disturbances, however, the incidence of recruitment for this species was very scarce during the 20th century. Our analysis revealed that plots with a higher proportion of healthy pecans are located in the southmost area, which is characterized by less frequent extraordinary floods and less stand competition. Conclusions We conclude that without oriented management efforts, pecans will eventually disappear from the northern extent of the species range soon, leading to a loss in tree diversity in the UMRS.
... In the wake of climate change-driven mass tree mortality events in Europe (Schuldt et al., 2020) and elsewhere (Hammond et al., 2022), there is a growing interest in the biotic and abiotic factors that influence tree vitality (Cailleret et al., 2017;Chakraborty et al., 2017). This is particularly true for competition, which is directly amenable to silvicultural measures such as thinning and therefore important for forest management (cf. ...
In times of more frequent global change‐type droughts and associated tree mortality events, competition release is one silvicultural measure discussed to have an impact on the resilience of managed forest stands. Understanding how trees compete with each other is therefore crucial, but different measurement options and competition indices (CI) leave users with a difficult choice, as no single competition index has proven universally superior.
To help users with the choice and computation of appropriate indices, we present the open‐source TreeCompR package, which handles 3D point clouds and classical forest inventory data, enabling the calculation of both innovative point cloud‐based indices and traditional distance‐dependent indices. It serves as a centralized platform for exploring and comparing different CIs, allowing users to test and select the most suitable CI for their specific research questions within a common interface.
To evaluate the package, we used TreeCompR to quantify the competition situation of 307 European beech trees from 13 sites in Central Europe. Based on this dataset, we discuss the interpretation, comparability and sensitivity of the different indices to their parameterization and identify possible sources of uncertainty and ways to minimize them.
The compatibility of TreeCompR with different data formats and different data collection methods makes it accessible and useful for a wide range of users, specifically ecologists and foresters. Due to the flexibility in the choice of input formats as well as the emphasis on tidy, well‐structured output, our package can easily be integrated into existing data‐analysis workflows both for 3D point cloud and classical forest inventory data.
... In regions with clearly defined growing seasons, trees typically form annual growth rings (commonly known as tree rings), whose variability over time is often linked to annual climate conditions that constrain growth, such as low temperature or drought. To date, numerous treering studies have highlighted the consequences of global warming on forest ecosystems, including mortality events (Cailleret et al., 2019(Cailleret et al., , 2017, changes in carbon and water dynamics (Babst et al., , 2014Buras et al., 2023;Frank et al., 2015), and reconstruction of past climatic variability (Büntgen et al., 2010;Esper et al., 2016;Popa and Kern, 2009;Rocha et al., 2021). Tree rings thus represent an invaluable climate change proxy, which integrates climate variability from various parts of the growing season (Babst et al., 2018) and can contribute greatly to Intergovernmental Panel on Climate Change (IPCC) assessment reports (Calvin et al., 2023). ...
Across much of Europe, climate change has caused a major dieback of Norway spruce (Picea abies L.), an economically important tree species. However, the southeasternmost fringe of this tree species-the Eastern Carpathians-has not yet suffered large-scale dieback. Studying temporal shifts of climate sensitivity (TSCS) over time may elucidate the degree to which Norway spruce may be vulnerable to climate-change induced decline in upcoming decades. Under this framework, we analyzed a regional tree-ring network comprising >3000 trees, with the aim of quantifying TSCS since 1950. We mathematically defined TSCS as the slope parameter of the regression of climate sensitivity (the correlation coefficient) over time. Given the often-observed contrasting shift of climate sensitivity at low versus high elevations, we were particularly interested in studying potentially divergent TSCS along elevational and spatial gradients. Our analyses revealed several indications of TSCS for Norway spruce in the Eastern Carpathians. First, at high elevations (>1100 m a.s.l.), we found that the positive link between summer temperature and spruce growth decreased significantly over the study period. In turn, these trees, over time, featured an increasing positive relationship with late winter temperatures. At low elevations (<800 m a.s.l.), the signal of positive summer Standardised Precipitation-Evapotranspiration Index (SPEI) correlation became more frequent among sites towards 2021, while the strength of the positive winter SPEI correlation from the previous growing season weakened. Our results revealed that TSCS was driven significantly by an elevational climate gradient and a longitudinal continentality gradient. Overall, our findings indicate that Norway spruce is increasingly affected by water limitations under climate change at low elevations, highlighting a potentially rising risk of decline of this species in the Eastern Carpathians.
... Therefore, the phenotypical plasticity of tree populations indicates their adaptive capacity under a changing environment, and it is ex-pected to influence xylem formation and tree-ring growth (Matisons et al. 2019). Variability in xylem anatomy through quantitative analysis, in combination with valuable information on the xylem formation and factors affecting it, offers a more holistic approach to trees' ecophysiological response to extreme weather events or contrasting environments (Diaconu et al. 2017;Deslauriers et al. 2018) and predicting questions relevant to other disciplines such as forest ecology and forest management (Beeckman 2016) or even the probability of tree mortality (Cailleret et al. 2017). ...
... Even though the dead trees were smaller compared to the vital trees, our results strongly highlight the similar long-term growth trend and variability between the cooccurring vital and dead trees. A long-term reduction in radial growth (vigour loss) is a common growth pattern of trees that died following severe droughts [4,62,63], pointing to the so-called 'legacy effect' as a framework for explaining tree death [64][65][66]. Physiological changes can lead to higher drought impact and reduce growth recovery (loss of resilience capacity) in subsequent droughts, ultimately resulting in tree death [11]. ...
Understanding the drivers of drought-induced tree mortality remains a significant scientific challenge. Here, we investigated an unexpected mortality event of Nothofagus dombeyi (Mirb.) Oerst. following the 2014–15 drought in a Valdivian rainforest, Argentina. Our focus was on long-term growth trend differences between vital and dead trees, and how the mixing of species in tree neighbourhoods drives tree growth during drought. The inter-annual variation of basal area increments of vital and 2014–15-dead N. dombeyi trees showed a similar pattern through the 1930–2015 period, while the climate–growth relationships indicated that precipitation during the growing season promoted growth in both vitality classes, regardless of whether they were in the wettest location. For the period 1990–2015, both vitality classes showed similar estimated growth regardless of competition level, whereas species mingling in the neighbourhood significantly affected the dead tree growth. Network analysis revealed that drought performance covaried positively with a neighbourhood dominated by species functionally different from the focal species only in vital trees. These findings suggest a nuanced response of N. dombeyi to drought, shaped by multifaceted interactions at both the individual tree and neighbourhood levels. This research underscores that species-specific relationships under different mixtures imply different tree responses within a stand, and add complexity to understanding drought response at the individual level.
... These negative effects on growth might even deteriorate as soil water availability decreases, as Mortality is usually preceded by growth declines indicative of inciting stress that can span several decades [102][103][104] . Smaller trees in the lower canopy may furthermore have limited access to deeper water layers due to their less developed root systems 105 and may thus be more prone to drought-induced stress. ...
Floodplain forests are currently undergoing substantial reorganization processes due to the combined effects of management-induced altered hydrological conditions, climate change and novel invasive pathogens. Nowadays, the ash dieback is one of the most concerning diseases affecting temperate floodplain forests, causing substantial tree mortality and threatening the loss of the dominant key tree species of the hardwood floodplain forest, Fraxinus excelsior. Understanding how the increased light availability caused by pathogen-driven mortality in combination with altered hydrological conditions and climate change affects growth responses in a diverse forest community is of crucial importance for conservation efforts. Thus, we examined growth of the main tree species in response to ash dieback and how it depended on altered hydrological conditions under novel climatic conditions for the lower and upper canopy in the floodplain forest of Leipzig, Germany. Our study period encompassed the consecutive drought years from 2018 to 2020. We found that tree growth responded mostly positively to increased light availability, but only on moist sites, while tree growth largely declined on dry sites, suggesting that water availability is a critical factor for tree species to be able to benefit from increased light availability due to canopy disturbances caused by ash dieback. This hydrological effect was species specific in the lower canopy but not in the upper canopy. While, in the lower canopy, some species such as the competitive shade-tolerant but flood-intolerant Acer pseudoplatanus and Acer platanoides benefited from ash dieback on moist sites, others were less affected or suffered disproportionally, indicating that floodplain forests might turn into a novel ecosystem dominated by competitive Acer species, which may have detrimental effects on ecosystem functioning. Our results give hints on floodplain forests of the future and have important implications for conservation measures, suggesting that a substantial revitalization of natural hydrological dynamics is important to maintain a tree composition that resembles the existing one and thus sustain their conservation status.
... Soil drought may damage the fine root system and reduce nutrient supply (Bréda et al., 2006), while heat can directly harm the photosynthetic apparatus (Húdoková et al., 2022), reducing investment in stem growth (Bigler et al., 2006;Berdanier & Clark, 2016;Obladen et al., 2021). Continued growth decline may serve as an early indicator of tree vitality loss and often hints at a higher mortality risk (Cailleret et al., 2017). ...
