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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.

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... However, the oldest trees, that were more than 100-years old, did not present missing rings. Thus, the increased frequency of missing rings could be an early sign of tree dieback since these events are preceded by growth reduction and suppression (Camarero et al. 2015;Cailleret et al. 2016). A missing ring is easily identified during crossdating (Bräuning et al. 2016), however when there are several missing rings over a short period, it is very difficult to assign them the correct calendar year, especially when these missing rings are near the end of the time series. ...
... Soil water repellence could exacerbate drought effects on tree health by reducing the infiltration of the first rains into the soil after prolonged dry spells. Reduced recovery indicates growth decline after the disturbance, which can be an early signal of forest dieback (Cailleret et al. 2016). ...
... Worldwide episodes of tree dieback have been related to drought and heat in many forest types (Allen et al. 2010). In some species, drought-induced mortality has been observed at the driest populations or driest and warmest edges of their distribution range (Carnicer et al. 2011;Cailleret et al. 2016;Anderegg et al. 2019), indicating that these populations are more susceptible to climate change (Camarero et al. 2021a). Populations at the edge of the species distribution are probably beyond the species' "fundamental niche" and existing outside where the species can physiologically survive (Anderegg et al. 2019). ...
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Key message Pinus halepensis trees at the southern limit of the species distribution show less growth, low relative resilience, and more missing rings in response to increased temperatures and droughts. Abstract Climate change is increasing the frequency and intensity of extreme droughts in the northern hemisphere, leading to forest decline and tree mortality. Species are more vulnerable to climate fluctuations at the rear-edge limits of their distribution ranges. Pinus halepensis Mill. is a Mediterranean conifer with its southern distribution limit in the High Atlas Mountains of Morocco. The purpose of this study was to analyse the climate response of P. halepensis tree rings from the High Atlas Mountains of Morocco, and to determine its resilience to extreme drought events. Climate data for the study site revealed that temperature has significantly increased in recent decades, but precipitation remained unchanged, resulting in increased aridity. The ring-width time series revealed several missing rings since 1999, possibly linked to the increased aridity. Tree-ring width responded negatively to spring and summer maximum temperature and positively to previous winter and spring precipitation. Moving correlation analysis revealed an increased negative relation with maximum temperature in April, June, and July, supporting the adverse effect of global warming on P. halepensis growth. Resilience analysis revealed that trees were able to recover from extreme droughts, but its detrimental effect remained in the following years. Pinus halepensis trees at the southern limit of their distribution are already suppressing growth in extreme drought years. If the frequency of extreme droughts increases, as predicted by climate change models, the recovery capacity of these trees will be compromised, resulting in habitat loss and in the potential contraction of the species southern range.
... A corollary to this theory, is that in the absence of fire, live tree density and biomass accumulate, increasing competition for growth resources which can reduce tree vigor (Das et al., 2016). Many factors can episodically reduce tree vigor (i.e., surface fire, dry years, bark beetle outbreaks), but inter-tree competition can create chronic growth reductions which, in gymnosperms (Cailleret et al., 2017(Cailleret et al., , 2019, increase tree susceptibility to stress and potential mortality (Franklin et al., 1987;Das et al., 2011). While dendrochronology studies and historical records document frequent-fire forests' resistance to stress 'pulses', these forests may not be as well adapted to continuous internal stress 'presses' that result from sustained and widespread competition for resources. ...
... While dendrochronology studies and historical records document frequent-fire forests' resistance to stress 'pulses', these forests may not be as well adapted to continuous internal stress 'presses' that result from sustained and widespread competition for resources. Recent meta-analyses of growth patterns and tree mortality found that long-term reductions in gymnosperm radial growth from density-dependent competition were associated with chronic deterioration of a tree's carbon and water economies (Cailleret et al., 2017(Cailleret et al., , 2019. This response may explain why some restoration studies in dense, fire-suppressed forests, have recorded mortality of large, longlived trees following low-intensity treatments such as thinning small neighboring trees or re-introduction of surface fire (Das et al., 2011;Collins et al., 2014;van Mantgem et al., 2018;Stevens et al., 2020;Steel et al., 2021b). ...
... Many dry western U.S. forests historically had similar frequent, low-intensity fire regimes suggesting our results may be more broadly applicable to a larger geographic extent than the Sierra Nevada. Ecological theory and empirical field studies have associated robust growth rates with forest resilience to a range of stresses (Das et al., 2011(Das et al., , 2016Cailleret et al., 2017;Zhang et al., 2019). In frequent-fire forests, the competitive environment can be indirectly but readily assessed using relative stand density index, a standard forest management metric that can be used to assess potential forest resilience. ...
Article
With the increasing frequency and severity of altered disturbance regimes in dry, western U.S. forests, treatments promoting resilience have become a management objective but have been difficult to define or operationalize. Many reconstruction studies of these forests when they had active fire regimes have documented very low tree densities before the onset of fire suppression. Building on ecological theory and recent studies, we suggest that this historic forest structure promoted resilience by minimizing competition which in turn supported vigorous tree growth. To assess these historic conditions for management practices, we calculated a widely-used measure of competition, relative stand density index (SDI), for two extensive historical datasets and compared those to contemporary forest conditions. Between 1911 and 2011, tree densities on average increased by six to seven fold while average tree size was reduced by 50%. Relative SDI for historical forests was 23–28% of maximum, in the ranges considered ‘free of’ (<25%) to ‘low’ competition (25–34%). In contrast, most (82–95%) contemporary stands were in the range of ‘full competition’ (35–59%) or ‘imminent mortality’ (≥60%). Historical relative SDI values suggest that treatments for restoring forest resilience may need to be much more intensive then the current focus on fuels reduction. With the contemporary increase in compounding stresses such as drought, bark beetles, and high-severity wildfire, resilience in frequent-fire forests may hinge on creating stands with significantly lower densities and minimal competition. Current management practices often prescribe conditions that maintain full competition to guide development of desired forest conditions. Creating stands largely free of competition would require a fundamental rethinking of how frequent-fire forests can be managed for resilience.
... These reports are paralleled by dendrochronological studies demonstrating recent decline in the growth rates of major timber species, which have been related to increases in the frequency and severity of droughts (Jump et al., 2006, Dulamsuren et al., 2010Scharnweber et al., 2011;Zimmermann et al., 2015). The mortality increase has triggered an intensive debate about the likely physiological mechanisms leading to drought-induced tree death (Anderegg and Callaway, 2012;McDowell et al., 2013;Anderegg et al., 2015a), while less is known about the drivers of long-term, non-lethal decline in radial stem growth, if not attributable to senescence or competition effects (Cailleret et al., 2017). It is likely that long-term growth decline and death are often inter-related and caused by similar factors (Liang et al., 2016;Vanoni et al., 2016;Wang et al. 2020). ...
... The study of the physiological and physical drivers of tree death and forest dieback is mostly concerned with the immediate causes of mortality, only rarely taking effects of the more distant past into account, while dendrochronological studies of growth declines predominantly address living trees and only exceptionally deal with tree death. However, increasing evidence suggests that many cases of tree death and forest dieback can only be fully understood, when a causal analysis of the growth patterns of the past is conducted (Bigler and Bugmann, 2004;Cailleret et al., 2017). ...
... Several authors have examined tree-ring series with the aim to identify parameters suited for predicting drought-induced tree mortality. Vanoni et al. (2016), Cailleret et al. (2017Cailleret et al. ( , 2019 and Wang et al. (2020) reported abrupt or gradual decreases in the annual stem increment for variable time spans prior to death; however, such growth declines were not consistently found. Long-term growth declines seem to be more typical for dying gymnosperms, whereas angiosperms rather tend to show smaller growth reductions for short periods before death (Cailleret et al., 2017). ...
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Widespread increases in tree mortality have fueled the debate on the mechanisms of heat- and drought-related tree death. While much research focused on the immediate causes of tree death, the role of predisposing factors for death is not well understood. We employed tree-ring and climate sensitivity analysis of growth to study the importance of legacy effects of past disturbances for growth patterns and a recent mortality burst in a protected European beech (Fagus sylvatica) forest, comparing groups of live and recently died trees in the same stand. Live and dead trees showed a decade-long decrease in radial growth since the severe 1976 drought, which indicates that the high mortality in the last two decades is not solely caused by recent drought spells, but that trees were apparently predisposed by previous events to death decades later. An even more distant event than the 1976 drought that has imprinted on the chronologies is the severe 1947 drought in combination with extensive logging at that time. It appears that stand opening 60 and 40 years ago due to high immediate drought mortality in conjunction with heavy logging resulted in much higher inter-annual ring width variability with more extreme negative and positive pointer years in the subsequent years, and led to permanent reduction in tree health. We identified the first derivative of the ring width curve as the most reliable early-warning signal of a predisposition to drought-induced death, while inter-annual growth variability and growth resistance and resilience to drought were less suited. We suggest that the causes of recent climate warming-related dieback of beech and other temperate hardwood forests may only be fully grasped, when the imprint of past stress events on growth and vitality is understood. The physiological mechanisms causing legacy effects with long delay of death require further study.
... Our understanding of ecological drought impacts comes from a combination of experimental and observational studies. In forests, there has been a recent recognition that these ecosystems substantially suffer from drought events (Allen et al., 2010(Allen et al., , 2015Brun et al., 2020;Buras et al., 2020;Cailleret et al., 2017;Schuldt et al., 2020;Senf et al., 2020), even though in absolute terms drought impacts in forests are less intense when compared to dryland ecosystems Shekhar et al., 2020;Teuling et al., 2010;Wolf et al., 2013). Forest drought responses vary across temporal scales (Cavender-Bares & Bazzaz, 2000). ...
... Short-time responses (i.e., minutes to hours) include changes in stomatal closure (Jiang et al., 2019), altered energy balance (Sippel et al., 2018), a regulation of photosynthesis (Clark et al., 2016), and lowered hydraulic conductivity (Adams et al., 2017;Kukowski et al., 2013). Seasonal responses include phenological alterations, such as early wilting , leaf discoloration, leaf loss (MeteoSchweiz, 2018), growth reductions (Cailleret et al., 2017), and changes in resource allocation and repair mechanisms (Sippel et al., 2018). Long-term responses (i.e., years to decades) or legacy effects include plant mortality (Berdanier & Clark, 2018;Buras et al., 2018;Clark et al., 2016;Wolf et al., 2016), an increased vulnerability of the forest to other stressors (Miralles et al., 2019), and reduced growth (stem and canopy) in recovering trees (Anderegg et al., 2015;Kannenberg et al., 2019Kannenberg et al., , 2020Schwalm et al., 2017). ...
