Developing alternatives to adaptive silviculture: Thinning and tree growth resistance to drought in a Pinus species on an elevated gradient in Southern Spain
To read the full-text of this research, you can request a copy directly from the authors.
Abstract
Forest plantations are more vulnerable to the stress induced by biotic and abiotic factors than are naturally regenerated forests. These effects can be aggravated by a lack of management in large reforestation areas, and thinning could, therefore, help trees to reduce dieback and tree mortality related to drought. We address this question using a dendrochronology and modelling approach to improve the understanding of the growth response of high-density planted pine forests to thinning in drought-prone areas of Southern Spain. An experimental trial was, therefore, carried out with three species (Pinus halepensis, P. nigra, and P. sylvestris) and three thinning treatments (unthinned, moderate, and heavy thinning), after which growth-climate relationships and drought vulnerability indices were assessed. Three separate generalized linear mixed-effects models (GLMM), one for each species and location, were fitted using BAI as the response variable, and post-thinning growth trajectories and drought vulnerability indices were also simulated. Ten-year basal area showed strong growth responses following the thinning treatment (BAI 10 , 72% for P. halepensis and 50% for P. sylvestris as regards heavy thinning and 51% for P. nigra as regards moderate thinning), with different responses to precipitation and temperature according to species and thinning intensity. The significant effects of thinning on drought vulnerability indices indicated that the thinning treatments had a positive effect, irrespective of the pine species, although this was more evident in the case of P. sylvestris (recovery F = 28.10, p < 0.001, and resilience F = 35.21, p < 0.001 respectively) and P. halepensis (recovery F = 10.97, p < 0.001 and resilience F = 16.91, p < 0.001). The models also showed that climatic effect was greater for P. nigra than for P. sylvestris. The simulation also provides information on the long-term effectiveness of thinning; in P. sylvestris the effect of thinning tended to be attenuated after 20 years, but this was not the case of P. nigra and P. halepensis. High values of modelled resilience were found after thinning, with a time to recovery of between two and four years after drought depending on thinning intensity. Our findings showed the advantages of thinning for growth under different climatic constraints, particularly drought. This work is a new contribution that demonstrates the urgent need for forest managers to take steps in order to help drought-vulnerable Mediterranean pine plantations to adapt to the risks posed by climate change.
To read the full-text of this research, you can request a copy directly from the authors.
... Improving both the structural and functional diversity of forests not only contributes to stabilizing ecosystem processes, adapting them to disturbances and changing climatic conditions, but also improves the delivery of ecosystem goods and services [11]. In pine-planted mountain forests, where there are generally highdensity stocks with a limited supply of water, radial growth responses to climate vary significantly depending on tree density [12][13][14]. Furthermore, according to one study [15], in order to predict the response of forest ecosystems to climate change, the modeling of radial increments serves as a well-established quantitative measure with which to explore variations in forest adaptation to thinning, thus shedding light on how future climatic scenarios will impact on tree growth and adaptation to stress [16]. ...
... Dendrochronology has commonly been used to precisely determine the impact of thinning on radial growth [14]. However, dendrochronological data operate on a tree to plot scale, collecting data over several years by means of a systematic design. ...
... Similar findings have been reported for trees with different competition intensities or thinning in Mediterranean climates [14,56,57]. The increased growth rates resulting from heavy thinning are typically associated with improved tree water status and illumination within the stand as a result of reduced inter-tree competition [58]. ...
Thinning focused on achieving growth and diameter management objectives has typically led to stands with reduced climate sensitivity compared to unthinned stands. We integrated dendrochronological with Airborne Laser Scanner (LiDAR) data and growth models to assess the long-term impact of thinning intensity on Canary pine (Pinus canariensis) radial growth. In 1988, 18 permanent treatment units were established in 73-year-old Canary pine plantations and three thinning treatments were applied (C–control-unthinned; 0% basal area removal; MT–moderate thinning: 10% and 15% basal area removal, and HT–heavy thinning: 46% and 45% basal area removal on the windward and leeward slopes, respectively). Dendrochronological data were measured in 2022 and expressed as basal area increment (BAI). The impact of climate on growth was examined by fitting linear regression models considering two different Representative Concentration Pathway (RCP) climate scenarios, RCP 2.6 and RCP 4.5. Finally, LiDAR data were used for standing segmentation to evaluate changes in overall growth under different climatic scenarios. The LiDAR–stand attributes differed between aspects. The BAI of the most recent 20 years (BAI20) after thinning was significantly higher for the moderate and heavy treatments on the leeward plots (F = 47.31, p < 0.001). On the windward plots, BAI decreased after moderate thinning. Considerable thinning treatments resulted in stronger changes in growth when compared to RCP climatic scenarios. From a silviculture perspective, the mapping of canopy structure and growth response to thinning under different climatic scenarios provides managers with opportunities to conduct thinning strategies for forest adaptation. Combining dendrochronological and LiDAR data at a landscape scale substantially improves the value of the separate datasets as forecasted growth response maps allow improving thinning management plans.
... The adaptation of forest management should be a dynamic process that takes into account the local interactions between climate and the species through a systematic learning process [15]. Adaptive stone pine management tools, including the establishment of mixed stands that enhance the species' resistance to extreme droughts [15], thinning [50], and genetic selection of drought-resistant trees [19,40], could help in transitioning towards efficient climate adaptation strategies. ...
... In general, our results showed that resilience was not correlated with recovery or resistance in the study area, probably due to the big difference in the frequency of drought years across sites. We suggest performing further studies on plantations subjected to recurrent droughts, which have cumulative stress effects [50]; those studies can provide information about tree resistance and recovery, but not about tree resilience due to the high growth variability. ...