... principisrupprechtii (Liu et al., 2017;Oberhuber et al., 2015;Yang et al., 2022;Zhang et al., 2022). Predicting drought-year tree growth is crucial to reduce the uncertainties in the estimation of carbon storage (Cailleret et al., 2017;Duan et al., 2022;Zang et al., 2014). Therefore, the aims of our study were to: (a) optimize the climatic variables selection from a variety of climate variables, (b) evaluate the predictive performance on tree growth of L. gmelinii var. ...
Drought induced great tree growth declines and high mortality, leading to high uncertainty in carbon storage estimation. The influences of droughts on tree growth had been extensively explored, however, how to predict tree growth during drought years to reduce uncertainty in carbon storage prediction is still challenging. We utilized a combined approach of random forest importance assessment and the "VSURF" package in R software to optimize the variable selection, and then used the selected variables in random forest and multiple linear regression (MLR) method to predict the tree growth based on 132 monthly climate variables and tree-ring network consisting of 1198 Larix gmelinii var. principis-rupprechtii trees from 48 sites. Forty-three out of 132 variables were selected if the random forest importance assessment was only used to selected climate variable, however, twelve important variables were identified by optimized variable selection, which further improve model efficiency with the least variables. The comparison between random forest and MLR showed that the predictions of the random forest model showed a better fit with the observed tree-ring values than MLR from 1989 to 2018. The predicted growth of L. gmelinii var. principis-rupprechtii is better in dense sites compared to sparse sites. During summer drought years, random forest performs well to predict tree growth in densely distributed sites. Our results highlighted the usage of optimized variable selection method combined with the random forest model to predict drought-year growth over the dense sites. Our study is crucial to predict drought-year carbon sink of planted larch forests under different climate changes scenarios in the future.
... Both species suffered significant growth losses during the drought period, but in the post drought period, spruce showed a delayed recovery (part of the spruce trees dying and some not showing signs of significant recovery), while beech showed a strong and early recovery in the first 2 years. A study using tree-ring data of 36 species showed that drought-induced mortality caused a median decrease of 40% in growth rates of trees before death when compared to trees that survived, and duration of reductions was shorter and more intense when drought was associated with biotic agents after the drought (Cailleret et al. 2017). We could not ensure that pixels included in the TR analysis did not include trees dying due to drought stress or biotic agents after the drought due to the large spatial extent of TR. ...
Enhancing forest drought resilience is important for preserving ecosystem services in the face of climate change, but operational management methods for boreal forests aiming to preserve ecosystem services under drought are still largely missing. This study explores the use of satellite remote sensing to monitor vegetation stress phases related to drought progression and to quantify both short-and long-term impacts on forest growth. Data from the Sentinel-2 satellite mission were used to calculate the response of vegetation indices across six forest types, using the time period from 2015 to 2017 as a reference dataset. The percentual difference between the drought year of 2018 and the reference period was used to represent disturbance intensity and the system's capability to resist it. Additionally, the recovery time was taken into consideration. Breakpoint detection with daily temporal resolution was used to quantify the response time of each forest type in relation to the onset date of meteorological drought. Results indicate that Sentinel-2 data can be used to monitor vegetation stress associated with drought progression and estimate the characteristics of forest resilience. High temporal resolution observations should be prioritized over annual maximum vegetation index values to determine the intensity of disturbance. Consistent severe impacts were observed in areas with limited soil moisture availability. The recovery time of forests took up to 4 years. Drought exposure during consecutive years could be especially damaging for species requiring a longer recovery time than a single growing season. Sentinel-2-based monitoring approach could benefit decision-support systems for forest management aiming to enhance drought resilience.
Climate models have predicted changes in woody plant growth, vitality, and species distribution. Those changes are expected mainly within the boundaries of species ranges. We studied the influence of changing hydrothermal and burning-rate regimes on relict pine stands at the southern edge of the Pinus sylvestris range in Siberia. We hypothesize that (1) warming has stimulated pine growth under conditions of sufficient moisture supply, and (2) increased burning rate has threatened forest viability. We found that the increase in air temperature, combined with the decrease in soil and air drought, stimulated tree growth. Since the “warming restart” around 2000, the growth index (GI) of pines has exceeded its historical value by 1.4 times. The GI strongly correlates with the GPP and NPP of pine stands (r = 0.82). Despite the increased fire rate, the GPP/NPP and EVI index of both pine stands and surrounding bush–steppes are increasing, i.e., the pine habitat is “greening” since the warming restart. These results support the prediction (by climatic scenarios SSP4.5, SSP7.0, and SSP8.5) of improvement in tree habitat in the Siberian South. Meanwhile, warming has led to a reduction in the fire-return interval (up to 3–5 y). Although the post-fire density of seedlings on burns (ca. 10,000 per ha) is potentially sufficient for pine forest recovery, repeated surface fires have eliminated the majority of the undergrowth and afforestation. In a changing climate, the preservation of relict pine forests depends on a combination of moisture supply, burning rate, and fire suppression.
Forests and scrubland comprise a large proportion of terrestrial ecosystems and, due to the long lifespan of trees and shrubs, their capacity to grow and store carbon as lasting woody tissues is particularly sensitive to warming-enhanced drought occurrence. Climate change may trigger a transition from forests to scrubland in many drylands during the coming decades due to the higher resilience of shrubs. However, we lack standardized frameworks to compare the response to drought of woody plants. We present a framework and develop an index to estimate the drought-induced vulnerability (DrVi) of trees and shrubs based on the radial growth trajectory and the response of growth variability to a drought index. We used tree-ring width series of three tree (Pinus halepensis Mill., Juniperus thurifera L., and Acer monspessulanum L.) and three shrub (Juniperus oxycedrus L., Pistacia lentiscus L., and Ephedra nebrodensis Tineo ex Guss.) species from semi-arid areas to test this framework. We compared the DrVi values between species and populations and explored their temporal changes. Across species, the strongest DrVi values were found in declining P. halepensis stands and J. oxycedrus from the same site, while the lowest DrVi values were found in A. monspessulanum, P. lentiscus, and E. nebrodensis. Across populations, J. oxycedrus presented higher vulnerability in one of the dry sites. The P. halepensis declining stand showed a steady increase in DrVi value after the 1980s as the climate shifted toward warmer and drier conditions. We conclude that the DrVi allows comparing species and populations using a standardized general framework.
Models of forest growth and yield are frequently used to inform adaptive management decisions aimed at increasing forest resilience or promoting long-term carbon storage. Despite the increasing ecological detail represented in growth and yield models, there remains large variability (uncertainty) in predictions of forest dynamics under global change. Quantifying this uncertainty and accounting for it when making management decisions is integral to sustainable management in the face of changing conditions. However, the structure and complexity of modern growth and yield models make it challenging to quantify uncertainty and propagate it to predictions of forest dynamics under alternative management strategies. To address this challenge, we develop a Bayesian dynamical model informed by continuous forest inventory data that supports the quantification, partitioning, and propagation of uncertainty in predictions of forest dynamics at a stand scale. The model predicts the temporal evolution of the size-species distribution using a matrix projection process model approximating growth, mortality, and regeneration. Disturbance is integrated through its effects on the size-species distribution within a stand providing a flexible framework to represent adaptive management. We apply the model to long-term inventory data from the Penobscot Experimental Forest in Maine, USA to predict multi-decadal biomass dynamics under five alternative management strategies. Predictions are used to identify the management strategy maximizing live aboveground biomass growth and yield over the model period. We conclude by discussing the benefits and challenges of connecting the model to large-scale inventory data and how its predictions can be used to better inform adaptive management decisions.
Introduction
Understanding the stress recovery of trees, particularly with respect to increasing droughts due to climate change, is crucial. An often-overlooked aspect is how short versus long drought events of high intensity (i.e., low and high stress dose) result in stress damage and affect post-stress recovery.
Methods
This study examines the stress and recovery dynamics of 3-year-old Picea abies following a short drought (n = 5) of 18 days or a long drought (n = 9) of 51 days during late summer. We particularly assessed how the recovery of canopy conductance and tree transpiration is linked to i) stress intensity in terms of minimum water potential, ii) stress duration inferred by days below a water potential related to 12% hydraulic conductance loss (dP12), iii) stress dose inferred by the cumulative tree water deficit on days below P12 (TWDP12) as well as the cumulative water potential (Ψcum), and iv) the percent loss of conductive xylem area (PLA).
Results
Both drought treatments resulted in stem and root embolism with a higher PLA of 49% ± 10% in the long drought treatment compared to 18% ± 6% in the short drought treatment consistent across the measured plant parts. Suffering from embolism and leaf shedding (long drought, 32%; short drought, 12%), canopy conductance in the long drought treatment recovered to 41% ± 3% of the control and in the short drought treatment to 66% ± 4% at 12 days after drought release. These recovery rates were well explained by the observed PLA (R² = 0.66) and the dP12 (R² = 0.62) but best explained by stress dose metrics, particularly the cumulative TWDP12 (R² = 0.88).
Discussion
Our study highlights that stress duration and intensity should be integrated to assess post-stress recovery rates. Here, the tree water deficit derived from point dendrometers appears promising, as it provides a non-destructive and high temporal resolution of the stress dose.