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Extreme events such as the summer drought of 2018 in Central Europe are projected to occur more frequently in the future and may cause major damages including increased tree mortality and negative impacts on forests ecosystem services. Here we quantify the response of > 1 million forest pixels of 10 x 10 m across Switzerland to the 2018 drought in terms of resistance, recovery, and resilience. We used the Normalized Difference Water Index (NDWI) derived from Sentinel‐2 satellite data as a proxy for canopy water content and analyzed its relative change. We calculated NDWI change between the 2017 pre‐drought and 2018 drought years (indicating resistance), 2018 and the 2019 post‐drought (indicating recovery), and between 2017–2019 (indicating resilience). Analyzing the data from this large natural experiment, we found that for 4.3% of the Swiss forest the NDWI declined between 2017 and 2018, indicating areas with low resistance of the forest canopy to drought effects. While roughly 50% of this area recovered, in 2.7% of the forested area NDWI continued to decline from 2018 to 2019, suggesting prolonged negative effects or delayed damage. We found differential forest responses to drought associated with site topographic characteristics and forest stand characteristics, and to a lesser extent with climatic conditions and interactions between these drivers. Low drought resistance and high recovery were most prominent at forest edges, but also on south‐facing slopes and lower elevations. Tree functional type was the most important driver of drought resilience, with most of the damage in stands with high conifer abundance. Our results demonstrate the suitability of satellite‐based quantification of drought‐induced forest damage at high spatial resolution across large areas. Such information is important to predict how local site characteristics may impact forest vulnerability to future extreme events and help in the search for appropriate adaptation strategies.
... G lobal environmental change is affecting ecosystems in many regions around the world. Forests are key terrestrial ecosystems where evidence increasingly points towards cascading impacts related to anthropogenic-induced climate change [1][2][3] , including far-reaching consequences for the water and carbon (C) cycles, and services to society 4 . Evolving questions related with those impacts can be best addressed through large-scale analyses, encompassing the full distribution range of key species 3 . ...
... Dendroecological analyses typically present local data and have provided valuable regional insight into growth responses to local habitat conditions and climate change 2,9 . Despite recent advances in tree-ring research 9 , spatio-temporal studies of actual and predicted growth are uncommon, particularly at scales incorporating species' geographic and climatic distributions 10 . ...
Article
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The growth of past, present, and future forests was, is and will be affected by climate variability. This multifaceted relationship has been assessed in several regional studies, but spatially resolved, large-scale analyses are largely missing so far. Here we estimate recent changes in growth of 5800 beech trees ( Fagus sylvatica L.) from 324 sites, representing the full geographic and climatic range of species. Future growth trends were predicted considering state-of-the-art climate scenarios. The validated models indicate growth declines across large region of the distribution in recent decades, and project severe future growth declines ranging from −20% to more than −50% by 2090, depending on the region and climate change scenario (i.e. CMIP6 SSP1-2.6 and SSP5-8.5). Forecasted forest productivity losses are most striking towards the southern distribution limit of Fagus sylvatica , in regions where persisting atmospheric high-pressure systems are expected to increase drought severity. The projected 21 st century growth changes across Europe indicate serious ecological and economic consequences that require immediate forest adaptation.
... Because riparian tree growth is limited by hydroclimatic factors, annual radial growth can be used as a measure of system performance and response to environmental stressors. For example, in riparian and floodplain systems, radial growth of poplars responds to drought stress resulting from changes in water table depth, precipitation, and streamflow because of the limitations that stomatal closure, leaf shedding, and branch sacrifice impose on carbon assimilation (Andersen, 2016;Cailleret et al., 2017;Stromberg & Patten, 1996). ...
... However, environmental stressors do not always reduce growth, as studies have observed plants to use stored carbon (McCarroll et al., 2017) or reallocate resources from other plant processes to maintain growth rates when water is limiting (Ogaya & Peñuelas, 2007), which can obscure interpretations of growth responses to drought. Furthermore, riparian tree growth can be permanently limited due to crown die-back (Andersen, 2016;Stella, Riddle, et al., 2013), and also influenced by biotic factors including competition, disease, and pest infestations (Cailleret et al., 2017), which can further complicate interpretations of climate response. ...
Article
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Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro‐isotopic analysis of radial growth and seasonal (semi‐annual) carbon isotope discrimination (Δ13C) to investigate the response of riparian cottonwood stands to the unprecedented California‐wide drought from 2012‐2019, along the largest remaining free‐flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ13C) during peak drought years. However, patterns of radial growth and Δ13C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ13C for individual trees, and higher inter‐correlation of Δ13C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m yr‐1 beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded ~3 m. These findings indicate that drought‐induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures.
... Most commonly, exceptional events of drought-related tree mortality are perceived as sudden events. However, in most cases, they are preceded by long-term signals, such as the long-term reduction in diameter growth (Cailleret et al., 2017) and/or crown condition decline with dessication symptoms (Carnicer et al., 2011). ...
... The progressive decline of xylem ring width and mean conduit lumen area at the stem base have been reported to characterize the growth of trees eventually succumbing even decades after the predisposing drought events (Cailleret et al., 2017;Pellizzari et al., 2016). Notably, the reduction in the xylem conductivity at the stem base unlikely would cause strong limitations to water transport, since the contribution of these tissues to the total hydraulic resistance is negligible compared with those towards the crown periphery (Lechthaler et al., 2020;Prendin, Mayr, et al., 2018). ...
Article
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Forest trees are experiencing increasing frequency and intensity of drought events with climate change. We investigated xylem and phloem traits from mature Fagus sylvatica and Picea abies trees after 5 years of complete exclusion of throughfall precipitation during the growing season. Xylem and phloem anatomy, leaf and branch biomass were analysed along top branches of ~1.5 m lenght in 5 throughfall precipitation excluded (TE) and 5 control (CO) trees of both beech and spruce. Xylem traits were analysed on wood cores extracted from the stem at breast height. In the top branches of both species, the lumen diameter (or area) of xylem and phloem conduits did not differ between TE and CO trees. At breast height, TE trees of both species produced narrower xylem rings and conduits. While allocation to branch (BM) and needle biomass (LM) did not change between TE and CO in P. abies, TE F. sylvatica trees allocated proportionally more biomass to leaves (LM) than BM compared to CO. Despite artificial drought increased the mortality in the TE plots, our results revealed no changes in both xylem and phloem anatomies, undermining the hypothesis that successful acclimation to drought would primarily involve increased resistance against air embolism.
... Our understanding of ecological drought impacts comes from a combination of experimental and observational studies. In forests, there has been a recent recognition that these ecosystems substantially suffer from drought events (Allen et al., 2010(Allen et al., , 2015Brun et al., 2020;Buras et al., 2020;Cailleret et al., 2017;Schuldt et al., 2020;Senf et al., 2020), even though in absolute terms drought impacts in forests are less intense when compared to dryland ecosystems Shekhar et al., 2020;Teuling et al., 2010;Wolf et al., 2013). Forest drought responses vary across temporal scales (Cavender-Bares & Bazzaz, 2000). ...
... Short-time responses (i.e., minutes to hours) include changes in stomatal closure (Jiang et al., 2019), altered energy balance (Sippel et al., 2018), a regulation of photosynthesis (Clark et al., 2016), and lowered hydraulic conductivity (Adams et al., 2017;Kukowski et al., 2013). Seasonal responses include phenological alterations, such as early wilting , leaf discoloration, leaf loss (MeteoSchweiz, 2018), growth reductions (Cailleret et al., 2017), and changes in resource allocation and repair mechanisms (Sippel et al., 2018). Long-term responses (i.e., years to decades) or legacy effects include plant mortality (Berdanier & Clark, 2018;Buras et al., 2018;Clark et al., 2016;Wolf et al., 2016), an increased vulnerability of the forest to other stressors (Miralles et al., 2019), and reduced growth (stem and canopy) in recovering trees (Anderegg et al., 2015;Kannenberg et al., 2019Kannenberg et al., , 2020Schwalm et al., 2017). ...
Article
Vegetation responds at varying temporal scales to changing soil water availability. These process dynamics complicate assessments of plant-water relations but also offer various access points to advance understanding of vegetation responses to environmental change. Remote sensing (RS) provides large capacity to quantify sensitive and robust information of vegetation responses and underlying abiotic change driver across observational scales. Retrieved RS based vegetation parameters are often sensitive to various environmental and plant specific factors in addition to the targeted plant response. Further, individual plant responses to water limitation act at different temporal and spatial scales, while RS sampling schemes are often not optimized to assess these dynamics. The combination of these aspects complicates the interpretation of RS parameter when assessing plant-water relations. We consequently aim to advance insight on the sensitivity of physiological, biochemical and structural RS parameter for plant adaptation in response to emerging soil water limitation. We made a field experiment in maize in Tuscany (Central Italy), while irrigation was stopped in some areas of the drip-irrigated field. Within a period of two weeks, we measured the hydraulic and physiological state of maize plants in situ and complemented these detailed measurements with extensive airborne observations (e.g. sun-induced chlorophyll fluorescence (SIF), vegetation indices sensitive for photosynthesis, pigment and water content, land surface temperature). We observe a double response of far-red SIF with a short-term increase after manifestation of soil water limitation and a decrease afterwards. We identify different response times of RS parameter representing different plant adaptation mechanisms ranging from short term responses (e.g. stomatal conductance, photosynthesis) to medium term changes (e.g. pigment decomposition, changing leaf water content). Our study demonstrates complementarity of common and new RS parameter to mechanistically assess the complex cascade of functional, biochemical and structural plant responses to evolving soil water limitation.
... Droughts cause abrupt growth reductions in tree growth (Schweingruber, 1986) that can last for several years creating carryover effects or drought legacies (see Anderegg et al., 2015) and impacting drought resilience capacity (Lloret et al., 2011). Importantly, tree growth responses to past droughts may determine the likelihood of dieback and mortality occurrence in the future and be used as early warning signals of tree vulnerability (Camarero et al., 2015a(Camarero et al., , 2021aCailleret et al., 2017;De Soto et al., 2020;Keen et al., 2022). ...
... All together, these results suggest that all species are vulnerable to drought occurrence but that P. halepensis might be in an advantageous position to thrive in a drier world and outperform the other species, at least in the study site or in other sites with similar climate conditions. It has been suggested that negative growth trends are common before drought-induced dieback in conifer species (e.g., Camarero et al., 2015aCamarero et al., , 2018Camarero et al., , 2021aCailleret et al., 2017;De Soto et al., 2020;Keen et al., 2022). We observed that growth trends for P. sylvestris and P. nigra were negative in the period 2008−2017 after accounting for age-and SPEI-effects (Figure 4). ...
Article
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Drought is an important driver of forest dynamics in the Mediterranean region. The forecasted increase in drought frequency and severity can notably influence tree growth, forest structure, composition and productivity. Understanding how coexisting tree species respond to drought is thus crucial to understand which are less vulnerable and will perform better in a warmer and drier world. To assess drought vulnerability, we used dendrochronology to study the radial growth trends and responses to a drought index of four pine species (Pinus halepensis, Pinus pinea, Pinus nigra, and Pinus sylvestris) coexisting in North-eastern Spain. We reconstructed the growth of each species and evaluated their short- and long-term growth response to drought for the common period 1980–2017. The growth of the four pine species depended on water availability and high early spring temperatures impacted the growth of P. nigra and P. sylvestris negatively. The occurrence of a severe drought between 2005 and 2007 lead to marked growth reductions in the four species, but it was greater in magnitude in P. pinea and P. halepensis in 2005, and in P. nigra in 2007. The results of basal area increment models at the individual tree level suggested that P. halepensis trees grow more than the rest of species. After accounting for age and drought effects, P. nigra and P. sylvestris displayed negative growth trends in the 2008–2017 period while P. pinea and P. halepensis displayed positive growth trends. P. sylvestris was the most resistant species and P. pinea the less resistant. Conversely, P. halepensis and P. pinea were slightly more resilient than P. sylvestris. Moreover, P. sylvestris was the species displaying the highest autocorrelation and the lowest coefficient of variation in ring-width indices. A marked drop in the autocorrelation of P. pinea ring-width index was observed in response to the 2005 drought. These results indicate that all study species are vulnerable to drought but in different degrees. The strong resilience capacity of P. halepensis suggests that it will better thrive in a drier future, but mixed pine forests, such as the one here studied, may contract or become rare due to the strong sensitivity of P. pinea to drought and the lower post-drought performance of P. nigra and P. sylvestris.