Abstract
The increasing occurrence of dry and hot summers generates chronic water deficits that negatively affect tree radial growth. This phenomenon has been widely studied in natural stands of native species but not in commercial plantations of exotic tree species. In central Chile, where the species is increasingly planted, the dynamics of stone pine (Pinus pinea L.) growth under drought have been little explored. We studied the impact of drought on four stone pine plantations growing in central Chile. We sampled and cross-dated a total of 112 trees from four sites, measured their tree-ring width (RWL) series, and obtained detrended series of ring width indices (RWIs). Then, we calculated three resilience indices during dry years (Rt, resistance; Rc, recovery; and Rs, resilience), and the correlations between the RWI series and seasonal climate variables. We found the lowest growth rate (1.94 mm) in the driest site (Peñuelas). Wet conditions in the previous winter and current spring favored growth. In the wettest site (Pastene), the growth rates were high (4.87 mm) and growth also increased in response to spring thermal amplitude. Overall, fast-growing trees were less resilient than slow-growing trees. Drought reduced stone pine stem growth and affected tree resilience to hydric deficit. At the stand level, growth rates and resistance were driven by winter and spring precipitation. Fast-growing trees were more resistant but showed less capacity to recover after a drought. In general, stone pine showed a high post-drought resilience due to a high recovery after drought events. The fact that we found high resilience in non-native habitats, opens new perspectives for stone pine cropping, revealing that it is possible to explore new areas to establish the species. We conclude that stone pine shows a good acclimation in non-native, seasonally dry environments.
Keywords: stone pine; tree radial growth; hydric deficit; resilience; recovery; climate change
... Although this study found that thinning is often not economically beneficial, thinning timely scheduled and properly done can have positive effects in the mid-and long-term on stand development (Moreau et al. 2022). Enhanced resistance to for example storm (Pukkala et al. 2016), snow (Valinger et al. 1994) and drought (Navarro-Cerrillo et al. 2023) could not be incorporated in this analysis but should be taken into account in areas where such abiotic disturbance agents are a potential risk to forest management (Halbritter et al. 2020). This holds also true for some biotic threats (Roberts et al. 2020). ...
... Thinning is an important silvicultural activity that was initially used for commercial timber production but is now increasingly used for sustainable forest management [6]. Thinning improves tree growth and biomass productivity by regulating the growth space and microsite environment [7,8]. However, its effects in Masson pine forests are far from conclusive, particularly concerning thinning intensity, community response, and assessment methods [9]. ...
Masson pine (Pinus massoniana Lamb.) is a tree species that is widely distributed throughout southern China and holds significant economic and ecological value. The main objective of our study was to assess the effects of thinning on aboveground biomass increments and tree diversity in both the overstory and understory. Additionally, the underlying factors and mechanisms responsible for driving changes in biomass increment were analyzed. Four different thinning treatments (control, light thinning, moderate thinning, and heavy thinning) were implemented in 214 plots (~1800 tree ha−1) in three Masson pine forests in Hunan Province, China. A robustly designed experiment was used with over six years of repeated measurements. The differences in biomass increment and tree diversity among the different treatments were compared using repeated measures ANOVAs. The Mantel test was used to determine environmental metrics correlated with biomass increments across tree strata. Structural equation modeling was utilized to explore the multivariate relationships among site environment, tree diversity, and post-treatment biomass increment. The results indicated that thinning overall increased biomass increment, the Shannon index, and the Gini index, while decreasing the Dominance index over time. Moderate thinning (25%–35% of trees removed) was found to promote overstory biomass increment to 9.72 Mg·ha−1·a−1 and understory biomass increment to 1.43 Mg·ha−1·a−1 six years post-thinning, which is significantly higher than that of other treatments. Environmental metrics such as light intensity, soil organic matter, and other soil physiochemical properties were positively correlated with biomass increments, and their effects on the overstory and understory differed. Structural equation modeling revealed that thinning treatments, environmental metrics, tree diversity, and their interactions could be the main drivers for biomass increments across tree strata. Specifically, thinning treatments, light intensity, and tree size diversity (Gini index) had significant effects on overstory biomass increment, while understory species richness (Shannon index) and soil organic matter affected understory biomass increment. In conclusion, moderate thinning is an effective silvicultural treatment for stimulating biomass increments of both the overstory and understory in Masson pine forests in southern China if a middle period (e.g., six years) is considered. Some factors, such as species richness, tree size diversity, and environmental metrics (e.g., light and soil), are suggested for consideration to improve the efficiency of thinning.
... Non-mature forests may require different management strategies from those used for mature forests. Thinning was reported to improve growth response to drought for several pines (Navarro-Cerrillo et al. 2023), including stone pine trees (Del Río et al. 2011;Pardos et al. 2015). ...
Background and aims
Growth of Pinus pinea forests and plantations in native countries is known to be affected by soil and climate characteristics. However, edaphoclimatic drivers of growth and fruiting have been scarcely studied outside the species’ native range; in addition, the role of soil nutrients, particularly in juvenile trees’ development, has been poorly explored.
Methods
Relationships between edaphoclimatic variables and the performance of 54 young plantations were studied in Chile. Vegetative growth and fruiting were measured in 100 randomly selected trees per plantation. Composite soil samples were taken from each site to analyze soil chemical characteristics and texture. A principal component analysis was performed including climatic, soil data, and growth and fruiting variables.