The increase in the frequency and severity of drought-induced tree mortality in many European low-elevation forests poses considerable challenges to forest management and requires an understanding of its causes. We propose a novel framework for integrating the factors underlying drought-induced tree mortality in a dynamic vegetation model.We evaluate whether this framework accurately reproduces drought-related mortality in six mesic beech-dominated stands in 2018-2020, and over multiple years in a xeric Scots pine-dominated stand in Switzerland. Additionally, we investigate its behavior along a large climatic gradient in central Europe. We employ a three-step approach.First, we evaluate multiple drought indices for capturing tree growth responses to extreme drought. We find that in contrast to widespread indices such as SPI and SPEI, the ForClim drought index captures growth responses to drought intensity during summer, the growing period, and annually.Second, we assess in detail the capability of the ForClim soil moisture model to simulate soil water dynamics, comparing it to the mechanistic soil-vegetation-atmosphere model LWFBrook90. The ForClim soil moisture model adequately simulates soil water dynamics, particularly in extreme drought years.Third, based on Manion’s Decline Disease Theory, we develop a novel mortality submodel that combines predisposing and inciting factors. Its integration in ForClim captures drought-induced mortality events in the mesic beech forests as well as the multi-year mortality at the xeric Scots pine-dominated stand. Along a climatic gradient in central Europe, the model provides good quantifications of Potential Natural Vegetation.The novel framework to capture drought-related tree mortality is simple yet produces accurate results. The underlying hypothesis regarding the factors leading to drought-induced tree mortality is promising but requires further tests for its generality
Over the past decades, the expansion of natural secondary forests has played a crucial role in offsetting the loss of primary forests and combating climate change. Despite this, there is a gap in our understanding of how tree species' growth and mortality patterns vary with elevation in these secondary forests. In this study, we analyzed data from two censuses (spanning a five-year interval) conducted in both evergreen broadleaved forests (EBF) and temperate coniferous forests (TCF), which have been recovering for half a century, across elevation gradients in a subtropical mountain region, Mount Wuyi, China. The results indicated that the relative growth rate (RGR) of EBF (0.028 ± 0.001 cm·cm−1·a−1) and the mortality rate (MR) (20.03% ± 1.70%) were 27.3% and 16.4% higher, respectively, than those of TCF. Interestingly, the trade-off between RGR and MR in EBF weakened as elevation increased, a trend not observed in TCF. Conversely, TCF consistently showed a stronger trade-off between RGR and MR compared to EBF. Generalized linear mixed models revealed that elevation influences RGR both directly and indirectly through its interactions with slope, crown competition index (CCI), and tree canopy height (CH). However, tree mortality did not show a significant correlation with elevation. Additionally, DBH significantly influenced both tree growth and mortality, whereas and CH and CCI had opposite effects on tree growth between EBF and TCF. Our study underscores the importance of elevation in shaping the population dynamics and the biomass carbon sink balance of mountain forests. These insights enhance our understanding of tree species' life strategies, enabling more accurate predictions of forest dynamics and their response to environmental changes.
Silver fir (Abies alba Mill.) has been considered a suitable tree species for drought-resilient mountain forests but
recent observations of increased mortality rates in Central Europe challenge this notion, prompting the need for
in-depth assessments of drought-induced mortality. Here we analyzed whether the death of individual silver fir
trees during drought was indicated by early warning signals in radial stem growth patterns and declines in
drought-tolerance of radial growth. For this purpose, we compared tree-ring data of silver fir trees with droughtrelated dieback and their nearest living neighbors in three forest reserves in the Black Forest region (500–1000 m
asl). Early-warning signals in radial growth (EWS) were successful in separating dead from living trees. Longlasting (ca. 30 years) growth reductions were found to be the strongest mortality signal in fir trees, and these
were accompanied by increases in first-order autocorrelation of radial growth series. Dying firs exhibited initially
faster growth rates and were larger compared to surviving neighbors at the onset of growth decline, which
coincided with a period of increased air pollution in the 1970–80 s. Growth of dead trees was more sensitive to
soil moisture fluctuations compared to that of living trees, and this was accompanied by a lower growth resil
ience to drought in the former. In addition, decreases in growth recovery and resilience to drought were syn
chronized with the appearance of EWS (reductions in growth) in trees that eventually died during drought. The
results of this study raise the question whether the long-term growth reductions preceding tree death, closely
linked with declining drought resilience, are contributing factors to tree mortality or merely symptoms of rapidly
decreasing vigor in initially fast-growing, larger trees—potentially due to factors such as higher exposure to pre1980s air pollution or mistletoe infestation.
Understanding the impacts of climate change on the future growth of tree species is particularly important for conserving endemic species with limited geographic distributions, such as Serbian spruce (Picea omorika (Pan�cic) Purk.). This study describes an approach to assessing the effects of future climate conditions on the growth and the implications for future management to conserve this endangered species on the IUCN Red List. To investigate the climate-growth relationship, age structure and diameter growth trends, we have sampled 231 trees across 11 locations at National Park "Tara" in western Serbia. The existence of heterogeneous age structures suggests that Serbian spruce poses considerable potential for continual regeneration in stands with open canopy. Conducted dendroclimatological analysis exhibits exceptional coherence in growth patterns within populations (Rxy 0.67–0.78), allowing the established climate-sensitive mixed-effect model to achieve conditional Rc 2 ¼ 0.683. It is revealed that the radial increment of Serbian spruce is dominantly regulated by water deficit in the summer season. The rainfall amount during the spring is another meaningful climatic factor for growth trends, while minimal winter temperatures and previous autumn water balance show varying influences. Finally, the growth projections under climate change scenarios RCP4.5 and RCP8.5 foreseen reductions of up to one-third and almost half from the historical mean growth rate. The given estimations should be seen as a critical warning signal calling for immediate conversion from passive to active protection to preserve this unique species
Detection of early warning signals (EWSs) for forest declines is a critical yet challenging in forest ecology and management. Although the EWSs have been investigated from the perspective of external stresses, changes in tree resistance in stresses before forest declines have received much less attention. In this study, we utilized tree ring-width data from 346 juniper trees in the Qilian Mountains to calculate tree resistance in stresses during non-decline period of forest and to explore the temporal relationships between the occurrence of forest declines and changes in resistance of trees. The forest declines, characterized by sustained slow growth rates, were identified independently. The results showed that, there was a decrease in tree resistance during the 12 years before forest declines. This phenomenon was particularly common at the study sites. Resistance of trees before forest declines was positively correlated with tree growth during forest declines, which implying that lower pre-decline resistance was associated with more severe forest decline. These observations suggest that decreasing resistance can be used as an EWS of forest declines. Our results provide insights into the relationships between changes in ecological resilience and forest stability and are useful for monitoring or predicting changes in forest health under future climate change.
In subalpine fir wave forests, strips of dead and weakened trees occur perpendicular to the slope next to strips of healthy trees. To reveal the transpiration by weakened Abies veitchii trees exposed to increased atmospheric evaporative demand, we investigated the ecophysiological traits closely related to the growth and transpiration, comparing them with those of the healthy trees and saplings in the fir wave of Mt. Shimagare in central Japan. The transpiration rate ( E ) was investigated using sap flux sensors to measure heat pulse velocity and compared with the surrogate for the needle water demand, which was computed using a multilayered gas and energy transfer model (modeled E , E mod ). Weakened trees exhibited smaller diameter growth and narrower sapwood than healthy trees, as well as lower heat pulse velocity compared with healthy saplings. However, needle‐level traits did not differ significantly between weakened and healthy trees. Needle water potential at midday was as negative as the needle turgor loss point, and the measured heat pulse velocity increased linearly with E mod but leveled off above a certain E mod value in weakened trees and healthy saplings, suggesting that trees restricted E to balance the needle water budget. Heat pulse velocity of weakened trees leveled off at E mod lower than that of healthy saplings, probably due to lower capacity for water supply to the needles. Restriction of E would occur less frequently but be necessary for both weakened and healthy A. veitchii on Mt. Shimagare to avoid hydraulic failure, sacrificing photosynthetic carbon assimilation.
Wood density (WD) is an important quality and functional trait of wood. However, despite the relationships between WD and abiotic factors being important to model or predict spatial distributions of functional traits, as well as responses of vegetation to climate changes, in current Earth system models or dynamic global vegetation models (ESMs/DGVMs), WD is often oversimplified, being defined as a globally uniform constant either for all plant functional types (PFTs) or for each individual PFT. Such oversimplifications may lead to simulation biases in the morphology of woody PFTs, as well as ecosystem transition and vegetation–atmosphere interactions. Moreover, existing conclusions about the relationships between WD and abiotic factors drawn from field observations remain mixed, making model parameterization improvements difficult.
This study systematically investigated the influences of climate and soil factors on WD across various PFTs. Optimal fitting models for predicting WD within each PFT were then constructed by utilizing our collated global database of 138 604 observations. For WDs of tree PFTs, climate emerges as a more influential factor than soil characteristics, whereas for shrub PFTs the effects of climate and soil are of equivalent significance. Across all six PFTs, correlation coefficients between predictions by fitting models and observed WD range from 0.49 to 0.93. The predicted and observed WD exhibit good agreement across climate space. It is expected that the incorporation of our research findings into DGVMs will improve the simulation of tree height and forest fractional coverage, particularly in the central forest areas and forest transition zones.