... Considering tree growth as a sensitive indicator of the trees carbon-water balance, as well as their nutritional status, the retrospective analysis of tree growth trends, assessed by the study of tree rings, provides reliable way to investigate the above-mentioned drought-induced unbalances. Common patterns, although more common in gymnosperms, have been identified as early warning signals of tree dieback and mortality including low long-term growth rates, high inter-annual growth variability or low growth resilience to drought events (Cailleret et al., 2017;Camarero et al., 2015Camarero et al., , 2018DeSoto et al., 2020;Serra-Maluquer et al., 2021a). However, the specific individual traits and environmental factors that predispose trees to growth decline and death still remain unclear (Kannenberg et al., 2020). ...
... The onset of growth decline in DD trees several decades ago (Figures 1 and 2) is aligned with previous studies reporting canopy dieback in sub-Mediterranean silver fir populations in the Spanish Pyrenees (Büntgen et al., 2014;Camarero et al., 2011Camarero et al., , 2015Gazol et al., 2018bGazol et al., , 2020Hevia et al., 2019). The occurrence of successive severe droughts and increasingly warmer temperatures during the 1980s-1990s may have impaired growth and resilience and led to very low growth rates that preceded dieback (Cailleret et al., 2017;Camarero et al., 2015Camarero et al., , 2018DeSoto et al., 2020). Indeed, the extreme 1985-1986 drought has been identified as a tipping point for the growth decline of several silver fir populations in the southwestern Spanish Pyrenees , being aggravated by successive droughts in 2005 and 2012 (Camarero et al., 2018). ...
Article
Rear‐edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in the southwestern of Europe. While silver fir drought‐induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon‐water balances. We performed a comparative analysis of radial growth, intrinsic water‐use efficiency (iWUE), oxygen isotopes (δ18O) and nutrient concentrations in leaves of declining (DD) and non‐declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical – chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate – growth relationships and lower δ18O than ND trees. The uncoupling between iWUE and δ18O, together with the positive correlations between P and K leaf concentrations and δ18O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.
... Across stands, drought mortality risk increases with higher competitive pressure (Knapp et al., 2021;Rigling et al., 2013;Young et al., 2017) and sensitivity to drought variability (Keen et al., 2022), which in turn is increased in stands with lower soil water availability (Fritts et al., 1965). It is generally accepted that background tree mortality, i.e. mortality that is not attributed to a specific event, is largely a function of tree size and relative growth rates (Cailleret et al., 2016;Gillner et al., 2013;Hülsmann et al., 2018). However, covariation between individual tree traits mentioned above within sites and across sites with varying environmental conditions (e.g., average water availability) can often confound mortality risk predictions (Trugman et al., 2021). ...
... Water limitation, particularly less access to deep soil water poolsderived through lower tree heights and δ 18 O enriched xylem waterwas previously found as a determining factor for drought-induced mortality in oaks in Italy (Colangelo et al., 2017;Ripullone et al., 2020). Lower growth rates for a prolonged period preceding tree mortality is a well-documented phenomenon (Cailleret et al., 2016). There are, however, contrasting results showing that trees, which were growing faster in the past, were also more prone to drought-induced mortality in extremely dry years (e.g., Gessler et al., 2018;Voltas et al., 2013). ...
Article
Ongoing climate warming is increasing evapotranspiration, a process that reduces plant-available water and aggravates the impact of extreme droughts during the growing season. Such an exceptional hot drought occurred in Central Europe in 2018 and caused widespread defoliation in mid-summer in European beech (Fagus sylvatica L.) forests. Here, we recorded crown damage in 2021 in nine mature even-aged beech-dominated stands in northwestern Switzerland along a crown damage severity gradient (low, medium, high) and analyzed tree-ring widths of 21 mature trees per stand. We aimed at identifying predisposing factors responsible for differences in crown damage across and within stands such as tree growth characteristics (average growth rates and year-to-year variability) and site-level variables (mean canopy height, soil properties). We found that stand-level crown damage severity was strongly related to soil water availability, inferred from tree canopy height and plant available soil water storage capacity (AWC). Trees were shorter in drier stands, had higher year-to-year variability in radial growth, and showed higher growth sensitivity to moisture conditions of previous late summer than trees growing on soils with sufficient AWC, indicating that radial growth in these forests is principally limited by soil water availability. Within-stand variation of post-drought crown damage corresponded to growth rate and tree size (diameter at breast height, DBH), i.e., smaller and slower-growing trees that face more competition, were associated with increased crown damage after the 2018 drought. These findings point to tree vigor before the extreme 2018 drought (long-term relative growth rate) as an important driver of damage severity within and across stands. Our results suggest that European beech is less likely to be able to cope with future climate change-induced extreme droughts on shallow soils with limited water retention capacity.
... The process by which plants' carbon economy has a role in mortality before, during and after lethal droughts involves multiple potential mechanisms. These include those associated with decreased photosynthetic gain 45 , reduced xylem and constitutive defensive compound production [110][111][112][113] , initial carbohydrate storage increases and then decreases prior to death 11,47-114 , a critical increasing role of carbohydrates in osmoregulation and cellular maintenance [115][116][117][118][119][120][121][122] , and possible carbohydrate feedbacks upon hydraulic and defensive function 47 . In this view, carbon starvation, or the steps by which metabolic functions are impaired by limitations in the supply rate of carbohydrates, is a mechanism within the larger process of carbon limitations upon mortality. ...
... There is evidence that water-carbon-related factors that occur prior to drought can promote mortality (Fig. 2). Decades-long reductions in carbon allocation to stem wood and resin ducts within dying conifer species indicate potential roles of pre-drought allocation to mortality [105][106][107][110][111][112][113] . Low growth preceding death can be a lingering consequence of prior climate or injuries that predispose trees to carbon constraints 79 (Fig. 2). ...
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Drought-associated woody-plant mortality has been increasing in most regions with multi-decadal records and is projected to increase in the future, impacting terrestrial climate forcing, biodiversity and resource availability. The mechanisms underlying such mortality, however, are debated, owing to complex interactions between the drivers and the processes. In this Review, we synthesize knowledge of drought-related tree mortality under a warming and drying atmosphere with rising atmospheric CO2. Drought-associated mortality results from water and carbon depletion and declines in their fluxes relative to demand by living tissues. These pools and fluxes are interdependent and underlay plant defences against biotic agents. Death via failure to maintain a positive water balance is particularly dependent on soil-to-root conductance, capacitance, vulnerability to hydraulic failure, cuticular water losses and dehydration tolerance, all of which could be exacerbated by reduced carbon supply rates to support cellular survival or the carbon starvation process. The depletion of plant water and carbon pools is accelerated under rising vapour pressure deficit, but increasing CO2 can mitigate these impacts. Advancing knowledge and reducing predictive uncertainties requires the integration of carbon, water and defensive processes, and the use of a range of experimental and modelling approaches. Enhanced drought frequency and magnitude have impacted tree mortality, leading to multiple examples of regional-scale dieback. This Review outlines the mechanisms leading to mortality, including carbon starvation and hydraulic failure.
... The aerial photo approach had the advantage of incorporating riparian forest expansion and con- (Schook et al., 2020). Here, we add that the DW reach also had higher tree mortality ( Figure 10) and fragmentation of the forest canopy ( Figure 8) cottonwoods, providing an alternate mechanism for tree-ring narrowing (Cailleret et al., 2017). This demonstrates that although SBAI provides a better indication of stand-level productivity than BAI alone, it too reflects factors other than drought. ...
... Hydrological drought appears to be the factor that differentiated cottonwood forest production in Snake Creek's DW reach relative to the RD reach. Hydrological drought can lead to gradual tree and forest decline (Cailleret et al., 2017;Schook et al., 2020) or cause immediate and widespread forest mortality (Cooper et al., 2003;Scott et al., 2000), depending upon the severity of the drought. A compounding atmospheric drought four decades after diversion likely triggered a second decline of DW riparian forest production that differentiated it from the RU reach. ...
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Riparian trees and their annual growth rings can be used to reconstruct drought histories related to streamflow. Because the death of individual trees reduces competition for survivors, however, tree‐ring chronologies based only on surviving trees may underestimate drought impacts. This problem can be addressed by calculating productivity at the stand scale to account for tree mortality and establishment. We calculated riparian wood production for 1946‐2016 in the semi‐arid Great Basin in the western USA along a stream where most flow has been removed by a diversion pipeline since 1961. The water table was found to be generally below the root zone of cottonwoods (Populus angustifolia and P. angustifolia x. trichocarpa) in the pipeline‐dewatered reach but within it in reference reaches. To reconstruct forest production through time, we combined measurements of tree‐ring basal area increment with changing forest area from aerial photos or with a census of cross‐dated living and dead cottonwoods. Both approaches revealed production declines in the dewatered reach relative to adjacent reference reaches, and the decline accelerated in the 2000s. Tree‐ring narrowing resulted in divergence between the dewatered reach and one reference reach within five years after diversion. However, the dewatered reach did not diverge from the other reference reach until 40 years later, when an unprecedented early 2000’s atmospheric drought coupled with diversion to cause extensive cottonwood mortality. We conclude that dendrochronological investigations of forest response to environmental stress should incorporate stand dynamics and that the full impacts of flow diversion can be delayed for decades.
... Future differences in the European beech forest productivity and radial tree growth dipole will further be modulated by the portion of the variability not explained by summer JSL. Among the factors integrating that variability are regional differences in the trees' climate sensitivity to other seasons 44,46 , legacy effects 58 , site conditions that buffer against climate variabilities such as forest composition 59 , genetic composition due to past demographic and phylogeographic processes 60 , and historical responses to forest management and disturbances 61 . Southeastern European beech forests are closer to their physiological limits (i.e., the warm/dry edge of their continental distribution), but they show a more plastic response to climate compared to forests at the core and cold edge of the distribution range (central-northern Europe) 44,62 . ...
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The mechanistic pathways connecting ocean-atmosphere variability and terrestrial productivity are well-established theoretically, but remain challenging to quantify empirically. Such quantification will greatly improve the assessment and prediction of changes in terrestrial carbon sequestration in response to dynamically induced climatic extremes. The jet stream latitude (JSL) over the North Atlantic-European domain provides a synthetic and robust physical framework that integrates climate variability not accounted for by atmospheric circulation patterns alone. Surface climate impacts of north-south summer JSL displacements are not uniform across Europe, but rather create a northwestern-southeastern dipole in forest productivity and radial-growth anomalies. Summer JSL variability over the eastern North Atlantic-European domain (5-40E) exerts the strongest impact on European beech, inducing anomalies of up to 30% in modelled gross primary productivity and 50% in radial tree growth. The net effects of JSL movements on terrestrial carbon fluxes depend on forest density, carbon stocks, and productivity imbalances across biogeographic regions. Here the authors show that extremes in the summer jet stream position over Europe create a beech forest productivity dipole between northwestern and southeastern Europe and can result in regional anomalies in forest carbon uptake and growth.