Results
Annual growth was 1.3 cm year⁻¹ for stem diameter, 28.3 cm year⁻¹ for crown diameter, and 38.9 cm year⁻¹ for height; cone production per crown area was 0.07 cones m⁻². Negative correlation of height growth, crown growth and cone production with EC, Na, and pH were found, along with positive correlations with OM, PP and the index PP × AT. Stem diameter growth was favored by less acidic soils with high sand content, and showed no correlation with cone production. Furthermore, vegetative growth was positively correlated with N, P, Mg and clay content. In young plantations, vegetative growth was higher than in the species’ native habitat.
Conclusion
Soil properties, in particular low EC and Na were identified as favorable for growth of young stone pine trees, along with high content of soil N, P and Mg, and PP.
... Recently, the effects of thinning on tree and shrub growth, biodiversity, carbon stocks, hydrological processes, soil physicochemical properties, soil microbial biomass and community structure, soil enzyme activities, and soil microclimate have been the subject of several comprehensive reviews and meta-analyses [9][10][11][12][13][14][15]. Among the results of these studies are that thinning can promote individual tree growth and alleviate drought stress by increasing the soil water availability to the remaining trees and by developing more extensive individual root systems over time, particularly in high-density stands that tend to have less developed root systems [16][17][18][19][20][21]. In fact, good thinning management can increase shrub and herb diversity [10,15,22]. ...
Pinus halepensis Miller is a widespread tree species in the western Mediterranean basin, where very dense monospecific stands can be found, especially in natural regeneration after forest fires. Silvicultural thinning can reduce the competition of trees for natural resources and favour their development, although its effect depends on the habitat. The present study aims to know the effects on the soil at the physicochemical and microbiological levels after a heavy thinning in a young pine forest stand with a high stocking density. The stand is on a slope where the soil depth tends to decrease with altitude, and shows changes in its physicochemical properties between the upper and lower zones. Several soil carbon fractions (i.e., soil organic carbon (SOC), water-soluble organic carbon (WSOC), and microbial biomass carbon (MBC)), microbial activity (basal soil respiration (BSR)) and enzyme activities (acid phosphatase (AP) and urease (UA)) were analysed at specific dates over a period of about five years after a heavy thinning. The changes in organic matter content were abrupt in the slope, conditioning the observed differences. It is highlighted that the SOC and WSOC contents in the mineral soil were 2.5- and 3.5-fold significantly higher, respectively, in the upper shallow zone compared to the lower deeper zone. This was also reflected in significantly higher levels of gravimetric water content (GWC) and MBC (both about 1.4-fold higher), with higher levels of BSR and UA, and 2.5-fold significantly higher levels of AP. As a result, most of the properties studied showed no significant differences between the thinning treatment and the untreated control. Results varying between dates, with a strong dependence on climate (soil temperature and humidity) of WSOC and UA. It can be concluded that the heavy thinning applied in this short-term case study favoured the growth conditions of the pine without negatively affecting the soil properties studied.
Among various forest management activities, thinning is a prevalent treatment that affects tree growth and living biomass. Increased moisture and light availability may also enhance the mineralization of litter and dead wood organic matter, impacting soil carbon stocks. Thinning may also affect the services forests provide, including water production and nutrient cycling. The impacts of thinning on water yield and carbon stocks have been well documented around the globe while targeting mainly one of these ecosystem services. Our experimental paired catchment study covers both and puts forward long term results. We assessed the carbon stock changes caused by two slight thinning treatments together with the impacts on water yield in experimental paired catchments of 71.9 (W–I) and 77.5 hectares (W–IV) in Istanbul, Türkiye. The null hypothesis was that the slight thinnings did not affect the water yield and carbon stocks significantly. On the carbon stock part, we calibrated and parametrized the CBM–CF3 model with field measurements to simulate changes in carbon stocks of mixed deciduous forest stands. The intensities of the treatments (thinnings) were 11% and 18% of the basal area, performed in 1986 and 2011, respectively. We found that, while C stocks decreased by around 30 tons per hectare during the 1986–2020 period, the water yield was enhanced by approximately 25 mm/yr in the treatment catchment compared to the control watershed during the four-year post-treatment period. This amount of streamflow increase was around 10 percent of the average water yield of the catchments. It was concluded that there was a detectable increase in water yield during the following four years of the slight thinning treatments, while the reduction in abovegound carbon stocks continued for more than three decades.
Global warming is leading to more frequent and intense drought events, exerting unprecedented pressure on forest growth. Although post-drought recovery in plantation growth has been studied enormously, the variation of planted populations across the whole distribution range of a species is not well understood. In this study, the growth suitability of the most widely planted conifer species in dry lands of northern China, Mongolian Scots pine (Pinus sylvestris var. mongolica), was estimated, based on tree-ring data sampled in 289 plots of afforestation across most of its planting range. According to changes in the hydrothermal condition during the last forty years (1980–2018), the distribution range was divided into three parts: warming, warming-wetting, and non-significant trends (NT) regions. Two severe droughts in 2000 and 2007 were identified using the Standardized Precipitation Evapotranspiration Index (SPEI). Then, the resistance (Rt), recovery (Rc), and resilience (Rs) growth indices were calculated and compared among these three parts. The growth of planted Mongolian Scots pine has overall significantly increased from 1980 to 2018 (p < 0.001). The pine plantations in warming regions were sensitive to annual precipitation, while warming-wetting regions were more influenced by the growing season climate. Pines reverted to pre-drought growth levels within 3 years after the drought event in 2000 but failed in 2007, and populations in the warming region showed significantly lower post-drought recovery. Furthermore, high pre-drought growth levels were linked to low Rs in both drought events (p < 0.001). Post-drought precipitation enhanced the Rc of plantations, compensating for the growth reduction caused by droughts. Soil moisture during the drought events enhanced Rt and Rs. The Rt and Rs indices decreased significantly with the age of plantations (p < 0.05). Overall, our results revealed that pre-drought growth and post-drought precipitation were key factors influencing the resilience of planted Mongolian Scots pine. Besides, the plantations experiencing wetting trends revealed favorable drought resilience and stable radial growth, which indicated promising potential for planted Mongolian Scots pine in those areas. Controlling the age of plantations seems to be crucial for maintaining drought resilience in large-scale plantations and maximizing ecological benefits.