Mistletoes are xylem-tapping hemiparasites that rely on their hosts for water and nutrient uptake. Thus, they impair tree performance in the face of environmental stress via altering the carbon and water relations and nutritional status of trees. To improve our understanding of physiological responses to mistletoe and ongoing climate change, we investigated radial growth, stable carbon and oxygen isotopic signals, and elemental composition of tree rings in silver fir (Abies alba Mill.) and Scots pine (Pinus sylvestris L.) forests infested with Viscum album L. We compared temporal series (1990–2020) of basal area increment (BAI), intrinsic water-use efficiency (iWUE), oxygen isotope composition (δ18O), nutrient concentrations and stoichiometric ratios between non-infested (NI) and severely infested (SI) fir and pine trees from populations located close to the xeric distribution limit of the species in north-eastern Spain. The SI trees showed historically higher growth, but the BAI trend was negative for more than three decades before 2020 and their growth rates became significantly lower than those of NI trees by the mid-2010s. Mistletoe infestation was related to an enhanced sensitivity of radial growth to vapour pressure deficit (atmospheric drought). The SI trees showed less pronounced iWUE increases (fir) and lower iWUE values (pine) than NI trees. The lower tree-ring δ18O values of SI trees may be the result of several superimposed effects operating simultaneously, including leaf-level evaporative enrichment, source water isotopic signals, and anatomical and phenological differences. We observed a deterioration of potassium (K) nutrition in tree-ring wood of both species in SI trees, along with accumulation of manganese (Mn). We suggest that such nutritional patterns are driven by the indirect effect of mistletoe-induced drought stress, particularly in pine. The combined analyses of different physiological indicators imprinted on tree rings provided evidence of the progressive onset of carbon, water and nutrient imbalances in mistletoe-infested conifers inhabiting seasonally dry regions.
Tree mortality is a key process shaping forest dynamics. Thus, there is a growing need for indicators of the likelihood of tree death. During the last decades, an increasing number of tree-ring based studies have aimed to derive growth–mortality functions, mostly using logistic models. The results of these studies, however, are difficult to compare and synthesize due to the diversity of approaches used for the sampling strategy (number and characteristics of alive and death observations), the type of explanatory growth variables included (level, trend, etc.), and the length of the time window (number of years preceding the alive/death observation) that maximized the discrimination ability of each growth variable. We assess the implications of key methodological decisions when developing tree-ring based growth–mortality relationships using logistic mixed-effects regression models. As examples, we use published tree-ring datasets from Abies alba (13 different sites), Nothofagus dombeyi (one site), and Quercus petraea (one site). Our approach is based on a constant sampling size and aims at (1) assessing the dependency of growth–mortality relationships on the statistical sampling scheme used, (2) determining the type of explanatory growth variables that should be considered, and (3) identifying the best length of the time window used to calculate them. The performance of tree-ring-based mortality models was reasonably high for all three species (area under the receiving operator characteristics curve, AUC > 0.7). Growth level variables were the most important predictors of mortality probability for two species (A. alba, N. dombeyi), while growth-trend variables need to be considered for Q. petraea. In addition, the length of the time window used to calculate each growth variable was highly uncertain and depended on the sampling scheme, as some growth–mortality relationships varied with tree age. The present study accounts for the main sampling-related biases to determine reliable species-specific growth–mortality relationships. Our results highlight the importance of using a sampling strategy that is consistent with the research question. Moving towards a common methodology for developing reliable growth–mortality relationships is an important step towards improving our understanding of tree mortality across species and its representation in dynamic vegetation models.
Drought- and heat-driven tree mortality, along with associated insect outbreaks, have been observed globally in recent decades and are expected to increase in future climates. Despite its potential to profoundly alter ecosystem carbon and water cycles, how tree mortality scales up to ecosystem functions and fluxes is uncertain. We describe a framework for this scaling where the effects of mortality are a function of the mortality attributes, such as spatial clustering and functional role of the trees killed, and ecosystem properties, such as productivity and diversity. We draw upon remote-sensing data and ecosystem flux data to illustrate this framework and place climate-driven tree mortality in the context of other major disturbances. We find that emerging evidence suggests that climate-driven tree mortality impacts may be relatively small and recovery times are remarkably fast (~4 years for net ecosystem production). We review the key processes in ecosystem models necessary to simulate the effects of mortality on ecosystem fluxes and highlight key research gaps in modeling. Overall, our results highlight the key axes of variation needed for better monitoring and modeling of the impacts of tree mortality and provide a foundation for including climate-driven tree mortality in a disturbance framework.
Significance
Predicting the impacts of climate extremes on plant communities is a central challenge in ecology. Physiological traits may improve prediction of drought impacts on forests globally. We perform a meta-analysis across 33 studies that span all forested biomes and find that, among the examined traits, hydraulic traits explain cross-species patterns in mortality from drought. Gymnosperm and angiosperm mortality was associated with different hydraulic traits, giving insight into the relative weights of different traits and mechanisms in mortality prediction. Our results provide a foundation for more mechanistic predictions of drought-induced tree mortality across Earth’s diverse forests.
A within-species trade-off between growth rates and lifespan has been observed across different taxa of trees, however, there is some uncertainty whether this trade-off also applies to shade-intolerant tree species. The main objective of this study was to investigate the relationships between radial growth, tree size and lifespan of shade-intolerant mountain pines. For 200 dead standing mountain pines (Pinus montana) located along gradients of aspect, slope steepness and elevation in the Swiss National Park, radial annual growth rates and lifespan were reconstructed. While early growth (i.e. mean tree-ring width over the first 50 years) correlated positively with diameter at the time of tree death, a negative correlation resulted with lifespan, i.e. rapidly growing mountain pines face a trade-off between reaching a large diameter at the cost of early tree death. Slowly growing mountain pines may reach a large diameter and a long lifespan, but risk to die young at a small size. Early growth was not correlated with temperature or precipitation over the growing period. Variability in lifespan was further contingent on aspect, slope steepness and elevation. The shade-intolerant mountain pines follow diverging growth trajectories that are imposed by extrinsic environmental influences. The resulting trade-offs between growth rate, tree size and lifespan advance our understanding of tree population dynamics, which may ultimately improve projections of forest dynamics under changing environmental conditions.
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought-induced forest dieback and tree death. Here we evaluate the relative role played by these two mechanisms in the long-term by quantifying wood-anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water-use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs. non-declining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of non-declining trees. Declining silver firs presented a lower iWUE than conspecific non-declining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long-lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2-5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long-term capacity of trees to withstand drought-induced dieback. This article is protected by copyright. All rights reserved.
Drought threatens tropical rainforests over seasonal to decadal timescales, but the drivers of tree mortality following drought remain poorly understood. It has been suggested that reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk through insufficient carbon supply to metabolism ('carbon starvation'). However, little is known about how NSC stores are affected by drought, especially over the long term, and whether they are more important than hydraulic processes in determining drought-induced mortality. Using data from the world's longest-running experimental drought study in tropical rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from mortality in the experimentally droughted forest increased substantially after >10 years of reduced soil moisture availability. The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees. However, we find no evidence that the droughted trees suffered carbon starvation, as their NSC concentrations were similar to those of non-droughted trees, and growth rates did not decline in either living or dying trees. Our results indicate that hydraulics, rather than carbon starvation, triggers tree death from drought in tropical rainforest.
p style="text-align: justify;">Drought-induced tree mortality has received much attention in the recent past. McDowell et al.’s (2008) hydraulic framework links tree hydraulics with carbon dynamics and proposes two non-exclusive mortality mechanisms: carbon starvation (CS) and hydraulic failure (HF). CS is often referred to as the (partial) depletion of non-structural carbohydrates (NSC) in response to stomatal closure, reduced C assimilation and sustained C storage dependency during longer droughts. HF describes a lethal level of xylem dysfunction from runaway embolism during severe droughts. While HF can be readily inferred from the percentage loss of conductivity in vascular tissues at the time of death, CS is much more difficult to assess.
Starvation is usually defined as a lack of food leading to suffering or death. In plants photosynthetic sugars play many functional roles, not only as a source of catabolic energy. For example, sugars are important for osmotic regulation of cell pressure and recent studies suggest a potential link between xylem parenchyma sugars and embolism repair following drought. Hence, carbon limitation could have a direct impact on tree hydraulics and HF; however, empirical evidence for such a mechanism is still inconclusive.
Although HF appears to be predominant during drought mortality, our limited understanding of the roles of NSC in hydraulic function precludes any premature refutation of CS as a mechanism in drought-induced tree mortality.</p
Parenchyma is an important tissue in secondary xylem of seed plants, with functions ranging from storage to defence and with effects on the physical and mechanical properties of wood. Currently, we lack a large‐scale quantitative analysis of ray parenchyma (RP) and axial parenchyma (AP) tissue fractions.
Here, we use data from the literature on AP and RP fractions to investigate the potential relationships of climate and growth form with total ray and axial parenchyma fractions (RAP).
We found a 29‐fold variation in RAP fraction, which was more strongly related to temperature than with precipitation. Stem succulents had the highest RAP values (mean ± SD: 70.2 ± 22.0%), followed by lianas (50.1 ± 16.3%), angiosperm trees and shrubs (26.3 ± 12.4%), and conifers (7.6 ± 2.6%). Differences in RAP fraction between temperate and tropical angiosperm trees (21.1 ± 7.9% vs 36.2 ± 13.4%, respectively) are due to differences in the AP fraction, which is typically three times higher in tropical than in temperate trees, but not in RP fraction.