... Carbon investment for canopy development in spring 2018 may have yielded a poor photosynthetic return during summer, when trees operated close to their dehydration thresholds, resulting in a likely reduction in tree carbon reserves over the year. Long-term legacy effects due to the depletion of carbohydrate reserves and damage to the hydraulic system 37,38 during HW2018 will very likely compromise tree growth, performance, and survival in the coming years 62 . Furthermore, contrasting stem water refilling behaviour between broadleaves and conifers links to differences in stomatal regulation 24 and hydraulic safety margins 43,45 observed between taxonomic clades and broadens our perspective on tree hydraulic functioning. ...
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Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.
... For this, understanding the role of carbohydrates on wood formation (regulatory or as substrate and at what developmental process is it restricting) will need to increase for example through manipulation experiments (e.g., Rademacher et al., 2019). Additionally, simulating future tree rings could help forecast tree mortality in conjunction with tree ring-based mortality algorithms (Cailleret et al., 2017). Finally, resolving growth processes as a carbon sink within the tree may help to answer questions on active vs. passive storage and can in the same context also help address the source-sink controversy (Schiestl-Aalto et al., 2015). ...
Article
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Wood formation has received considerable attention across various research fields as a key process to model. Historical and contemporary models of wood formation from various disciplines have encapsulated hypotheses such as the influence of external (e.g., climatic) or internal (e.g., hormonal) factors on the successive stages of wood cell differentiation. This review covers 17 wood formation models from three different disciplines, the earliest from 1968 and the latest from 2020. The described processes, as well as their external and internal drivers and their level of complexity, are discussed. This work is the first systematic cataloging, characterization, and process-focused review of wood formation models. Remaining open questions concerning wood formation processes are identified, and relate to: (1) the extent of hormonal influence on the final tree ring structure; (2) the mechanism underlying the transition from earlywood to latewood in extratropical regions; and (3) the extent to which carbon plays a role as “active” driver or “passive” substrate for growth. We conclude by arguing that wood formation models remain to be fully exploited, with the potential to contribute to studies concerning individual tree carbon sequestration-storage dynamics and regional to global carbon sequestration dynamics in terrestrial vegetation models.
... Radial increments are an established quantitative proxy for investigating spatial and temporal changes in tree vitality, and highlighting the effects of natural and anthropogenic factors on tree growth (Bert, 1993;Badeau et al., 1996;Gillner et al., 2013;Bachtobji Bouachir et al., 2017;Cailleret et al., 2016;Cavin and Jump, 2016;Preisler et al., 2019). However, tree growth is the result of many biological processes that respond strongly, but differently, to climate events, such as droughts. ...
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The low-latitudinal range margins of many temperate and boreal tree species consist of scattered populations that persist locally in climate refugia. Recent studies have shown that such populations can be remarkably resilient, yet their past resilience does not imply that they are immune to threats from future climate change. The functioning of refugial tree populations therefore needs to be better understood if we are to anticipate their prospects correctly. We performed a detailed study of tree radial growth and vigour in a long-term climate refugial population of beech (Fagus sylvatica), comparing the observed trends with those of co-occurring pedunculate oak (Quercus robur). Annual growth rates (basal area increment, BAI) for both species were similar to those observed in range-core populations, but natural lifespan was half that in the mountains. The master chronologies spanning 1870 to 2015 revealed 22% (Fagus) and 20% (Quercus) increases in BAI until the 1980s and a smaller decrease (-6% for Fagus, -9% for Quercus) since then. Stable carbon isotope measurements (δ¹³C) revealed no effect of cambial age and an increase in water-use efficiency (iWUE) from 1870 to 2015 of about 50% for Fagus and 20% for Quercus. The trend continued until 2015 in Fagus, whereas Quercus reached its maximum in the 1980s. A detailed analysis of the relationship between climate and annual growth based on a 118-year meteorological record revealed a major role of water availability in the current and previous year. We used the observed climatic relationships to model future growth trends until 2100 for the IPCC scenarios RCP4.5 and RCP8.5. Most projections revealed no change in current growth rates, suggesting that this climate refugium will be able to provide suitable conditions for the persistence of Fagus and Quercus over the coming decades even under warmer and drier regional climate conditions. Overall, our study provides valuable insight into the precise climatic and biological mechanisms enhancing the persistence of refugial tree populations under ongoing climate change.
... One general consequence of drought is the reduction of growth (Klein et al. 2014;Piper et al. 2017). The magnitude of this reduction is used as an indicator of drought resistance, i.e., the ability of individuals to maintain their functioning during drought Gazol et al. 2018;Bose et al. 2020;Gessler et al. 2020;Bottero et al. 2021), and in some cases, it can predict the survival/mortality response of trees (Cailleret et al. 2017;Rodríguez-Catón et al. 2019). Drought causes growth decline because the process of tissue formation (i.e., mitosis, cell differentiation, cell elongation) requires sufficient turgor (Muller et al. 2011;Körner 2015). ...
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Key message: At the sixth and seventh years of a drought event in south of Chile, non-structural carbohydrate (NSC) concentrations were similar between healthy and unhealthy trees of Araucaria araucana (Molina) K. Koch, and growth did not decrease, suggesting that leaf loss prevented C shortage in unhealthy trees. Context: Tree drought resistance and resilience may be impaired by decreasing growth and non-structural carbohydrates (NSC). During a 7-year drought, the isohydric species Araucaria araucana (Araucariaceae) evidenced decline (foliage loss and browning). Aims: To determine whether tree decline was related to an impaired carbon status and reduced growth. Methods: In two sites of southern Chile, we selected healthy- and unhealthy-looking trees to study drought effects on NSC and growth. We measured the basal area increment (BAI) and NSC concentrations of needles and roots after 6 years of drought (2016) and following one less severe year in terms of drought (2017). Results: At both years, healthy and unhealthy trees had similar NSC and sugar concentrations in needle and roots, and furthermore, they maintained their growth rates. In 2017, NSC, starch, and sugar concentrations of needles (but not roots) increased in both healthy and unhealthy trees at one of the study sites, while growth did not vary. Conclusion: Unhealthy trees likely prevented C shortage through an acclimation mechanism such as foliage loss. The remarkable similar NSC concentrations found between healthy and unhealthy trees indicates the absence of C starvation in trees that lost a substantial fraction of their foliage under drought. Keywords: Climate change, Isohydric, Monkey puzzle tree, Soluble sugars, Starch
... The research on the impact of climate change on tree species diversity has, therefore, not received due attention. At present, more attention is paid to the impact of single climate factors on tree species diversity [4], and current research focuses on the radial growth of tree species and the impact of climate change on the phenological period [5], and it is rare to study the response of tree species diversity to climate change, especially on a fine scale. ...
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Localized climate is sensitive to terrain, underlying surface material, building distribution, green coverage and CO2 emissions. The Regional Climate Model (RegCM) was used to make a statistical detailed analysis of the climate change data in a specific study area to obtain fine-scale distribution of climatic elements data over time. The effects of climate change factors on height growth trends of a climate-sensitive tree species (Cyclobalanopsis glauca) were simulated based on historical climate base line data (1961–2010) and future climate change (2010–2100) predictions. Cyclobalanopsis glauca growth trends were simulated and analyzed by using a nonlinear mixed effect model (NLME). The results showed that under the RCP8.5 emissions scenario, the growth promotion effect on the height growth of Cyclobalanopsis glauca will be obvious. Under RCP4.5 and RCP2.6 emissions scenarios, although the inhibition intensity is not exactly the same, height growth will still be inhibited to a certain extent, which may lead to the gradual extinction of this species, affecting the composition of dominant tree species in the study area. The results indirectly reflect the impact of climate change on tree species diversity in the future.
... The two main known physiological mechanisms leading to tree mortality are hydraulic failure (embolism) and carbon starvation, which have been investigated in many previous studies over the past decade Cailleret et al., 2017;Choat et al., 2012;Hartmann, 2015;Hartmann, Ziegler, & Trumbore, 2013;Kono et al., 2019;McDowell et al., 2008;Meir, Mencuccini, & Dewar, 2015;Sala, Piper, & Hoch, 2010;Sevanto, Mcdowell, Dickman, Pangle, & Pockman, 2014). Identifying early warning indicators to determine when trees cross the "point of no return," beyond which mortality is inevitable, has been a major focus of recent studies, which highlight plant water content, leaf wilting and loss of hydraulic conductivity as key parameters (Anderegg, Berry, & Field, 2012;Hartmann et al., 2018;Sapes et al., 2019). ...
... Range shifts are symbolized by curved arrows directed toward higher altitudes (on land) or deeper zones (at sea). Range shifts toward higher latitudes also occur (not shown here) mortality, through change in the organism's environment beyond its physiological limits, or an indirect effect through changes in species interactions such as competition or parasitism (Allen et al., 2010;Cahill et al., 2013;Cailleret et al., 2017;Garrabou et al., 2009). Adaptation can counteract these outcomes via five main interrelated adaptive mechanisms ( Figure 1). ...
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Potential for, and limits to, adaptation to environmental changes are critical for resilience and risk mitigation. The Mediterranean basin is a mosaic of biodiversity-rich ecosystems long affected by human influence, whose resilience is now questioned by climate change. After reviewing the different components of biological adaptation, we present the main characteristics of marine and terrestrial biodiversity in the Mediterranean basin and of the pressures they face. Taking climatic trends into consideration, we discuss the adap-tive potential of a range of ecosystems dominated by species without active dispersal. We argue that the high heterogeneity of Mediterranean landscapes and seascapes constitutes a laboratory for the study of adaptation when environmental conditions change rapidly and may provide opportunities for adaptation and adaptability of species and ecosystems. Adaptive management in the Mediterranean can and should harness the nature-based solutions offered
... As a consequence, recent research has particularly focused on understanding both tree response to extreme environmental conditions such as drought and late-frost (Anderegg et al., 2015;Bose et al., 2020;Kannenberg et al., 2020;Lloret et al., 2011;Meyer et al., 2020;Príncipe et al., 2017;Rehschuh et al., 2017;Sangüesa-Barreda et al., 2021;Zang et al., 2014), and the drivers of tree mortality (e.g. Buras et al., 2018;Cailleret et al., 2017). In turn, this empirically derived evidence allows for refining our understanding of the mechanisms that underpin projections of tree-species performance and future species distributions under anticipated climate change (Buras and Menzel, 2019;Lasch-Born et al., 2020;Reyer et al., 2020;. ...