The increase in the frequency, duration and severity of disturbances can disrupt forest composition and structure, posing threats to ecosystem services. Adaptive forest management has the potential to adjust forests to future disturbance regimes, by reducing their susceptibility to these threats and, therefore, increasing their adaptation capacity. However, few comprehensive studies have integrated the factors involved in adaptive forest management by comparing the biomass of national parks of the same region, or examined how global change might affect them in the near future. In this study, we integrate data of forest inventories, wildfires, high-resolution climate and pests information in climate model projections to assess climate-sensitive risks associated with key abiotic disturbances (such as fires, droughts and pests) in coniferous Mediterranean mountain forests, considering various forest management approaches and climate scenarios in three Spanish national parks. We employed the conceptual framework introduced by Lecina-Diaz et al. (2021a) to evaluate risks to forests and considered Exposed values (E), Hazard Magnitude (HM), Susceptibility (S) and Lack of Adaptive Capacity (LAC). We applied statistical weights using expert weights and the analytic hierarchy process (AHP) as the basis for determining the global ranking score of each indicator of the risk components. We extracted information about the climatic scenarios from the Coupled Model Intercomparison Project Phase 6. Three forest management scenarios were generated, named “low intervention», «traditional silviculture”, and "reduction of climate vulnerability", and mapped the risks of carbon sequestration for the different scenarios. The importance of each component was estimated using the AHP. HM was most significantly affected by the Aridity Index and the Fire Risk Index, with weights of 27.7 % and 24.2 %, respectively; S was affected by fuel load (29.0 %) and the Suppression Difficulty index (21.0 %), and LAC was influenced by forest management (32.0 %) and resprouting capacity (22.0 %). The three forest management scenarios exhibited different risk values that decreased with increasing intensity of the intervention in the three protected areas. Likewise, when two climate scenarios were considered, risk increased in the protected areas (Teide National Park, 10.69 %; Sierra Nevada National Park, 10.9 %; Sierra de las Nieves National Park, 6.17 %). Patterns of spatial arrangement of risk, which included HM, S and LAC, were consistent with the risk level, with the highest risk values being concentrated in the buffer areas (natural parks), with areas with a greater presence of coniferous plantations. The risk maps allowed us to identify areas that are critical under extreme conditions and where the greatest disturbance events may occur. Our study underscores that risks of disturbances in mountain ecosystems dominated by conifers within protected areas can be mitigated by forest management. The application of forest management adaptation strategies in national parks and buffer areas has proven effective in reducing risks of disturbances and preserving certain ecosystem services, such as carbon sequestration.
Integrating climate change concerns into forest management strategies remains challenging. Among management strategies, thinning has been proven to alter major components underlying the carbon and water cycles over the short term. However, the functional adaptability of managed forests to cope with long-term drought remains unclear. This study aims to quantify the influence of thinning on key plant functional traits for two pine species, and their impacts on the fundamental processes of tree growth and transpiration. We conducted an experimental silvicultural trial with varying thinning intensities in two Pinus sylvestris and Pinus nigra plantations with the following specific objectives: i) to assess the impact of thinning on major traits involved in the leaf economic spectrum (LES), and hydraulic relations, and ii) to understand which of these traits reflect fundamental aspects of plant growth and transpiration, and their relations with environment. We used a combination of dendrochronological (basal area increment, BAI), sapflow (Fd), and tree ring isotope (δ13C) data, along with key functional and physiological traits including photosynthesis (A), leaf mass area (LMA), Huber value (Hv), sapwood density (Wd), and predawn to midday water potential (Ψpd, Ψmd). Our results revealed species-specific responses, with P. nigra exhibiting a conservative growth strategy, whereas P. sylvestris displayed increased growth after thinning. Despite thinning influenced both BAI and wood δ13C with notable species-dependent variations, we evidenced a general convergence in the relationships between LES traits and Hv across species, treatments and seasons (p <0.01). BAI responses to thinning translated into predictable changes in key hydraulic traits and adaptive changes in physiological performance. For instance, BAI was inversely related to LMA and Hv. Consistent with LES, variations in LMA influenced A. In turn, seasonal variations in LMA scaled negatively with Ψpd, and Ψmd (p <0.01). Such a functional adaptation suggests that thinning promotes growth while favouring forest resilience in the long term. This study provides practical implications for the implementation of a trait-based approach into silvicultural frameworks and anticipating the consequences of climate change for managed forest ecosystems.