Our results illustrate that both temperature and growth form are important drivers of RAP fractions. These findings should help pave the way to better understand the various functions of RAP in plants.
Drought and bark-beetle infestation are major and often interconnected drivers of forest dieback and tree death. These two stressors may interact and accelerate forest mortality, since warmer and drier conditions boost beetle attacks and reduce tree growth. However, the way in which drought and bark-beetle infestation interact and affect declining or dying trees is still poorly understood. To disentangle the long-term interaction between the two stressors, we quantified radial growth (basal area increment), resin production, mortality and intrinsic water-use efficiency (iWUE) inferred from wood-carbon isotope discrimination. We compared trees infested and recently killed by two abundant bark beetle species (. Orthotomicus erosus and Tomicus piniperda) with non-infested living trees in a drought-prone Aleppo pine plantation. Growth and iWUE showed similar values in infested and non-infested trees. Since bark-beetle-infested trees did not grow less than non-infested trees, our results did not support the hypothesis of higher costs of resin production at the expense of stem-wood formation. Radial growth was enhanced by cool and wet winter conditions prior to the growing season. However, infested trees showed lower growth responsiveness than did non-infested trees with respect to this climatic driver of growth. Infested trees also showed a lower resin-duct production two years prior to death than non-infested trees. The growth responsiveness to climate should be characterized in bark-beetle-infested trees, since a weak correlation between climate and growth can be regarded as a predisposing factor of infestation-induced tree death. Such reduced responsiveness to climate stress could be linked to the tree vulnerability to beetle attacks in drought-prone forests.
Tree growth rate is frequently used to estimate mortality probability. Yet, growth metrics can vary in form, and the justification for using one over another is rarely clear. We tested whether a growth index (GI) that scales the realized diameter growth rate against the potential diameter growth rate (PDGR) would give better estimates of mortality probability than other measures. We also tested whether PDGR, being a function of tree size, might better correlate with the baseline mortality probability than direct measurements of size such as diameter or basal area. Using a long-term dataset from the Sierra Nevada, California, U.S.A., as well as existing species-specific estimates of PDGR, we developed growth–mortality models for four common species. For three of the four species, models that included GI, PDGR, or a combination of GI and PDGR were substantially better than models without them. For the fourth species, the models including GI and PDGR performed roughly as well as a model that included only the diameter growth rate. Our results suggest that using PDGR can improve our ability to estimate tree survival probability. However, in the absence of PDGR estimates, the diameter growth rate was the best empirical predictor of mortality, in contrast to assumptions often made in the literature.
Patterns, mechanisms, projections, and consequences of tree mortality and associated broad- scale forest die-off due to drought accompanied by warmer temperatures—‘‘hotter drought’’, an emerging characteristic of the Anthropocene—are the focus of rapidly expanding literature. Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortality- relevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO2] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.
The impacts of climate extremes on terrestrial ecosystems are poorly understood but important for predicting carbon cycle feedbacks to climate change. Coupled climate-carbon cycle models typically assume that vegetation recovery from extreme drought is immediate and complete, which conflicts with the understanding of basic plant physiology. We examined the recovery of stem growth in trees after severe drought at 1338 forest sites across the globe, comprising 49,339 site-years, and compared the results with simulated recovery in climate-vegetation models. We found pervasive and substantial "legacy effects" of reduced growth and incomplete recovery for 1 to 4 years after severe drought. Legacy effects were most prevalent in dry ecosystems, among Pinaceae, and among species with low hydraulic safety margins. In contrast, limited or no legacy effects after drought were simulated by current climate-vegetation models. Our results highlight hysteresis in ecosystem-level carbon cycling and delayed recovery from climate extremes.
Copyright © 2015, American Association for the Advancement of Science.
Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects - bark beetles and defoliators - which are responsible for substantial tree mortality. We then discuss cross-biome patterns of insect-driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree-insect interactions will better inform projections of forest ecosystem responses to climate change.
© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
Tree mortality is a vital component of forest management in the context of prescribed fires; however, few studies have examined the effect of prefire tree health on postfire mortality. This is especially relevant for sugar pine (Pinus lambertiana Douglas), a species experiencing population declines due to a suite of anthropogenic factors. Using data from an old-growth mixed-conifer forest in Sequoia National Park, we evaluated the effects of fire, tree size, prefire radial growth, and crown condition on postfire mortality. Models based only on tree size and measures of fire damage were compared with models that included tree size, fire damage, and prefire tree health (e.g., measures of prefire tree radial growth or crown condition). Immediately following the fire, the inclusion of different metrics of prefire tree health produced variable improvements over the models that included only tree size and measures of fire damage, as models that included measures of crown condition performed better than fire-only models, but models that included measures of prefire radial growth did not perform better. However, 5 years following the fire, sugar pine mortality was best predicted by models that included measures of both fire damage and prefire tree health, specifically, diameter at breast height (DBH, 1.37 m), crown scorch, 30-year mean growth, and the number of sharp declines in growth over a 30-year period. This suggests that factors that influence prefire tree health (e.g., drought, competition, pathogens, etc.) may partially determine postfire mortality, especially when accounting for delayed mortality following fire.
Background and aims:
There is a growing concern about how forests will respond to increased herbivory associated with climate change. Carbon (C) and nitrogen (N) limitation are hypothesized to cause decreasing growth after defoliation, and eventually mortality. This study examines the effects of a natural and massive defoliation by an insect on mature trees' C and N storage, which have rarely been studied together, particularly in winter-deciduous species.
Methods:
Survival, growth rate, carbon [C, as non-structural carbohydrate (NSC) concentration] and nitrogen (N) storage, defences (tannins and total polyphenols), and re-foliation traits were examined in naturally defoliated and non-defoliated adult trees of the winter-deciduous temperate species Nothofagus pumilio 1 and 2 years after a massive and complete defoliation caused by the caterpillar of Ormiscodes amphimone (Saturniidae) during summer 2009 in Patagonia.
Key results:
Defoliated trees did not die but grew significantly less than non-defoliated trees for at least 2 years after defoliation. One year after defoliation, defoliated trees had similar NSC and N concentrations in woody tissues, higher polyphenol concentrations and lower re-foliation than non-defoliated trees. In the second year, however, NSC concentrations in branches were significantly higher in defoliated trees while differences in polyphenols and re-foliation disappeared and decreased, respectively.
Conclusions:
The significant reduction in growth following defoliation was not caused by insufficient C or N availability, as frequently assumed; instead, it was probably due to growth limitations due to factors other than C or N, or to preventative C allocation to storage. This study shows an integrative approach to evaluating plant growth limitations in response to disturbance, by examining major resources other than C (e.g. N), and other C sinks besides storage and growth (e.g. defences and re-foliation).
Dynamic models are pivotal for projecting forest dynamics in a changing climate, from the local to the global scale. They encapsulate the processes of tree population dynamics with varying resolution. Yet, almost invariably, tree mortality is modeled based on simple, theoretical assumptions that lack a physiological and/or empirical basis. Although this has been widely criticized and a growing number of empirically derived alternatives are available, they have not been tested systematically in models of forest dynamics.
We implemented an inventory‐based and a tree‐ring‐based mortality routine in the forest gap model ForClim v3.0. We combined these routines with a stochastic and a deterministic approach for the determination of tree status (alive vs. dead). We tested the four new model versions for two Norway spruce forests in the Swiss Alps, one of which was managed (inventory time series spanning 72 years) and the other was unmanaged (41 years). Furthermore, we ran long‐term simulations (~400 years) into the future under three climate scenarios to test model behavior under changing environmental conditions.
The tests against inventory data showed an excellent match of simulated basal area and stem numbers at the managed site and a fair agreement at the unmanaged site for three of the four empirical mortality models, thus rendering the choice of one particular model difficult. However, long‐term simulations under current climate revealed very different behavior of the mortality models in terms of simulated changes of basal area and stem numbers, both in timing and magnitude, thus indicating high sensitivity of simulated forest dynamics to assumptions on tree mortality.
Our results underpin the potential of using empirical mortality routines in forest gap models. However, further tests are needed that span other climatic conditions and mixed forests. Short‐term simulations to benchmark model behavior against empirical data are insufficient; long‐term tests are needed that include both nonequilibrium and equilibrium conditions. Thus, there is the potential to greatly improve the robustness of future projections of forest dynamics via more reliable tree mortality submodels.
Stomatal conductance (gs) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of gs in predictions of global water and carbon cycle changes, a global-scale database and an associated globally applicable model of gs that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed gs obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model and the leaf and wood economics spectrum. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of gs across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate.