Article
Detecting pointer years in tree-ring data is a central aspect of dendroecology. Pointer years are usually represented by extraordinary secondary tree growth, which is often interpreted as a response to abnormal environmental conditions such as late-frosts or droughts. Objectively identifying pointer years in larger tree-ring networks and relating those to specific climatic conditions will allow for refining our understanding of how trees perform under extreme climate and consequently, under anticipated climate change. Recently, Buras et al. (2020) demonstrated that frequently used pointer-year detection methods were either too sensitive or insensitive for such large scale analyses. In their study, Buras et al. (2020) proposed a novel approach for detecting pointer years – the standardized growth change (SGC) method which outperformed other pointer-year detection methods in pseudopopulation trials. Yet, the authors concluded that SGC could be improved further to account for the inability to detect pointer years following successive growth decline. Under this framework, we here present a refined version of the SGC-method – the bias-adjusted standardized growth change method (BSGC). The methodological adjustment to the SGC approach comprises conflated probabilities derived from standardized growth changes with probabilities derived from a time-step specific global standardization of growth changes. In addition, BSGC allows for estimating the length of the deflection period, i.e. the period before extraordinary growth values have reached normal levels. Application of BSGC to simulated and measured tree-ring data indicated an improved performance in comparison to SGC which allows for the identification of pointer years following years of successive growth decline. Also, deflection period lengths were estimated well and revealed plausible results for an existing tree-ring data set. Based on these validations, BSGC can be considered a further refinement of pointer-year detection, allowing for a more accurate identification and consequently better understanding of the radial growth response of trees to extreme events.
... In turn, during measurement periods when plots had higher productivity, they experienced lower mortality probabilities, which may reflect niche complementarity between species and reduce the potential for intraspecific competition-driven mortality on more-diverse sites. However, this link is unlikely to be completely causal: High tree growth rates are often correlated with low mortality probabilities over short time periods (27) because trees often experience a decline in growth prior to senescence. Therefore, this decline in mortality with increasing productivity is likely confounded by favorable climate and stand conditions during the census interval. ...
Article
Significance Despite a significant amount of recent research describing how tree species diversity improves forest productivity, few studies examine how tree diversity affects tree mortality, which is a key ecosystem function that drives succession, composition, and competition. Using a plot network from across Canada and the United States, here we show that plots with higher tree diversity also experience higher tree mortality. This effect becomes even more prominent when tree diversity effects are modeled holistically; in particular, more-diverse plots have higher stem densities, translating into higher mortality probabilities. We call for the use of integrated model frameworks when examining the response of forest ecosystem functions to diversity, thus ensuring proper accounting of direct and indirect effects.
... Deadly effects on vegetation may be evident even after years from drought stress periods [4], but stress-induced tree mortality is usually preceded by functional and structural changes (see, for instance, growth changes [5]). Vegetation response to drought can be Land 2022, 11, 825 2 of 18 different on a local scale because several factors, such as soils, topographic position, tree density, and root systems, may play a different role [6,7]. ...
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Climate-induced drought events are responsible for forest decline and mortality in different areas of the world. Forest response to drought stress periods may be different, in time and space, depending on vegetation type and local factors. Stress analysis may be carried out by using field methods, but the use of remote sensing may be needed to highlight the effects of climate-change-induced phenomena at a larger spatial and temporal scale. In this context, satellite-based analyses are presented in this work to evaluate the drought effects during the 2000s and the possible climatological forcing over oak forests in Southern Italy. To this aim, two approaches based on the well-known Normalized Difference Vegetation Index (NDVI) were used: one based on NDVI values, averaged over selected decaying and non-decaying forests; another based on the Robust Satellite Techniques (RST). The analysis of the first approach mainly gave us overall information about 1984–2011 rising NDVI trends, despite a general decrease around the 2000s. The second, more refined approach was able to highlight a different drought stress impact over decaying and non-decaying forests. The combined use of the RST-based approach, Landsat satellite data, and Google Earth Engine (GEE) platform allowed us to identify in space domain and monitor over time significant oak forest changes and climate-driven effects (e.g., in 2001) from the local to the Basilicata region scale. By this way, the decaying status of the Gorgoglione forest was highlighted two years before the first visual field evidence (e.g., dryness of apical branches, bark detachment, root rot disease). The RST exportability to different satellite sensors and vegetation types, the availability of suitable satellite data, and the potential of GEE suggest the possibility of long-term monitoring of forest health, from the local to the global scale, to provide useful information to different end-user classes.
... Investigations relying upon tree-ring width span across the full range of inter-disciplinary applications including archaeological, climatological, ecological, and geological sciences. Examples include: timber trade and resource utilization Domínguez-Delmás 2020); reconstructions of temperature primarily in higher latitude and/or altitude environments where cooler conditions limit radial tree-growth (Jacoby and D'Arrigo 1989;Esper et al. 2002;Salzer et al. 2014); assessments of past insect activity / outbreaks (Swetnam and Lynch 1989;Speer et al. 2001;Esper et al. 2007); reconstructions of drought (Cook et al. 2004;Fang et al. 2010), precipitation (Büntgen et al. 2011;Griffin and Anchukaitis 2014), streamflow (Woodhouse et al. 2006), and snowpack (Pederson et al. 2011;Belmecheri et al. 2016) variability primarily from locations where moisture limitations, potentially exacerbated by high thermal stress, limits radial growth; quantifying soil erosion rates (Gärtner et al. 2001;Rubiales et al. 2008); dating debris flows and avalanches (Stoffel and Bollschweiler 2008) as well as the construction of ancient (Douglass 1929) and historical structures (Boswijk et al. 2016); quantifying phenotypic traits, variability, and associations with genetic lineage/provenance (Housset et al. 2018); assessments of large-scale ecological disturbances (Pederson et al. 2014), and the resilience of ecosystems to climatic stressors (Chamagne et al. 2017;Kannenberg et al. 2019); understanding and predicting tree mortality (Cailleret et al. 2017); quantifying the climate sensitivity and drivers of forest growth in the past (Babst et al. 2013;St. George and Ault 2014), and making projections about tree growth in the future (Williams et al. 2013;Klesse et al. 2020); reconstructing larger-scale pressure and oceanic and atmospheric circulation indices (Trouet et al. 2009;Villalba et al. 2012;Li et al. 2013); quantifying forest carbon stocks and fluxes (Babst et al. 2014b); and integrating tree-ring and remotely sensed metrics of forest productivity (Coulthard et al. 2017;Seftigen et al. 2018). ...
Chapter
This chapter overviews long-standing foundations, methods, and concepts of dendrochronology, yet also pays attention to a few related paradigm shifts driven by isotope measurements in tree-rings. The basics of annual ring formation are first reviewed, followed by structural descriptions of tree-rings at the macroscopic-to-microscopic scale including earlywoodandlatewoodin conifers (gymnosperms) and hardwoods (angiosperms), as well as wood anatomical features. Numerous examples of inter-disciplinary applications connected to various tree-ring parameters are provided. With the foundation of tree-rings established, this chapter then describes the process and necessity for crossdating—the process by which each and every ring is assigned to a specific year. Methods and terminology related to field sampling also briefly described. The long-standing paradigm of site selection criteria—well shown to maximize common signals in tree-ring width datasets—is challenged in a brief discussion of newer tree-ring isotope literature demonstrating that robust chronologies with high signal-to-noise ratios can be obtained at non-ecotonal locations. Opportunities for isotope measurements to enable crossdating in otherwise challenging contexts are likewise highlighted. The chapter reviews a conceptual framework to disaggregate tree-ring time-series, with special attention to detrending and standardization methods used to mitigate tree-age/size related noise common to many applications such as dendroclimatic reconstruction. Some of the drivers of long-term trends in tree-ring isotope data such as the increase in the atmospheric concentration of CO 2 , age/size/height trends, and climate variation are presented along with related debates/uncertainties evident in literature in order to establish priorities for future investigations. The development of tree-ring chronologies and related quality control metrics used to assess the common signal and the variance of tree-ring data are described, along with the limitations in correlation based statistics to determine the robustness of tree-ring datasets particularly in the low frequency domain. These statistical methods will gain relevance as tree-ring isotope datasets increasingly approach sample replications and dataset structures typical for tree-ring width measurements.
... BAI reduction can be due to any one of the above factors. Although drought and hot drought impact on carbohydrate reserves (total non-structural carbohydrate) was not found to have a universal role in tree mortality (Adams et al., 2017), carbon allocation to bole growth declines prior to tree death (Cailleret et al., 2017;Manion, 1981). This paper addresses the effect of forest treatments on BAI within the context of hydrological drought in both upland and lowland topographic positions in a dry, ponderosa pine-dominated forest. ...
Article
Forest treatments reduce wildfire risk and can promote the vigor and production of remaining trees, but they are also a disturbance. Understanding the type, timing, and longevity of tree response to treatment, as well as the potential for interactive effects of treatments and drought, could help managers plan and evaluate forest management practices. Environmental drivers, biological modifiers, and tree capacity to respond to prior disturbances were concurrently tested to predict ponderosa pine basal area increment (BAI) in a lowland and upland dry pine forest in south central Oregon, USA. Environmental drivers included current year and lags or running averages of a drought index, SPEI, and the sum or count of growing degree days >0°C or 10°C. Biological modifiers of environmental drivers considered pre-treatment response to disturbance, tree vigor, and tree-to-tree competition. A model was developed to predict BAI in both topographic positions for applicability to the landscape level, and then was used to test for specific differences in BAI between paired forest treatments differing by one treatment. Forest treatments tested included no management (NM), undercut and even spacing harvest (HE), prescribed fire (Rx), and their combinations. HE significantly increased BAI shortly after treatment. Post-harvest, one or two Rx did not provide additional BAI benefits, nor in the absence of HE, did 2Rx vs. 1Rx treatment. The 1Rx treatment was imposed between multi-year droughts; BAI significantly increased after the treatment and was resistant to droughts. Upland trees were affected by a single year of drought; lowland trees responded only after sequential drought years. A single treatment, HE or 1Rx appeared to be as effective as multiple or mixed treatments in improving BAI in dry pine forest stands. HE appeared to generate the largest effect. Timing of forest treatments relative to site water balance may affect short term (decadal) wood production.
... For gymnosperms, legacy effects of seasonal extreme drought might persist for a longer time than for angiosperms owing to a more conservative drought response strategy and longer organ lifetimes (Anderegg, Schwalm, et al., 2015;Blackman et al., 2019;DeSoto et al., 2020;Gazol et al., 2017;Zweifel & Sterck, 2018). The more conservative stomatal regulation and slower rate of desiccation in gymnosperms may partially explain why gymnosperms are more sensitive to legacy effects of previous growth conditions ( Figure 5) (Au et al., 2020;Cailleret et al., 2017;Gazol et al., 2020;Zweifel & Sterck, 2018). The long-term memory effects of preceding tree growth in gymnosperms could also be associated with the longer lifetimes of functional organs and reserves (e.g., leaf, stem, and root), their internal carbon use economy and hydraulic functioning (DeSoto et al., 2020;Zweifel & Sterck, 2018). ...