Forest management plays a crucial role in preserving and enhancing the delivery of ecosystem services, whereas dendrochronological methods can play a significant role in this regard. In the Azores archipelago, with a temperate oceanic climate, with low thermal amplitude, and mild and relatively wet Summers, Cryptomeria japonica (Thunb. ex L.f.) D.Don is the main timber production species. It has been extensively planted for timber production and erosion control, being the mainstay of the regional forestry chain, with a rotation period of 30 years. Although dendrochronological studies have targeted this species elsewhere, this dendrochronological study in the Azores aimed to better understand its climate-growth relationships in the archipelago. For this purpose, we sampled 140 trees, in a total of 361 wood cores samples. Following standard dendrochronological
methods, we obtained four site chronologies from different volcanic complexes in S˜ao Miguel island. We used a stepwise modelling approach, with Random Forest, Principal Component Analysis and Generalized Linear Models. Our results suggest that despite year-round precipitation in the Azores, C. japonica is adversely affected by warm Winter and Summers, while on site SC it benefits from these conditions. As previously observed, precipitation may be overshadowed by temperature-driven mechanisms in C. japonica. Our spatial analysis resulted into climate-growth associations with similar directions for all site chronologies. In the future, higher Winters and Summer temperatures could lead to increased water stress and reduced growth rates. This should be considered when projecting the future distribution and productivity of C. japonica forests under different climate change scenarios. To ensure the long-term survival of this economically important tree species, adaptation responses should include genetic and conservation measures. Our findings provide baseline information for defining management approaches for this strategic species.
Understanding how we can increase the resilience of forest systems to future extreme drought events is increasingly important as these events become more frequent and intense. Diversifying production forests using intimate mixtures of trees with complementary functional traits is considered as one promising silvicultural approach that may increase drought resilience. However, the direction and magnitude of the drought response of mixed-species stands relative to monospecific stands of the same species can vary with species identity, relative abundance and levels of competition in a focal tree's immediate neighbourhood. Using a long-term experiment where tree-level mortality and the neighbourhood composition of each tree was known, we assessed the radial growth response of 24-year-old Scots pine (Pinus sylvestris) and Sitka spruce (Picea sitchensis) trees in intimately mixed and monospecific stands to a short-duration, high-intensity spring drought in Scotland. Mixing proportions included 25:75, 50:50 and 75:25 of P. sylvestris and P. sitchensis. At the species level, Scots pine was more drought resistant and resilient than Sitka spruce, while Sitka spruce showed higher recovery. Surprisingly, neither pre-drought tree size nor neighbourhood competition were significantly associated with resistance or resilience to drought, and trees of both species within monospecific stands showed higher recovery and resilience than trees growing in mixed stands. Our study suggests intimate mixtures of these two species may not be an effective way to mitigate the negative impacts of future extreme spring drought events. Given that these two species comprise almost 70% of coniferous forests in the UK, our results highlight the pressing need to better understand their vulnerability to drought and the conditions under which intimate mixtures of these species could be beneficial or detrimental. Such knowledge is essential if we are to enable forest managers to effectively plan how to adapt these forests to the challenges of a changing climate.
We reviewed recent literature to identify the positive and negative effects of thinning on both stand-and tree-level resistance and resilience to four stressors that are expected to increase in frequency and/or severity due to global change: (1) drought, (2) fire, (3) insects and pathogens, and (4) wind. There is strong evidence that thinning, particularly heavy thinning, reduces the impact of drought and also the risk and severity of fire when harvest slash is burned or removed. Thinning also increases the growth and vigor of residual trees, making them less susceptible to eruptive insects and pathogens, while targeted removal of host species, susceptible individuals and infected trees can slow the spread of outbreaks. However, the evidence that thinning has consistent positive effects is limited to a few insects and pathogens, and negative effects on root rot infection severity were also reported. At this point, our review reveals insufficient evidence from rigorous experiments to draw general conclusions. Although thinning initially increases the risk of windthrow, there is good evidence that thinning young stands reduces the long-term risk by promoting the development of structural roots and favouring the acclimation of trees to high wind loads. While our review suggests that thinning should not be promoted as a tool that will universally increase the resistance and resilience of forests, current evidence suggests that thinning could still be an effective tool to reduce forest vulnerability to several stressors, creating a window of opportunity to implement longer term adaptive management strategies such as assisted migration. We highlight knowledge gaps that should be targeted by future research to assess the potential contribution of thinning to adaptive forest management. One of these gaps is that studies from boreal and tropical regions are drastically underrepresented, with almost no studies conducted in Asia and the southern hemisphere. Empirical evidence from these regions is urgently needed to allow broader-scale conclusions.
Proactive silviculture treatments (e.g., thinning) may increase C sequestration contributing to climate change mitigation, although, there are still questions about this effect in Mediterranean pine forests. The aim of this research was to quantify the storage of biomass and soil organic carbon in Pinus forests along a climatic gradient from North to South of the Iberian Peninsula. Nine experimental Pinus spp trials were selected along a latitudinal gradient from the pre-Pyrenees to southern Spain. At each location, a homogeneous area was used as the operational scale, and three thinning intensity treatments: unthinned or control (C), intermediate thinning (LT, removal of 30–40% of the initial basal area) and heavy thinning (HT, removal of 50–60%) were conducted. Growth per unit area (e.g., expressed as basal area increment-BAI), biomass, and Soil Organic Carbon (SOC) were measured as well as three sets of environmental variables (climate, soil water availability and soil chemical and physical characteristics). One-way ANOVA and Structural Equation Modelling (SEM) were used to study the effect of thinning and environmental variables on C sequestration. Biomass and growth per unit area were higher in the control than in the thinning treatments, although differences were only significant for P. halepensis. Radial growth recovered after thinning in all species, but it was faster in the HT treatments. Soil organic carbon (SOC10, 0–10 cm depth) was higher in the HT treatments for P. halepensis and P. sylvestris, but not for P. nigra. SEM showed that Pinus stands of the studied species were beneficed by HT thinning, recovering their growth quickly. The resulting model explained 72% of the variation in SOC10 content, and 89% of the variation in silvicultural condition (basal area and density) after thinning. SOC10 was better related to climate than to silvicultural treatments. On the other hand, soil chemical and physical characteristics did not show significant influence over SOC10- Soil water availability was the latent variable with the highest influence over SOC10. This work is a new contribution that shows the need for forest managers to integrate silviculture and C sequestration in Mediterranean pine plantations.