Drought-related tree die-off episodes have been observed in all vegetated continents. Despite much research effort, however, the multiple interactions between carbon starvation, hydraulic failure and biotic agents in driving tree mortality under field conditions are still not well understood. We analysed the seasonal variability of non-structural carbohydrates (NSCs) in four organs (leaves, branches, trunk and roots), the vulnerability to embolism in roots and branches, native embolism (percentage loss of hydraulic conductivity (PLC)) in branches and the presence of root rot pathogens in defoliated and non-defoliated individuals in a declining Scots pine (Pinus sylvestris L.) population in the NE Iberian Peninsula in 2012, which included a particularly dry and warm summer. No differences were observed between defoliated and non-defoliated pines in hydraulic parameters, except for a higher vulnerability to embolism at pressures below −2 MPa in roots of defoliated pines. No differences were found between defoliation classes in branch PLC. Total NSC (TNSC, soluble sugars plus starch) values decreased during drought, particularly in leaves. Defoliation reduced TNSC levels across tree organs, especially just before (June) and during (August) drought. Root rot infection by the fungal pathogen Onnia P. Karst spp. was detected but it did not appear to be associated to tree defoliation. However, Onnia infection was associated with reduced leaf-specific hydraulic conductivity and sapwood depth, and thus contributed to hydraulic impairment, especially in defoliated pines. Infection was also associated with virtually depleted root starch reserves during and after drought in defoliated pines. Moreover, defoliated and infected trees tended to show lower basal area increment. Overall, our results show the intertwined nature of physiological mechanisms leading to drought-induced mortality and the inherent difficulty of isolating their contribution under field conditions.
Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought‐induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought‐related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir ( A bies alba ), S cots pine ( P inus sylvestris ) and Aleppo pine ( P inus halepensis ), to a severe drought event.
We compared basal area increment ( BAI ) trends and responses to climate and drought in declining (very defoliated and dying) vs. non‐declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non‐structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood‐inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death.
Declining silver fir and Scots pine trees showed less growth than non‐declining trees one to three decades, respectively, before the drought event, whereas A leppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood‐inhabiting fungi were found in post‐drought declining trees apart from blue‐stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death.
Synthesis . Early warning signals of drought‐triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for S cots and A leppo pines. Longer growth and defoliation series, additional vigour parameters and multi‐species comparisons are required to understand and predict drought‐induced tree death.
Many species have the ability to resprout vegetatively after a substantial loss of biomass induced by environmental stress, including drought. Many of the regions characterised by ecosystems where resprouting is common are projected to experience more frequent and intense drought during the 21st Century. However, in assessments of ecosystem response to drought disturbance there has been scant consideration of the resilience and post-drought recovery of resprouting species. Systematic differences in hydraulic and allocation traits suggest that resprouting species are more resilient to drought-stress than nonresprouting species. Evidence suggests that ecosystems dominated by resprouters recover from disturbance more quickly than ecosystems dominated by nonresprouters. The ability of resprouters to avoid mortality and withstand drought, coupled with their ability to recover rapidly, suggests that the impact of increased drought stress in ecosystems dominated by these species may be small. The strategy of resprouting needs to be modelled explicitly to improve estimates of future climate-change impacts on the carbon cycle, but this will require several important knowledge gaps to be filled before resprouting can be properly implemented.
Stem respiration plays a role in species coexistence and forest dynamics. Here we examined the intra- and inter-specific variability of stem CO2 efflux (E) in dominant and suppressed trees of six deciduous species in a mixed forest stand: Fagus sylvatica L., Quercus petraea [Matt.] Liebl, Quercus pyrenaica Willd., Prunus avium L., Sorbus aucuparia L. and Crataegus monogyna Jacq. We conducted measurements in late autumn. Within species, dominants had higher E per unit stem surface area (Es ) mainly because sapwood depth was higher than in suppressed trees. Across species, however, differences in Es corresponded with differences in the proportion of living parenchyma in sapwood and concentration of nonstructural carbohydrates (NSC). Across species, Es was strongly and NSC marginally positively related with an index of drought tolerance, suggesting that slow growth of drought-tolerant trees is related to higher NSC concentration and Es . We conclude that, during the leafless period, E is indicative of maintenance respiration and is related with some ecological characteristics of the species, such as drought resistance; that sapwood depth is the main factor explaining variability in Es within species; and that the proportion of NSC in the sapwood is the main factor behind variability in Es among species.
Global warming is expected to increase the frequency and intensity of droughts in the twenty-first century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project, projected twenty-first century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and Penman-Monteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit. Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of ≤-1) by the end of the twenty-first century from 12 to 30 %. PET induced moderate drying is even more severe in the SPEI projections (SPEI standard deviation of ≤-1; 11 to 44 %), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system.
Widespread dieback of aspen forests, sometimes called sudden aspen decline, has been observed throughout much of western North America, with the highest mortality rates in the southwestern United States. Recent aspen mortality has been linked to drought stress and elevated temperatures characteristic of conditions expected under climate change, but the role of individual aspen tree growth patterns in contributing to recent tree mortality is less well known. We used tree-ring data to investigate the relationship between an individual aspen tree's lifetime growth patterns and mortality. Surviving aspen trees had consistently higher average growth rates for at least 100 years than dead trees. Contrary to observations from late successional species, slow initial growth rates were not associated with a longer lifespan in aspen. Aspen trees that died had slower lifetime growth and slower growth at various stages of their lives than those that survived. Differences in average diameter growth between live and dead trees were significant (α = 0.05) across all time periods tested. Our best logistical model of aspen mortality indicates that younger aspen trees with lower recent growth rates and higher frequencies of abrupt growth declines had an increased risk of mortality. Our findings highlight the need for species-specific mortality functions in forest succession models. Size-dependent mortality functions suitable for late successional species may not be appropriate for species with different life history strategies. For some early successional species, like aspen, slow growth at various stages of the tree's life is associated with increased mortality risk.
The processes leading to drought-associated tree mortality are poorly understood, particularly long-term predisposing factors, memory effects, and variability in mortality processes and thresholds in space and time. We use tree rings from four sites to investigate Pinus edulis mortality during two drought periods in the southwestern USA. We draw on recent sampling and archived collections to (1) analyze P. edulis growth patterns and mortality during the 1950s and 2000s droughts; (2) determine the influence of climate and competition on growth in trees that died and survived; and (3) derive regression models of growth-mortality risk and evaluate their performance across space and time. Recent growth was 53% higher in surviving vs. dying trees, with some sites exhibiting decades-long growth divergences associated with previous drought. Differential growth response to climate partly explained growth differences between live and dead trees, with responses wet/cool conditions most influencing eventual tree status. Competition constrained tree growth, and reduced trees' ability to respond to favorable climate. The best predictors in growth-mortality models included long-term (15-30 year) average growth rate combined with a metric of growth variability and the number of abrupt growth increases over 15 and 10 years, respectively. The most parsimonious models had high discriminatory power (ROC>0.84) and correctly classified ∼70% of trees, suggesting that aspects of tree growth, especially over decades, can be powerful predictors of widespread drought-associated die-off. However, model discrimination varied across sites and drought events. Weaker growth-mortality relationships and higher growth at lower survival probabilities for some sites during the 2000s event suggest a shift in mortality processes from longer-term growth-related constraints to shorter-term processes, such as rapid metabolic decline even in vigorous trees due to acute drought stress, and/or increases in the attack rate of both chronically stressed and more vigorous trees by bark beetles.
The drivers of background tree mortality rates – the typical low rates of tree mortality found in forests in the absence of acute stresses like drought – are central to our understanding of forest dynamics, the effects of ongoing environmental changes on forests, and the causes and consequences of geographical gradients in the nature and strength of biotic interactions. To shed light on factors contributing to background tree mortality, we analyzed detailed pathological data from 200,668 tree-years of observation and 3729 individual tree deaths, recorded over a 13-year period in a network of permanent forest plots in California's Sierra Nevada mountain range. We found that: (1) Biotic mortality factors (mostly insects and pathogens) dominated (58%), particularly in larger trees (86%). Bark beetles were the most prevalent (40%), even though there were no outbreaks during the study period; in contrast, the contribution of defoliators was negligible. (2) Relative occurrences of broad classes of mortality factors (biotic, 58%; suppression, 51%; and mechanical, 25%) are similar among tree taxa, but may vary with tree size and growth rate. (3) We found little evidence of distinct groups of mortality factors that predictably occur together on trees. Our results have at least three sets of implications. First, rather than being driven by abiotic factors such as lightning or windstorms, the “ambient” or “random” background mortality that many forest models presume to be independent of tree growth rate is instead dominated by biotic agents of tree mortality, with potentially critical implications for forecasting future mortality. Mechanistic models of background mortality, even for healthy, rapidly-growing trees, must therefore include the insects and pathogens that kill trees. Second, the biotic agents of tree mortality, instead of occurring in a few predictable combinations, may generally act opportunistically and with a relatively large degree of independence from one another. Finally, beyond the current emphasis on folivory and leaf defenses, studies of broad-scale gradients in the nature and strength of biotic interactions should also include biotic attacks on, and defenses of, tree stems and roots. This article is protected by copyright. All rights reserved.