Article
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Plain Language Summary The occurrence patterns of seasonal extreme drought and wetness events are dramatically shifting with climate warming. However, how will different seasonal extreme climate regimes affect the bioclimatic sensitivity of tree growth remains poorly understood. In this study, we investigated the sensitivity of tree growth to different seasonal climate factors and preceding tree growth conditions during 1951–2013 under different seasonal extreme drought/wetness regimes, using 1,032 tree ring chronologies covering 121 gymnosperm and angiosperm species. We found the magnitude in tree growth reduction caused by seasonal extreme drought events is much larger than that in tree growth stimulation by seasonal extreme wetness events in arid and temperate regions. Tree growth in arid and temperate dry regions is more negatively impacted by extreme drought events in pre‐growing‐seasons (PGSs) than in growing‐seasons. We further found that angiosperms are more sensitive to PGS water availability, while gymnosperms are more sensitive to legacy effects of the preceding tree growth conditions in temperate dry and humid regions. These findings highlight divergent bioclimatic legacy effects on tree growth under different seasonal extreme climate regimes, and provide valuable insights into the future trajectories of forest growth across diverse ecoregions and functional groups in a more extreme climate.
... However, this study is the first to detect this response in an apparently healthy oldgrowth forest, in which no trees showed visual signs of decline, such as discolored foliage and thinning crowns. When comparing variation in individual ring-width indices from 1900 to 2015, we found some yellow-cedar suffered intermittent growth reductions after 1950 that may be consistent with thaw-freeze events and provide an additional early warning sign of decline (Cailleret et al., 2017;Comeau et al., 2021). We recommend careful monitoring of forest health to determine if canopy dieback or tree mortality of yellow-cedar results in future as winter climate continues to warm. ...
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Old-growth forests in the Pacific Northwest are being fundamentally altered by climate change. A primary example of this is yellow-cedar (Callitropsis nootkatensis ), a culturally and economically important species, which has suffered widespread decline across its range since the beginning of the twentieth century. We used tree rings to compare the climate-growth response of yellow-cedar to two co-occurring species; western hemlock ( Tsuga heterophylla ) and Sitka spruce ( Picea sitchensis ), in an old-growth forest on Haida Gwaii, Canada, to better understand the unique climatic drivers of a species that is declining across its range. We developed three species-specific chronologies spanning 560–770 years, reconstructing a long-term record of species growth and dynamics over time. The climate is strongly influenced by the Pacific Decadal Oscillation (PDO), a multi-decadal pattern of ocean-atmospheric climate variability. Climate varied across three time periods that have coincided with major shifts in the PDO during the twentieth century [1901–1945 (neutral/positive), 1946–1976 (negative) and 1977–2015 (positive)]. Conditions were significantly warmer and wetter during positive phases, with the greatest maximum temperatures in the most recent period. We used complimentary methods of comparison, including Morlet wavelet analysis, Pearson correlations, and linear-mixed effects modeling to investigate the relations between climate and species growth. All three species exhibited multi-decadal frequency variation, strongest for yellow-cedar, suggesting the influence of the PDO. Consistent with this, the strength and direction of climate-growth correlations varied among PDO phases. Growing season temperature in the year of ring formation was strongly positively correlated to yellow-cedar and western hemlock growth, most significantly in the latter two time periods, representing a release from a temperature limitation. Sitka spruce growth was only weakly associated with climate. Yellow-cedar responded negatively to winter temperature from 1977 to 2015, consistent with the decline mechanism. Increased yellow-cedar mortality has been linked to warmer winters and snow loss. This study provides new insights into yellow-cedar decline, finding the first evidence of decline-related growth patterns in an apparently healthy, productive coastal temperate rainforest.
... Biotic agents like plant pathogens and insects may intensify negative drought effects and reduce plant vitality to surpass drought conditions and to recover healthy conditions (Gaylord et al. 2013, 2015, Hartmann et al. 2015. The ultimate consequence is tree weakening, which may lead to higher tree mortality after drought or in subsequent years (Cailleret et al. 2017, Morcillo et al. 2019. ...
Article
Drought-related tree mortality is a global phenomenon that currently affects a wide range of forests. Key functional variables on plant hydraulics, carbon economy, growth and allocation have been identified and play a role in tree drought responses. However, tree mortality thresholds based on such variables are difficult to identify, especially under field conditions. We studied several Aleppo pine populations differently affected by an extreme drought event in 2014, with mortality rates ranging from no mortality to 90% in the most severely affected population. We hypothesized that mortality is linked with high levels of xylem embolism, i.e., hydraulic dysfunction, which would also lead to lower tree resistance to drought in subsequent years. Despite not finding any among-populations differences in the vulnerability curves to xylem embolism, there were large differences in the hydraulic safety margin and the hydraulic dysfunction level. High mortality rates were associated with a negative hydraulic safety margin when xylem embolism reached values over 60%. We also found forest weakening and post-drought mortality related to a low hydraulic water transport capacity, reduced plant growth, low carbohydrate contents and high pest infestation rates. Our results highlight the importance of drought severity and the hydraulic dysfunction level on pine mortality, as well as post-drought conditions during recovery processes.
... Forecasts indicate that many trees experiencing decline will have median growth and/or 95% confidence intervals of growth that drop below zero ( Figure S7). This is interpreted as representing the magnitude of stress experienced, which in real trees would translate into growth cessation and an increased risk of mortality (Cailleret et al., 2017;Keane et al., 2001). With greater warming, more trees are forecasted to have median growth that is negative: 6.9% of trees, 10.4% of trees, 25.1% of trees, and 52.1% of trees for RCPs 2.6, 4.5, 6.0, and 8.5, respectively. ...
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Robust ecological forecasting of tree growth under future climate conditions is critical to anticipate future forest carbon storage and flux. Here, we apply three ingredients of ecological forecasting that are key to improving forecast skill: data fusion, confronting model predictions with new data, and partitioning forecast uncertainty. Specifically, we present the first fusion of tree-ring and forest inventory data within a Bayesian state-space model at a multi-site, regional scale, focusing on Pinus ponderosa var. brachyptera in the southwestern US. Leveraging the complementarity of these two data sources, we parsed the ecological complexity of tree growth into the effects of climate, tree size, stand density, site quality, and their interactions, and quantified uncertainties associated with these effects. New measurements of trees, an ongoing process in forest inventories, were used to confront forecasts of tree diameter with observations, and evaluate alternative tree growth models. We forecasted tree diameter and increment in response to an ensemble of climate change projections, and separated forecast uncertainty into four different causes: initial conditions, parameters, climate drivers, and process error. We found a strong negative effect of fall-spring maximum temperature, and a positive effect of water-year precipitation on tree growth. Furthermore, tree vulnerability to climate stress increases with greater competition, with tree size, and at poor sites. Under future climate scenarios, we forecast increment declines of 22%-117%, while the combined effect of climate and size-related trends results in a 56%-91% decline. Partitioning of forecast uncertainty showed that diameter forecast uncertainty is primarily caused by parameter and initial conditions uncertainty, but increment forecast uncertainty is mostly caused by process error and climate driver uncertainty. This fusion of tree-ring and forest inventory data lays the foundation for robust ecological forecasting of aboveground biomass and carbon accounting at tree, plot, and regional scales, including iterative improvement of model skill.
... Previous dendrochronological studies have already shown that dead trees usually experience a marked growth reduction lasting a few years to decades prior to death (Bigler and Bugmann, 2004;Cailleret et al., 2019Cailleret et al., , 2017. Moreover, a low growth recovery and resistance in angiosperms to single extreme non-lethal drought events can increase the mortality risk at a later stage (DeSoto et al., 2020). ...
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European beech (Fagus sylvatica L.) has strongly suffered from the exceptional 2018 drought and subsequent dry years that hit Central Europe. While many trees showed severe signs of crown dieback or died following the 2018 extreme drought, other co-occurring and neighboring trees showed no sign of dieback or only minor damage. The reasons why some trees were more severely impacted than others and which predisposing factors make some trees more vulnerable than others are still poorly understood. Here, we analyzed differences in long-term growth trends, neighborhood composition (competition and species diversity), early-warning signals, and growth responses to past severe droughts of co-occurring vital and severely declining beech trees at six sites in Switzerland. We aimed to connect tree vitality after 2018 with past long-term growth trajectories and investigated whether declining trees had already been more susceptible to drought than vital trees before dieback occurred. Overall, trees that showed severe crown dieback had a stronger growth decline than vital trees in the last 50 years. Declining trees exhibited stagnating and then decreasing growth trajectories even before signs of crown dieback occurred. Interestingly, we did not find significant differences in growth response to past severe droughts between the vitality classes, with the exception that vital trees recovered faster from past more severe droughts. Further, we could neither detect any difference in the effect of competition and neighborhood species composition on growth response, nor predict crown dieback based on early-warning signals which try to predict regime shifts by sudden changes in the autoregressive coefficient with lag 1, standard deviation and skewness. Our results indicate that unlike vital trees, declining beech trees showed predisposing signs for crown dieback by having lower growth rates during the last 50 years.
... In addition to these named indices, we estimated climate sensitivity for 1970-2012 by calculating the standard deviation in growth for each individual tree. Trees with a high deviation from the mean were characterized as highly sensitive with a high variability in growth depending on climate conditions, which has been associated with higher mortality (Cailleret et al., 2017 (Mangiafico, 2017). Furthermore, we plotted the correlation between climate sensitivity and tree height using the R function ggscatter in ggpubr version 0.4.0 (Kassambara & Kassambara, 2020). ...
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Climate change is increasing the frequency and intensity of drought events in many boreal forests. Trees are sessile organisms with a long generation time, which makes them vulnerable to fast climate change and hinders fast adaptations. Therefore, it is important to know how forests cope with drought stress and to explore the genetic basis of these reactions. We investigated three natural populations of white spruce (Picea glauca) in Alaska, located at one drought‐limited and two cold‐limited treelines with a paired plot design of one forest and one treeline plot. We obtained individual increment cores from 458 trees and climate data to assess dendrophenotypes, in particular the growth reaction to drought stress. To explore the genetic basis of these dendrophenotypes, we genotyped the individual trees at 3,000 SNPs in candidate genes and performed genotype‐phenotype association analysis using linear mixed models and Bayesian sparse linear mixed models. Growth reaction to drought stress differed in contrasting treeline populations. Therefore, the populations are likely to be unevenly affected by climate change. We identified 40 genes associated with dendrophenotypic traits, that differed among the treeline populations. Most genes were identified in the drought‐limited site, indicating comparatively strong selection pressure of drought‐tolerant phenotypes. Contrasting patterns of drought‐associated genes among sampled sites and in comparison to Canadian populations in a previous study suggest that drought adaptation acts on a local scale. Our results highlight genes that are associated to wood traits which in turn are critical for the establishment and persistence of future forests under climate change.
... Seasonal water deficits vary across Amazonia but are typically most intense in the southern edge (Brando et al., 2014;Malhi et al., 2015;Marimon et al., 2020), explaining the strong influence of this variable on the death of shorter trees in this region. Under high MCWD, trees are more susceptible to hydraulic failure and carbohydrate deficiency due to the high evaporative demand and stomatal closure, respectively (McDowell et al., 2011(McDowell et al., , 2018, which may affect tree growth (Cailleret et al., 2017;Dobbertin, 2005;Feldpausch et al., 2016;Phillips et al., 2009) and may be exacerbated by high maximum temperatures in the region . ...