Forests are key elements in mitigating the effects of climate change due to the fact of their carbon sequestration capacity. Forest management can be oriented to optimise the carbon sequestration capacity of forest stands, in line with other productive objectives and the generation of ecosystem services. This research aimed to determine whether thinning treatments have a positive influence on the growth patterns of some of the main Mediterranean pine species and, therefore, on their Carbon (C) fixation capacity, both in terms of living biomass and soil organic carbon. The results obtained show that C sequestration capacity (biomass and SOC) increased at higher thinning intensities due to the induced alterations in tree growth patterns. We observed almost a 1.5-fold increase in P. nigra and P. sylvestris, respectively, and over a two-fold increase in P. pinaster under heavy thinning treatments; SOC stocks were affected by the intensity of the thinning treatments. These results can contribute to improving silvicultural practices aimed at C sequestration in forest plantations located in dry areas of the Mediterranean.
In Mediterranean seasonally dry regions, the rise in dieback and mortality episodes observed in pine afforestations has been related to higher drought intensity and lack of appropriate management, which enhance competition between trees for water and light. However, there is little understanding of the benefits of silviculture for plantations under seasonal drought stress. A combination of dendrochronology and wood C and O isotope analyses was used in three Black pine (Pinus nigra) plantations to work out the responses of radial growth (BAI, basal area increment) and water-use efficiency (WUEi) to thinning treatments (removal of 40% of the stand basal area). Thinning had a positive effect on BAI and WUEi, reduced drought sensitivity, and reduced the temporal dependence on the previous year’s growth. These results were significant even 13–14 years after thinning and coherent for the three study sites. Differences were found between the sites regarding the physiological mechanisms of adaptation. In two sites, we inferred the enhanced WUEi was due to increased photosynthetic rates (A) at constant stomatal conductance (gs). In the third site, which had higher tree density and therefore competition, we inferred increases in both A and gs, with the former being proportionally larger than the latter.
Drought will increasingly threaten forest ecosystems worldwide. Understanding how competition influences tree growth response to drought is essential for forest management aiming at climate change adaptation. However, published results from individual case studies are heterogeneous and sometimes contradictory. We reviewed 166 cases from the peer-reviewed literature to assess the influence of stand-level competition on tree growth response to drought. We monitored five indicators of tree growth response: mean sensitivity (inter-annual tree ring width variability); association between inter-annual growth variability and water availability; resistance; recovery; and resilience to drought. Vote counting did not indicate a consistent effect of competition on mean sensitivity. Conversely, higher competition for resources strengthened the association between water availability and inter-annual growth rates. Meta-analysis showed that higher competition reduced resistance (p < 0.001) and improved recovery (p < 0.05), but did not consistently affect resilience. Species, site and stand characteristics, and drought intensity were insignificant or poor predictors for the large variability among the investigated cases. Our review and meta-analysis show that competition does not affect the response of tree growth to drought in a unidirectional and universal way. Although density reduction (thinning) can alleviate growth declines during drought, the effects on growth after stress are uncertain. The large variability among investigated cases suggests that local-scale processes play a crucial role in determining such responses and should be explicitly evaluated and integrated into specific strategies for adaptation of forests to climate change.
Drought negative effects on forest ecosystems are projected to increase under global
warming all over the world. On this context, forest management can be an effective
option for reducing drought impacts and increasing tree growth stability to extreme
drought events. Here, we aim to evaluate black pine (Pinus nigra subsp. salzmannii)
growth response to climatic variability and drought events in managed and unmanaged
stands under similar Mediterranean climatic conditions. Drought events were identified
using long-term climatic data, and basal area increments were calculated for 100-120-
year old trees cored in managed and unmanaged plots. Results showed that tree size,
temperature, and the interaction between management treatment and water availability
significantly influenced tree growth. Basal area increment was reduced in response to
the 1983, 1991, 1994‒1995, 1999-2000 and 2005 drought events. Trees in managed
plots showed lower growth reductions in response to drought than those located in
unmanaged plots, probably experiencing higher competition for soil water, whereas the
reverse happened under wet climate conditions. Black pines showed greater resilience
and resistance, but generally lower recovery to drought events in managed than in
unmanaged stands. Our results suggest that forest management enhances drought
tolerance in black pine stands, which may help to ameliorate the negative impacts of
global warming across Mediterranean forest ecosystems.
Pinus halepensis dominates coastal to mountain areas throughout the Mediterranean Basin. Its growth plasticity, based on polycyclic shoot formation and dynamic cambial activity, and tolerance to extreme drought and exceptional frosts, allows it to colonize a vast array of environments. We used tree-rings from codominant pines to compare lifespan, growth rates, age and size distribution in a typical coastal (i.e., prolonged drought, occasional low-intensity fires) vs. inland hilly (i.e., moister conditions, recurrent frosts) population. BAI trends, growth-limiting climate factors and tree-ring anatomical anomalies were analyzed considering the differences in climate and phenology obtained from multispectral satellite images. The species maximum lifespan was 100–125 years. Mortality was mainly due to fire on the coast, or heart-rot in the inland site. Populations differed in productivity, which was maintained over time despite recent warming. Site conditions affected the growing season dynamics, the control over ring formation by summer drought vs. winter cold and the frequency of anatomical anomalies. Recurrent frost rings, associated with temperatures below −10 °C, occurred only at the inland site. Pinus halepensis confirmed its remarkable growth plasticity to diverse and variable environmental conditions. Its ability to survive extreme events and sustain productivity confirmed its adaptability to climate change in coastal areas as well as on Mediterranean mountains.