Recent forest diebacks, combined with threats of future drought, focus attention on the extent to which tree death is caused by catastrophic events as opposed to chronic declines in health that accumulate over years. While recent attention has focused on large-scale diebacks, there is concern that increasing drought stress and chronic morbidity may have pervasive impacts on forest composition in many regions. Here we use long-term, whole-stand inventory data from southeastern U.S. forests to show that trees exposed to drought experience multiyear declines in growth prior to mortality. Following a severe, multiyear drought, 72% of trees that did not recover their pre-drought growth rates died within 10 yr. This pattern was mediated by local moisture availability. As an index of morbidity prior to death, we calculated the difference in cumulative growth after drought relative to surviving conspecifics. The strength of drought-induced morbidity varied among species and was correlated with drought tolerance. These findings support the ability of trees to avoid death during drought events but indicate shifts that could occur over decades. Tree mortality following drought is predictable in these ecosystems based on growth declines, highlighting an opportunity to address multiyear drought-induced morbidity in models, experiments, and management decisions.
Premise of the study:
Concentrations of nonstructural carbohydrates (NSCs) are used as proxies for the net carbon balance of trees and as indicators of carbon starvation resulting from environmental stress. Woody organs are the largest NSC-storing compartments in forest ecosystems; therefore, it is essential to understand the factors that affect the size of this important storage pool. In wood, NSC are predominantly deposited in ray and axial parenchyma (RAP); however, direct links between nutrient storage and RAP anatomy have not yet been established. Here, we tested whether the NSC storage capacity of wood is influenced by the amount of RAP.
Methods:
We measured NSC concentrations and RAP fractions in root and stem sapwood of 12 temperate species sampled at the onset of winter dormancy and in stem sapwood of four tropical trees growing in an evergreen lowland rainforest. The patterns of starch distribution were visualized by staining with Lugol's solution.
Key results:
The concentration of NSCs in sapwood of temperate trees scales tightly with the amount of RAP and living fibers (LFs), with almost all RAP and LFs being densely packed with starch grains. In contrast, the tropical species had lower NSC concentrations despite their higher RAP and LFs fraction and had considerable interspecific differences in starch distribution.
Conclusions:
The differences in RAP and LFs abundance affect the ability of sapwood to store NSC in temperate trees, whereas a more diverse set of functions of RAP might be pronounced in species growing in a tropical environment with little seasonality.
Recent forest diebacks, combined with threats of future drought, focus attention on the extent to which tree death is caused by catastrophic events as opposed to chronic declines in health that accumulate over years. While recent attention has focused on large-scale diebacks, there is concern that increasing drought stress and chronic morbidity may have pervasive impacts on forest composition in many regions. Here we use long-term, whole-stand inventory data from southeastern U.S. forests to show that trees exposed to drought experience multiyear declines in growth prior to mortality. Following a severe, multiyear drought, 72% of trees that did not recover their pre-drought growth rates died within 10 yr. This pattern was mediated by local moisture availability. As an index of morbidity prior to death, we calculated the difference in cumulative growth after drought relative to surviving conspecifi cs. The strength of drought-induced morbidity varied among species and was correlated with drought tolerance. These fi ndings support the ability of trees to avoid death during drought events but indicate shifts that could occur over decades. Tree mortality following drought is predictable in these ecosystems based on growth declines, highlighting an opportunity to address multiyear drought-induced morbidity in models, experiments, and management decisions.
Although the analysis of flux data has increased our understanding of the interannual variability of carbon inputs into forest ecosystems, we still know little about the determinants of wood growth. Here, we aimed to identify which drivers control the interannual variability of wood growth in a mesic temperate deciduous forest.
We analysed a 9-yr time series of carbon fluxes and aboveground wood growth (AWG), reconstructed at a weekly time-scale through the combination of dendrometer and wood density data.
Carbon inputs and AWG anomalies appeared to be uncorrelated from the seasonal to interannual scales. More than 90% of the interannual variability of AWG was explained by a combination of the growth intensity during a first ‘critical period’ of the wood growing season, occurring close to the seasonal maximum, and the timing of the first summer growth halt. Both atmospheric and soil water stress exerted a strong control on the interannual variability of AWG at the study site, despite its mesic conditions, whilst not affecting carbon inputs.
Carbon sink activity, not carbon inputs, determined the interannual variations in wood growth at the study site. Our results provide a functional understanding of the dependence of radial growth on precipitation observed in dendrological studies.
Although disturbances such as fire and native insects can contribute to natural dynamics of forest health, exceptional droughts, directly and in combination with other disturbance factors, are pushing some temperate forests beyond thresholds of sustainability. Interactions from increasing temperatures, drought, native insects and pathogens, and uncharacteristically severe wildfire are resulting in forest mortality beyond the levels of 20th-century experience. Additional anthropogenic stressors, such as atmospheric pollution and invasive species, further weaken trees in some regions. Although continuing climate change will likely drive many areas of temperate forest toward large-scale transformations, management actions can help ease transitions and minimize losses of socially valued ecosystem services.
Copyright © 2015, American Association for the Advancement of Science.
Temporal increases of tree mortality have been observed in regions where global warming has decreased long-term water availability and/or induced droughts. However, temporal decreases in water availability are not a global phenomenon. Understanding how water deficit-free forests respond to the recent effects of climate change is paramount towards a full appreciation of the impacts of climate change on global forests. Here, we reveal temporally increasing tree mortality across all study species over the last three decades in the central boreal forests of Canada, where long-term water availability has increased without apparent climate change-associated drought. In addition, we find that the effects of conspecific tree-to-tree competition have intensified temporally as a mechanism for the increased mortality of shade-intolerant tree species. Our results suggest that the consequences of climate change on tree mortality are more profound than previously thought.
© 2015 John Wiley & Sons Ltd/CNRS.
Bark beetle outbreaks are an important cause of tree death, but the process by which trees die remains poorly understood. The effect of beetle attack on whole-tree nonstructural carbohydrate (NSC) dynamics is particularly unclear, despite the potential role of carbohydrates in plant defense and survival. We monitored NSC dynamics of all organs in attacked and protected lodgepole pines (Pinus contorta) during a mountain pine beetle (Dendroctonus ponderosae) outbreak in British Columbia, starting before beetle flight in June 2011 through October 2012, when most attacked trees had died. Following attack, NSC concentrations were first reduced in the attacked region of the bole. The first NSC reduction in a distant organ appeared in the needles at the end of 2011, while branch and root NSC did not decline until much later in 2012. Attacked trees that were still alive in October 2012 had less beetle damage, which was negatively correlated with initial bark sugar concentrations in the attack region. The NSC dynamics of dying trees indicate that trees were killed by a loss of water conduction and not girdling. Further, our results identify locally reduced carbohydrate availability as an important mechanism by which stressors like drought may increase tree susceptibility to biotic attack.
For plants to grow they need resources and appropriate conditions that these resources are converted into biomass. While acknowledging the importance of co-drivers, the classical view is still that carbon, that is, photosynthetic CO2 uptake, ranks above any other drivers of plant growth. Hence, theory and modelling of growth traditionally is carbon centric. Here, I suggest that this view is not reflecting reality, but emerged from the availability of methods and process understanding at leaf level. In most cases, poorly understood processes of tissue formation and cell growth are governing carbon demand, and thus, CO2 uptake. Carbon can only be converted into biomass to the extent chemical elements other than carbon, temperature or cell turgor permit.
Copyright © 2015. Published by Elsevier Ltd.
1 Dendroecological techniques were used to examine the patterns of canopy recruitment in relation to disturbance history for two dominant, yet ecologically contrasting, tree species, Pinus strobus (white pine; disturbance dependent) and Tsuga canadensis (hemlock; late successional), in a 300-year-old primary forest. 2 Most tree recruitment in both species occurred between 1690 and 1810. All of the white pine, which dominated recruitment during the first 40 years due to more rapid height growth, recruited in this period. Low levels of hemlock recruitment continued until 1900. Most of the younger trees comprised several northern hardwood species. No trees were less than 50 years old and the forest was devoid of an understorey due to intense deer browsing. 3 Radial growth chronologies were determined for 27 cores across all species and age classes. These exhibited 1-11 major and/or moderate releases (indicative of disturbance) in most decades between 1730 and 1990. Peak releases were recorded in the 1950s when a series of severe windstorms impacted the site. Species recruitment patterns were related to earlier growth releases observed in the oldest cores. 4 White pine exhibited a degree of plasticity in initial radial growth (1-5 mm year) depending on the time of establishment, as well as the ability to survive through prolonged periods of depressed growth (
Sudden and widespread forest die-back and die-off (e.g., Huang & Anderegg, 2012) and increased mortality rates (e.g., Peng et al., 2011) in many forest ecosystems across the globe have been linked to drought and elevated temperatures (Allen et al., 2010, Fig. 1). Furthermore, these observations have caused a focus on the physiological mechanisms of drought-induced tree mortality (e.g. McDowell et al., 2008) and many studies, both observational and manipulative, have been carried out to explain tree death during drought from a physiological perspective.
The formation of emboli in xylem conduits can dramatically reduce hydraulic capacity and represents one of the principal mechanisms of drought‐induced mortality in woody plants. However, our understanding of embolism formation and repair is constrained by a lack of tools to directly and nondestructively measure these processes at high spatial resolution.
Using synchrotron‐based microcomputed tomography (micro CT ), we examined embolism in the xylem of coast redwood ( Sequoia sempervirens ) saplings that were subjected to cycles of drought and rewatering.