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Tree death is a key process for our understanding of how forests are and will respond to global change. The extensive forests across the southern Amazonia edge—the driest, warmest and most fragmented of the Amazon regions—provide a window onto what the future of large parts of Amazonia may look like. Understanding tree mortality and its drivers here is essential to anticipate the process across other parts of the basin. Using 10 years of data from a widespread network of long‐term forest plots, we assessed how trees die (standing, broken or uprooted) and used generalised mixed‐effect models to explore the contribution of plot‐, species‐ and tree‐level factors to the likelihood of tree death. Most trees died from stem breakage (54%); a smaller proportion died standing (41%), while very few were uprooted (5%). The mortality rate for standing dead trees was greatest in forests subject to the most intense dry seasons. While trees with the crown more exposed to light were more prone to death from mechanical damage, trees less exposed were more susceptible to death from drought. At the species level, mortality rates were lowest for those species with the greatest wood density. At the individual tree level, physical damage to the crown via branch breakage was the strongest predictor of tree death. Synthesis. Wind‐ and water deficit‐driven disturbances are the main causes of tree death in southern Amazonia edge which is concerning considering the predicted increase in seasonality for Amazonia, especially at the edge. Tree mortality here is greater than any in other Amazonian region, thus any increase in mortality here may represent a tipping point for these forests. A mortalidade de árvores é um processo fundamental para nossa compreensão de como as florestas são e como responderão às mudanças climáticas globais. As florestas ao longo da extensa borda sul da Amazônia—a mais seca, quente e fragmentada das regiões amazônicas—são uma janela para o futuro de outras partes da Amazônia. Compreender a mortalidade das árvores e seus fatores aqui é essencial para antecipar o processo em outras partes da bacia. Usando 10 anos de dados de uma ampla rede de parcelas florestais de longo prazo, avaliamos como as árvores morrem (em pé, quebradas ou desenraizadas) e usamos modelos generalizados de efeitos mistos para explorar a contribuição de fatores ao nível de parcelas, espécies e árvores para o risco de morte de árvores. A maioria das árvores morreu quebrada (54%); uma proporção menor morreu em pé (41%), enquanto poucas morreram desenraizadas (5%). A taxa de mortalidade para árvores mortas em pé foi maior em florestas sujeitas a intensa sazonalidade e secas mais intensas. Enquanto as árvores com copa mais exposta à luz eram mais propensas à morte por danos mecânicos, as árvores menos iluminadas eram mais suscetíveis à morte por seca. Ao nível de espécie, as taxas de mortalidade foram mais baixas para as espécies com maior densidade de madeira. Ao nível da árvore, o dano físico à copa por meio da quebra de galhos na copa foi o preditor mais importante da morte de árvores. Síntese. Perturbações causadas pelo déficit de água e vento são as principais causas de morte de árvores na borda sul da Amazônia, o que é preocupante, considerando o prolongamento previsto na sazonalidade para a Amazônia, especialmente na borda sul. A mortalidade de árvores aqui é maior do que em qualquer outra região amazônica, portanto, qualquer aumento na mortalidade pode representar um ponto de inflexão “não retorno” para essas florestas. Tree death is a key process for our understanding of how forests are responding to global change. Our long‐term research shows that more than 70% of all trees dying in this critical region at the edge of Amazonia already had markedly broken and damaged crowns years before they died. The damage caused by climate extremes—especially wind and drought—is concerning in light of widespread increases in dry‐season intensity in the Amazon, and as tree mortality is already greater here than any in other Amazonian region, increases in mortality here may herald a tipping point for these forests.
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The high tree mortality during the dry and hot years of 2018–2019 in Europe has triggered concerns on the future of European beech (Fagus sylvatica L.) forests under climate change and raised questions as to whether forest management may increase tree mortality. We compared long‐term mortality rates of beech between managed and unmanaged stands including the years 2018‐2019 at 11 sites in Hessen, Germany. We hypothesized that mortality would increase with climate water deficits during the growing season, initial stand density, decreasing dominance of trees, and decreasing intensity of tree removals. Initial stand density, tree removals, the climate water balance and the competitive status of trees were used as predictor variables. Mean annual natural mortality rates ranged between 0.5% and 2.1%. Even in the drought years, we observed no signs of striking canopy disintegration. The significantly higher mortality (1.6–2.1%) in unmanaged stands during the drought years 2018 and 2019 was largely confined to suppressed trees. There was no significant increase of mortality in managed stands during the drought years, but a shift in mortality towards larger canopy trees. Our study did not confirm a general influence of management, in the form of tree removals, on mortality rates. Yet, we found that during drought years, management changed the distribution of mortality within the tree community. To analyse the effects of management on mortality rates more comprehensively, a wider gradient in site moisture conditions, including sites drier than in this study, and longer post‐drought periods should be employed. During the recent drought years mortality of suppressed beech trees increased in unmanaged closed stands, while in managed partially harvested stands the focus of mortality shifted towards the more dominant beech trees.
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Tree species display a wide variety of water use strategies, growth rates and capacity to tolerate drought. However, if we want to forecast species capacity to cope with increasing aridity and drought, we need to identify which measurable traits confer resilience to drought across species. Here, we use a global tree ring network (65 species; 1931 site series of ring‐width indices ‐ RWI) to evaluate the relationship of long‐term growth‐drought sensitivity (RWI‐SPEI drought index relationship) and short‐term growth response to extreme drought episodes (resistance, recovery and resilience indices) with functional traits related to leaf, wood and hydraulic properties. Furthermore, we assess the influence of climate (temperature, precipitation and climatic water deficit) on these trait‐growth relationships. We found close correspondence between the long‐term relationship between RWI and SPEI and resistance and recovery of tree growth to severe drought episodes. Species displaying a stronger RWI‐SPEI relationship to drought and low resistance and high recovery to extreme drought episodes tended to have higher wood density (WD) and more negative leaf minimum water potential (Ψmin). Such associations were largely maintained when accounting for direct climate effects. Our results indicate that, at cross‐species level and global scale, wood and hydraulic functional traits explain species’ growth responses to drought at short‐ and long‐term scales. These trait‐growth response relationships can improve our understanding of cross‐species capacity to withstand climate change and inform models to better predict drought effects on forest ecosystem dynamics.
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The global rise in temperature and associated changes in climate have led to decline of forests around the globe, across multiple species and ecosystems. A particularly severe example of this is yellow-cedar ( Callitropsis nootkatensis ) decline along the coast of British Columbia and Alaska, where anthropogenic climate change has led to reduced insulating snowpack, leaving yellow-cedar roots vulnerable to thaw-freeze events, resulting in freezing damage to fine roots and water stress during the subsequent growing season. This includes abundant evidence of tree decline and mortality on the islands of Haida Gwaii. Yellow-cedar decline is complex, with the potential for freezing injury over multiple years and damage that can accumulate over time. We found trees in various stages of decline, from long dead to currently declining, and multiple growth patterns at each study site. We conducted a principal component analysis and identified patterns of divergent growth and divergent response to climate among yellow-cedars within the same stands, across all sites, including three distinct periods of an onset of growth decline (1960s, 1990s, 2000s). Yellow-cedars affected by decline were decreasing in growth and negatively associated with warmer drier winter conditions, whereas yellow-cedars not affected by decline were increasing in growth and positively associated with warmer growing season temperatures. The limiting factors for declining trees, warm dry winter conditions, are consistent with the hypothesis from the mainland that climate warming has led to root freezing. Our research highlights the need to consider multiple signals within a site that would be masked by a single site-level chronology. This is especially relevant within the context of forest decline, where stressors may have differing effects on individual trees. Graphical abstract
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Abstract 1. To assess the impacts of climate change on vegetation from stand to global scales, models of forest dynamics that include tree demography are needed. Such models are now available for 50 years, but the currently existing diversity of model formulations and its evolution over time are poorly documented. This hampers systematic assessments of structural uncertainties in model‐based studies. 2. We conducted a meta‐analysis of 28 models, focusing on models that were used in the past five years for climate change studies. We defined 52 model attributes in five groups (basic assumptions, growth, regeneration, mortality and soil moisture) and characterized each model according to these attributes. Analyses of model complexity and diversity included hierarchical cluster analysis and redundancy analysis. 3. Model complexity evolved considerably over the past 50 years. Increases in complexity were largest for growth processes, while complexity of modelled establishment processes increased only moderately. Model diversity was lowest at the global scale, and highest at the landscape scale. We identified five distinct clusters of models, ranging from very simple models to models where specific attribute groups are rendered in a complex manner and models that feature high complexity across all attributes. 4. Most models in use today are not balanced in the level of complexity with which they represent different processes. This is the result of different model purposes, but also reflects legacies in model code, modelers’ preferences, and the ‘prevailing spirit of the epoch’. The lack of firm theories, laws and ‘first principles’ in ecology provides high degrees of freedom in model development, but also results in high responsibilities for model developers and the need for rigorous model evaluation. 5. Synthesis. The currently available model diversity is beneficial: convergence in simulations of structurally different models indicates robust projections, while convergence of similar models may convey a false sense of certainty. The existing model diversity – with the exception of global models – can be exploited for improved projections based on multiple models. We strongly recommend balanced further developments of forest models that should particularly focus on establishment and mortality processes, in order to provide robust information for decisions in ecosystem management and policymaking.
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An estimated 128 M trees died during the 2012–2016 California drought, largely in the southern Sierra Nevada Range. Prescribed burning and mechanical thinning are widely used to reduce fuels and restore ecosystem properties, but it is unclear if these treatments improve tree growth and vigor during extreme drought. This study examined tree growth responses after thinning, prescribed burning, and extreme drought at the Teakettle Experimental Forest, a historically frequent fire mixed-conifer forest in the southern Sierra Nevada of California, USA. Mechanical thinning (no thin, understory thin, and overstory thin) and prescribed burning (unburned, fall burning) were implemented in 2000–2001. Using annual growth data from increment cores, over 10,000 mapped and measured trees, and lidar-derived metrics of solar radiation and topographic wetness, we had two primary questions. First, what were the growth responses to thinning and prescribed burning treatments, and did these responses persist during the 2012–2016 drought? Second, what tree-level attributes and environmental conditions influenced growth responses to treatments and drought? Thinning increased residual tree growth and that response persisted through extreme drought 10–15 years after treatments. Growth responses were higher in overstory versus understory thinning, with differences between thinning types more pronounced during drought. Species-specific growth responses were strongest with overstory thinning, with sugar pine (Pinus lambertiana) and incense-cedar (Calocedrus decurrens) having higher growth responses compared to white fir (Abies concolor) and Jeffery pine (Pinus jeffreyi). For individual trees, factors associated with higher growth responses were declining pretreatment growth trend, smaller tree size, and post-treatment low neighborhood basal area. Growth responses were initially not influenced by topography, but topographic wetness became important during extreme drought. Mechanical thinning resulted in durable increases in residual tree growth rates during extreme drought over a decade after thinning occurred, indicating treatment longevity in mitigating drought stress. In contrast, tree growth did not improve after prescribed burning, likely due to fire effects that reduced surface fuels, but had little effect on reducing tree density. Thinning treatments promoted durable growth responses, but focusing on stand-level metrics may ignore important tree-level attributes such as localized competition and topography associated with higher water availability. Mechanical thinning was effective at improving growth in trees that had been experiencing declining growth trends, but was less effective in improving growth responses in large old trees of higher ecological importance.