Variations in seasonal responses in water relations and total carbohydrate content (TCC) in one-yearold shoots from precommercially thinned (PCT) and unthinned Anatolian black pine stands were assessed during three seasons (sampling in May, July and September) in 2015–2017. Three different treatments were established: unthinned control with 4 941 stems·ha–1 and two thinned spacing levels (2–2.5 and 3–3.5 m) where 2 133 stems·ha–1 and 1 093 stems·ha–1 were left, respectively. Differences in osmotic potential at turgor loss point (ΨπTLP) between the thinned and unthinned plots appeared only during a water shortage (September) in the second season, with the thinned stands showing lower ΨπTLP than the unthinned stands. Seasonal variation in terms of ΨπTLP was detected in the 3–3.5 m spacing trees. PCT were effective on osmotic potential at full turgor (Ψπ100), relative water content (RWC), symplastic water at saturated point per dry weight of the shoot and dry weight fraction. In both the thinned and unthinned plots, a gradual decline was observed in RWC from May to September in all three years. Generally, although TCC was found to be higher in the 3–3.5 m spacing, control plots were also high in carbohydrates in some periods. Results reflect the ability of this species to survive in changing environments by PCT during dry periods in the three studied seasons. Although PCT has an effect on water potential components and TCC, it is not clear exactly how effective the PCT density is in osmotic adjustment.
Purpose of Review
Despite the rapidly increasing use of resilience indices to analyze responses of trees and forests to disturbance events, there is so far no common framework to apply and interpret these indices for different purposes. Therefore, this review aims to identify and discuss various shortcomings and pitfalls of commonly used resilience indices and to develop recommendations for a more robust and standardized procedure with a particular emphasis on drought events.
Recent Findings
Growth-based resilience indices for drought responses of trees are widely used but some important drawbacks and limitations related to their application may lead to spurious results or misinterpretation of observed patterns. The limitations include (a) the inconsistency regarding the selection and characterization of drought events and the climatic conditions in the pre- and post-drought period and (b) the calculation procedure of growth-based resilience indices.
Summary
We discuss alternative options for metrics, which, when used in concert, can provide a more comprehensive understanding of drought responses in cases where common growth-based resilience indices are likely to fail. In addition, we propose a new analytical framework, the “line of full resilience,” that integrates the three most commonly used resilience indices and show how this framework can be used for comparative drought tolerance assessments such as rankings of different tree species or treatments. The suggested approach could be used to harmonize quantifications of tree growth resilience to drought and it may thus facilitate systematic reviews and development of the urgently needed evidence base to identify suitable management options or tree species and provenances to adapt forests for changing climatic conditions.
Key message
Decreasing stand density increases resistance, resilience, and recovery of
Quercus petraea
trees to severe drought (2003), particularly on dry sites, and the effect was independent of tree social status.
ContextControlling competition is an advocated strategy to modulate the response of trees to predicted changes in climate.AimsWe investigated the effects of stand density (low, medium, high; relative density index 0.20, 0.53, 1.04), social status (dominant, codominant, suppressed), and water balance (dry, mesic, wet; summer water balance − 182, − 126, − 96 mm) on the climate-growth relationships (1997–2012) and resistance (Rt), resilience (Rs), and recovery (Rc) following the 2003 drought.Methods
Basal area increments were collected by coring (269 trees) in young stands (28 ± 7.5 years in 2012) of sessile oak (Quercus petraea) in a French permanent network of silvicultural plots.ResultsWe showed that the climate-growth relationships depend on average site-level water balance with trees highly dependent on spring and summer droughts on dry and mesic sites and not at all on wet sites. Neither stand density nor social status modulated mean response to climate. Decreasing stand density increased Rt, Rs, and Rc particularly on dry sites. The effect was independent of tree social position within the stand.Conclusion
Reducing stand density mitigates more the effect of extreme drought events on drier sites than on wet sites.
The negative impacts of drought on forest growth and productivity last for several years generating legacies, although the factors that determine why such legacies vary across sites and tree species remain unclear.
We used an extensive network of tree‐ring width (RWI, ring‐width index) records of 16 tree species from 567 forests, and high‐resolution climate and normalized difference vegetation index (NDVI) datasets across Spain during the common period 1982‒2008 to test the hypothesis that climate conditions and growth features modulate legacy effects of drought on forests. Legacy effects of drought were calculated as the differences between detrended‐only RWI and NDVI series (i.e. after removing long‐term growth trends) and pre‐whitened RWI and NDVI series predicted by a model including drought intensity. Superposed Epoch Analysis (SEA) was used to estimate whether legacy effects differed from random. Finally, legacy effects were related to water balance, growth persistence and variability, and tree species identity.