Embolism formation was observed occurring by three different mechanisms: as tracheids embolizing in wide tangential bands; as isolated tracheids in seemingly random events; and as functional groups connected to photosynthetic organs. Upon rewatering, stem water potential recovered to predrought stress levels within 24 h; however, no evidence of embolism repair was observed even after a further 2 wk under well‐watered conditions.
The results indicate that intertracheid air seeding is the primary mechanism by which embolism spreads in the xylem of S. sempervirens , but also show that a small number of tracheids initially become gas‐filled via another mechanism. The inability of S. sempervirens saplings to reverse drought‐induced embolism is likely to have important ecological impacts on this species.
Bark beetle outbreaks are an important cause of tree death, but the process by which trees die remains poorly understood. The effect of beetle attack on whole-tree nonstructural carbohydrate (NSC) dynamics is particularly unclear, despite the potential role of carbohydrates in plant defense and survival. We monitored NSC dynamics of all organs in attacked and protected lodgepole pines (Pinus contorta) during a mountain pine beetle (Dendroctonus ponderosae) outbreak in British Columbia, starting before beetle flight in June 2011 through October 2012, when most attacked trees had died. Following attack, NSC concentrations were first reduced in the attacked region of the bole. The first NSC reduction in a distant organ appeared in the needles at the end of 2011, while branch and root NSC did not decline until much later in 2012. Attacked trees that were still alive in October 2012 had less beetle damage, which was negatively correlated with initial bark sugar concentrations in the attack region. The NSC dynamics of dying trees indicate that trees were killed by a loss of water conduction and not girdling. Further, our results identify locally reduced carbohydrate availability as an important mechanism by which stressors like drought may increase tree susceptibility to biotic attack.
© 2015 The Authors. New Phytologist © 2015 New Phytologist Trust.
Drought-induced forest mortality is an increasing global problem with wide-ranging consequences, yet mortality mechanisms remain poorly understood. Depletion of non-structural carbohydrate (NSC) stores has been implicated as an important mechanism in drought-induced mortality, but experimental field tests are rare. We used an ecosystem-scale precipitation manipulation experiment to evaluate leaf and twig NSC dynamics of two co-occurring conifers that differ in patterns of stomatal regulation of water loss and recent mortality: the relatively desiccation-avoiding piñon pine (Pinus edulis) and the relatively desiccation-tolerant one-seed juniper (Juniperus monosperma). Piñon pine experienced 72% mortality after 13-25 months of experimental drought and juniper experienced 20% mortality after 32-47 months. Juniper maintained three times more NSC in the foliage than twigs, and droughted juniper converted NSC to glucose and fructose, consistent with osmoregulation requirements to maintain higher stomatal conductance during drought than piñon. Despite these species differences, experimental drought caused decreased leaf starch content in dying trees of both species (p<0.001). Average dry-season leaf starch content was also a good predictor of drought-survival time for both species (R2 = 0.93). These results, along with observations of drought-induced reductions to photosynthesis and growth, support carbon limitation as an important process during mortality of these two conifer species.
Multiple hypotheses have been put forward to explain the rise of angiosperms to ecological dominance following the Cretaceous. A unified scheme incorporating all these theories appears to be an inextricable knot of relationships, processes and plant traits. Here, we revisit these hypotheses, categorising them within frameworks based on plant carbon economy, resistance to climatic stresses, nutrient economy, biotic interactions and diversification. We maintain that the enigma remains unresolved partly because our current state of knowledge is a result of the fragmentary nature of palaeodata. This lack of palaeodata limits our ability to draw firm conclusions. Nonetheless, based on consistent results, some inferences may be drawn. Our results indicate that a complex multidriver hypothesis may be more suitable than any single-driver theory. We contend that plant carbon economy and diversification may have played an important role during the early stages of gymnosperms replacement by angiosperms in fertile tropical sites. Plant tolerance to climatic stresses, plant nutrition, biotic interactions and diversification may have played a role in later stages of angiosperm expansion within temperate and harsh environments. The angiosperm knot remains partly tied, but to unravel it entirely will only be feasible if new discoveries are made by scientific communities.
Forest succession depends strongly on the life history strategies of individual trees. An important strategic element is the ability to survive unfavourable environmental conditions that result in strongly reduced tree growth. In this study, we investigated whether the relationship between growth and mortality differs among tree species and site conditions. We analysed 10 329 trees of nine tree species (Picea abies, Taxus baccata, Fagus sylvatica, Tilia cordata, Carpinus betulus, Fraxinus excelsior, Quercus robur, Betula spp. and Alnus glutinosa) from unmanaged forests of Europe: the continental Białowieża forest (Poland) and several oceanically influenced Swiss forest reserves. For each species, we calculated a set of flexible logistic regression models with the explanatory variables growth (as measured by relative basal area increment), tree size and site. We selected the species-specific model with the highest goodness-of-fit and calculated its discriminatory power (area under the receiver operating characteristic curve, AUC) and calibration measures. Most models achieved at least a good discriminatory power (AUC>0.7) and the AUC ranged from 0.62 to 0.87; calibration curves did not indicate any overfitting. Almost all growth–mortality relationships differed among species and sites, i.e. there is no universal growth–mortality relationship. Some species such as F. excelsior showed reduced survival probabilities for both unfavourable and very good growth conditions. We conclude that the growth–mortality relationships presented here can contribute to the life-history classification of trees and that they should also help to improve projections of forest succession models.
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Tree-rings offer one of the few possibilities to empirically quantify and reconstruct forest growth dynamics over years to millennia. Contemporaneously with the growing scientific community employing tree-ring parameters, recent research has suggested that commonly applied sampling designs (i.e., how and which trees are selected for dendrochronological sampling) may introduce considerable biases in quantifications of forest responses to environmental change. However, a systematic assessment of the consequences of sampling design on dendroecological and -climatological conclusions has not yet been performed. Here, we investigate potential biases by sampling a large population of trees and replicating diverse sampling designs. This is achieved by retroactively sub-setting the population and specifically testing for biases emerging for climate reconstruction, growth response to climate variability, long-term growth trends, and quantification of forest productivity. We find that commonly applied sampling designs can impart systematic biases of varying magnitude to any type of tree-ring based investigations, independent of the total number of samples considered. Quantifications of forest growth and productivity are particularly susceptible to biases, whereas growth responses to short-term climate variability are less affected by the choice of sampling design. The world's most frequently applied sampling design, focusing on dominant trees only, can bias absolute growth rates by up to 459% and trends in excess of 200%. Our findings challenge paradigms, where a subset of samples is typically considered to be representative for the entire population. The only two sampling strategies meeting the requirements for all types of investigations are the i) sampling of all individuals within a fixed area, and ii) fully randomized selection of trees. This result advertises the consistent implementation of a widely-applicable sampling design to simultaneously reduce uncertainties in tree-ring based quantifications of forest growth and increase the comparability of datasets beyond individual studies, investigators, laboratories, and geographical boundaries.This article is protected by copyright. All rights reserved.
Summary
1. Ageing and senescence in plants remain poorly understood. Although meristem totipotency may allow woody perennials to be immortal, relative growth and photosynthetic rates typically decline with age.
2. Trees of ages between 129 and 534 years were selected in one of the oldest extant populations of Scots pine. Apical branches were propagated by grafting onto homogeneous juvenile rootstock to eliminate the effects of size and environmental variability and isolate those due to age. The hormonal profile of leaves and seeds along with markers of the physiological status of leaves and their pattern of DNA cytosine methylation were measured 15 years after grafting.
3. The percentage of total methylated loci in nuclear DNA increased with increasing meristematic age. However, only very few significant relationships were found between levels of phyto-hormones, pigments or physiological markers either in leaves or seeds and age of the meristem. In addition, shoots grafted from old trees grew as fast as those from younger trees and produced the same number of germinable seeds.
4. Synthesis. We conclude that changes in DNA methylation can occur in old trees. The lack of apparent physiological deterioration in the grafted plants suggests that meristem senescence is not the main factor triggering whole-plant ageing in Scots pine
Key message
Mountain pines in the Swiss National Park show evidence of partial cambial mortality, which affects the precision of tree-ring-based death dates, followed by lagged crown mortality.
Abstract
The time of tree death is commonly reconstructed by dating the outermost ring of tree-ring series. However, due to the occurrence of partial cambial mortality, the date of the outermost tree ring may vary between different locations on the tree stem. Furthermore, a tree may continue to live following the formation of the most recent tree ring. In this study, we quantified precision and accuracy of tree-ring-based death dates from 229 dead mountain pines (Pinus montana) from a 28 km2 study area in the Swiss National Park. For almost two-thirds of the trees, a maximum difference of just 0–4 years between the dates of cambial mortality from three increment cores was observed, however, for a few trees the difference reached 30–65 years. Higher maximum differences between the dates of cambial mortality are expected for trees on steep slopes, for old trees or for trees that died a long time ago. For 84 % of dead mountain pines, which were sampled in a permanent sample plot with 2-year remeasurement intervals, the difference between the date of observed crown mortality and the death date determined from three cores was 0–5 years. Sampling two or just one core per tree decreases the accuracy of tree-ring-based death dates. Based on the findings of our study, we recommend a prior assessment of the precision and accuracy of tree-ring-based death dates for any dendroecological study dealing with the reconstruction of tree mortality.