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Unprecedented tree dieback across Central Europe caused by recent global change‐type drought events highlights the need for a better mechanistic understanding of drought‐induced tree mortality. While numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is still debated. It further remains largely unresolved how the local neighborhood composition affects individual mortality risk. We studied 9,435 young trees of 12 temperate species planted in a diversity experiment in 2013 to assess how hydraulic traits, carbon dynamics, pest infestation, tree height and neighborhood competition influence individual mortality risk. Across species, hydraulic safety margins were negatively and a shift towards a higher sugar fraction in the non‐structural carbohydrate (NSC) pool positively associated with mortality risk. Moreover, trees infested by bark beetles had a higher and taller trees a lower mortality risk, respectively. Most neighborhood interactions were beneficial, though neighborhood effects were highly species‐specific. Species that suffered more from drought, especially Larix spp. and Betula spp., tended to increase the survival probability of their neighbors and vice versa. While severe tissue dehydration marks the final stage of drought‐induced tree mortality, we show that hydraulic failure is interrelated with a series of other, mutually inclusive processes. These include shifts in NSC pools driven by osmotic adjustment and/or starch depletion as well as pest infestation, and are modulated by the size and species identity of a tree and its neighbors. A more holistic view that accounts for multiple causes of drought‐induced tree mortality is required to improve predictions of trends in global forest dynamics and to identify mutually beneficial species combinations.
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Global climate change and the increase in the frequency and intensity of drought have led to widespread forest decline and tree mortality. Studying the resilience components of tree growth to drought, including resistance (Rt), recovery (Rc), and resilience (Rs) and the influencing factors, helps assess forests' production and ecological stability under a changing climate. This study analyzed the responses of three resilience components of natural Mongolian pine (Pinus sylvestris var. mongolica) to drought events by examining individual-tree characteristics in two sites of Hulunbuir using the linear mixed effect model. The result showed that drought severity, diameter at breast height (dbh), pre-drought growth, and growth variability prior to drought had significant effects on the three resilience components of Mongolian pine growth. Specifically, as drought severity, dbh and growth variability increased, the Rt and Rs decreased, but Rc increased, showing a trade-off relationship with Rt. However, the Rt, Rc, and Rs decreased with pre-drought growth. Inter-tree competition and tree age also significantly impacted two resilience components. Besides, the interaction term between tree competition and tree age negatively affects Rt and Rs but positively affects Rc. Our findings highlight the influence of drought severity and individual-tree characteristics on drought resilience components, which can serve the adaptive management of natural Mongolian pine forests in the future.
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Tree‐ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes in forest dynamics. Here, we related tree‐ring growth chronologies and stand‐level forest changes in basal area from two independent datasets to test if tree‐ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree‐ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981‐2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index (SPEI), and their impacts on tree growth by quantifying tree‐ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought‐prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought‐induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought‐prone gymnosperm forests we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought‐induced growth reductions and their cumulative impacts have strong potential to be used as early‐warning indicators of regional forest vulnerability.
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Climate change is expected to increase the frequency and intensity of summer droughts. Sufficient drought resistance, the ability to acclimate to and/or recover after drought, is thus crucial for forest tree species. However, studies on the hydraulics of mature trees during and after drought in natura are scarce. In this study, we analyzed trunk water content (electrical resistivity ER) and further hydraulic (water potential, sap flow density, specific hydraulic conductivity, vulnerability to embolism) as well as wood anatomical traits (tree ring width, conduit diameter, conduit wall reinforcement) of drought‐stressed (artificially induced summer drought via throughfall‐exclusion) and unstressed Picea abies and Fagus sylvatica trees. In P. abies, ER indicated a strong reduction in trunk water content after five years of summer drought, corresponding to significantly lower pre‐dawn leaf water potential and xylem sap flow density. Vulnerability to embolism tended to be higher in drought‐stressed trees. In F. sylvatica, only small differences between drought‐stressed and control trees were observed. Re‐watering led to a rapid increase in water potentials and xylem sap flow of drought‐stressed trees, and to increased growth rates in the next growing season ER analyses revealed lower trunk water content in P. abies trees growing on throughfall‐exclusion plots even one year after re‐watering, indicating a limited capability to restore internal water reservoirs. Results demonstrated P. abies to be more susceptible to recurrent summer drought than F. sylvatica, and to exhibit long lasting and pronounced legacy effects in trunk water reservoirs.
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The Mediterranean-type Ecosystems of Central Chile is one of the most threatened regions in South America by global change, particularly evidenced by the historical megadrought that has occurred in central Chile since 2010. The sclerophyllous forest stands out, whose history and relationship with drought conditions has been little studied. Cryptocarya alba and Beilschmiedia miersii (Lauraceae), two large endemic trees, represent an opportunity to analyze the incidence of intense droughts in the growth of sclerophyllous forests by analyzing their tree rings. Here, we considered > 400 trees from nineteen populations of C. alba and B. miersii growing across a latitudinal gradient (32°–35° S). To study the influence of local and large-scale climatic variability on tree growth, we first grouped the sites by species and explored the relationships between tree-growth patterns of C. alba and B. miersii with temperature, precipitation, and climate water deficit (CWD). Second, we performed Principal Component Analysis to detect common modes of variability and to explore relationships between growth patterns and their relationship to Palmer Drought Severity Index (PDSI), ENSO and SAM indices. We detected a breaking point as of 2002 at regional level, where a persistent and pronounced decrease in tree growth occurred, mainly influenced by the increase in CWD and the decrease in winter-spring rainfall. In addition, a positive (negative) relationship was showed between PC1 growth-PDSI and PC1 growth-ENSO (growth-SAM), that is, growth increases (decreases) in the same direction as PDSI and ENSO (SAM). Despite the fact that sclerophyllous populations are highly resistant to drought events, we suggest that the sclerophyllous populations studied here experienced a generalized growth decline, and possibly the natural dynamics of their forests have been altered, mainly due to the accumulating effects of the unprecedented drought since 2010. Graphical abstract
Chapter
Tree-ring studies in the Mediterranean Basin and in regions characterized by Mediterranean climates are scarce because of a lack of old trees and difficulties related to the clear identification of individual rings.
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Los bosques de Araucaria araucana son uno de los ecosistemas más importantes en el ámbito evolutivo, biológico y cultural del extremo sur de América. Han sobrevivido a todo tipo de cambios en el paisaje y hoy son los únicos representantes del género que habita climas templados. En la cordillera de los Andes, sobre los 1000 m s.n.m., esta especie se asocia a Nothofagus pumilio y en conjunto generan un ecosistema destacado por su belleza y resiliencia. Este bosque ha sido esencial para la supervivencia y desarrollo de las comunidades humanas tanto las originarias como las más recientes. Sin embargo, los conflictos bélicos, la colonización, la expansión de las plantaciones forestales, el crecimiento poblacional y el desarrollo de la nación chilena afectaron grandes superficies de este tipo forestal. Hoy el bosque remanente resiste al fuego, a la ganadería, al piñoneo y al madereo. Pero ¿hasta qué punto es capaz de soportar estas presiones antrópicas? ¿Cuál es el punto de quiebre para que se afecte su regeneración y capacidad de crecer? Este libro analiza el efecto de algunos factores de alteración de hábitat con diferentes niveles de intensidad en los bosques de araucaria-lenga de Curacautín, Lonquimay y Melipeuco. La situación actual de los bosques indica que con perturbaciones de baja intensidad, manteniendo la estructura de tamaños referencial, se tienen tasas de regeneración que aseguran la continuidad de los bosques. Esta condición se presenta en terrenos manejados por comunidades pehuenches desde tiempos ancestrales. Sin embargo, por cercanía a centros poblados, la subdivisión de la propiedad y la presión de uso por ganadería, piñoneo, leña y cambio de uso de suelo estos ecosistemas se van degradando día a día. El desafío es conservar estos impresionantes bosques, considerando la cosmovisión ancestral y una gestión sostenible.
Chapter
Worldwide forests are being impacted by several components of global change such as climate warming, increased aridification, land-use changes modifying regimes of disturbances, increased carbon and nitrogen availability, and new pests which are often invasive species. In many cases, forests are responding to global change by showing a loss of vitality manifested as increased canopy defoliation and dieback, rising mortality rates, productivity loss, and growth decline. Forest health is a multidimensional and complex concept difficult to quantify but existing tree defoliation and mortality networks offer an opportunity to define and characterize dieback hotspots: sites or stands in which tree mortality surpasses historical background mortality rates. We present a study case on how drought-triggered forest dieback hotspots could be detected by using defoliation and mortality in European stands of Scots pine. The proposed framework allows deepening into the understanding of forest health by disentangling the spatiotemporal components of dieback hotspots.
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Plain Language Summary Droughts are increasingly severe and widespread globally, but their impacts on the global carbon cycle remain unclear due to the complexity of vegetation response to climate under and after droughts. In particular, how much plant activity decreases during drought and how quickly it recovers in the following months or years is not well understood. Here, by employing large‐scale satellite‐derived vegetation information, we identify the existence of widespread drought legacy effects on vegetation activity in the Northern Hemisphere ecosystems. We find that vegetation becomes greener in 2–6 growing‐season months after severe droughts with a stronger response to climate. These findings are in general more prominent at mid‐ and high‐latitudes, in particular for the grass‐dominated ecosystems compared with the tree‐dominated ones. Our analyses extend the picture for understanding how vegetation responds to severe droughts and could provide useful information to assist the development of dynamic vegetation models toward assessing changes in ecosystem resilience under changing climates.
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Key message: Climate envelope analysis of nine tree species shows that Fagus sylvatica L. and Picea abies H. Karst could lose 58% and 40% of their current distribution range. Quercus pubescens Willd and Quercus cerris L. may win areas equal with 47% and 43% of their current ranges. The ratio of poorly predictable areas increases by 105% in southern and south-eastern Europe. Context: Climate change requires adaptive forest management implementations. To achieve climate neutrality, we have to maintain and expand forest areas. Impact assessments have great importance. Aims: The study estimates the potential climate envelopes of nine European tree species for a past period (1961– 1990) and for three future periods (2011–2040, 2041–2070, 2071–2100) under two emission scenarios (RCP4.5 and RCP8.5) based on the current species distribution. Methods: Climate envelopes were estimated simultaneously using the random forest method. Multi-resolution seg- mentation was used to determine the climatic characteristics of each species and their combinations. Models were limited to the geographical area within which the climatic conditions correspond to the climatic range of the training areas. Results: Results showed remarkable changes in the extent of geographic areas of all the investigated species’ climate envelopes. Many of the tree species of Central Europe could lose significant portions of their distribution range. Adhering to the shift in climate, these tree species shift further north as well as towards higher altitudes. Conclusion: European forests face remarkable changes, and the results support climate envelope modelling as an important tool that provides guidelines for climate adaptation to identify threatened areas or to select source and destination areas for reproductive material. Availabiliy: It is an open access article. Click on DOI in the header or follow this link: https://rdcu.be/cTncj
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