We found a widespread occurrence of drought legacy effects on both RWI and NDVI, but they were seldom significant. According to SEA, first‐year drought legacies were negative and different from random in 9% and 5% of the RWI and NDVI series respectively. The number of significant second‐ and third‐year legacies was substantially lower. Differences between RWI and NDVI legacies indicate that canopy greenness and radial growth responses to drought are decoupled. We found variations in legacies between tree species with gymnosperms presenting larger first‐year drought legacies than angiosperms, which were exposed to less severe droughts. Greater growth variability can explain the presence of first‐year RWI legacies in gymnosperms from dry sites despite that the relationship between growth variability and legacies was complex.
Synthesis. Accounting for species and site responses to drought provides a better understanding of the magnitude and duration of drought legacies on forest growth and productivity. Despite the widespread occurrence of growth reductions in the years during and after drought occurrence, significant legacies were not very common, mostly lasted one year, and were more widespread in gymnosperms. These are relevant factors to be considered in the future when studying the consequences of drought on forest productivity and tree growth.
Climate change is increasing the vulnerability of Mediterranean coniferous plantations. Here, we integrate a Landsat time series with a physically-based distributed hydrological model (Watershed Integrated Management in Mediterranean Environments—WiMMed) to examine spatially-explicit relationships between the mortality processes of Pinus pinaster plantations and the hydrological regime, using different spectral indices of vegetation and machine learning algorithms. The Normalized Burn Ratio (NBR) and Moisture Stress Index (MSI) show the highest correlations with defoliation rates. Random Forest was the most accurate model (R2 = 0.79; RMSE = 0.059), showing a high model performance and prediction. Support vector machines and neural networks also demonstrated a high performance (R2 > 0.7). The main hydrological variables selected by the model to explain defoliation were potential evapotranspiration, winter precipitation and maximum summer temperature (lower Out-of-bag error). These results show the importance of hydrological variables involved in evaporation processes, and on the change in the spatial distribution of seasonal rainfall upon the defoliation processes of P. pinaster. These results underpin the importance of integrating temporal remote sensing data and hydrological models to analyze the drivers of forest defoliation and mortality processes in the Mediterranean climate.
Extreme drought events and increasing aridity are leading to forest decline and tree mortality, particularly in populations near the limits of the species distribution. Therefore, a better understanding of the growth response to drought and climate change could show the vulnerability of forests and enable predictions of future dieback. In this study, we used a dendrochronological approach to assess the response to drought in natural and planted forests of the maritime pine (Pinus pinaster Aiton) located in its southernmost distribution (south of Spain). In addition, we investigated how environmental variables (climatic and site conditions) and structural factors drive radial growth along the biogeographic and ecological gradients. Our results showed contrasting growth responses to drought of natural and planted stands, but these differences were not significant after repeated drought periods. Additionally, we found differences in the climate–growth relationships when comparing more inland sites (wet previous winter and late spring precipitation) and sites located closer to the coast (early spring precipitation). Response functions emphasized the negative effect of defoliation and drought, expressed as the June standard precipitation-evapotranspiration index calculated for the 12-month temporal scale and the mean temperature in the current February, on growth. The strong relationship between climatic variables and growth enabled acceptable results to be obtained in a modeling approach. The study and characterization of this tree species’ response to drought will help to improve the adaptive management of forests under climate change.
The Paris Agreement advances forest management as one of the pathways to halt climate warming through carbon dioxide (CO2) emission reduction1. The climate benefits from carbon sequestration from forest management may, however, be reinforced, counteracted, or even offset by concurrent management-induced changes in surface albedo, surface roughness, biogenic volatile organic compound emissions, transpiration, and sensible heat flux2–4. Forest management could, thus, offset CO2 emissions without halting global temperature rise. It remains, therefore, to be confirmed that sustainable forest management portfolios for the end of the 21st-century for Europe would comply with the Paris Agreement, i.e., reduce the growth rate of atmospheric CO2, reduce the radiative imbalance at the top of the atmosphere, and neither increase the near-surface air temperature nor decrease precipitation. Here we show that a spatially-optimized portfolio that maximises the carbon sink through carbon sequestration, wood use and product and energy substitution, reduces the growth rate of atmospheric CO2 but does not meet any of the other criteria. The portfolios that maximise the carbon sink or forest albedo pass only one, albeit different, criterion. Managing the European forests with the objective to reduce near-surface air temperature, on the other hand, will also reduce the atmospheric CO2 growth rate, thus meeting two out of four criteria. Our results demonstrate that if present-day forest cover is sustained, the additional climate benefits through forest management would be modest and local rather than global. Based on these findings we argue that if adaptation would require large-scale changes in species composition and silvicultural systems over Europe5,6, these changes could be implemented with little unintended climate effects.
The assessment of the long-term impacts of drought on tree growth decline using tree-ring analyses may be used to test if plantations are more vulnerable to warming after successive droughts, leading to a “cumulative stress” effect. We selected 76 Pinus pinaster trees (declining and non-declining trees), and basal area increments over the last 20 years (BAI20) were calculated to build the chronologies for the stand types and vigor classes. Resistance, recovery and resilience indices were calculated. Pearson correlations, analyses and Partial Least-Squares regression were used to analyze the relationships among the response and environmental variables. We found a negative and significant relationship between mean temperature for May and June of the current year and growth in the naturally regenerated stands. This negative effect on growth under warm spring conditions was more noticeable in plantations than in naturally regenerated stands. A negative trend along time was found for the resilience index in planted stands. Evapotranspiration, maximum temperature and annual radiation showed significant and negative correlations with the growth of declining trees from planted stands, indicating they are susceptible to drought stress. Declining trees in planted stands showed a loss of growth resilience, specifically a negative trend after successive droughts.
Aim of the study: Thinning experiments in Scots pine (Pinus sylvestris L.) stands have been carried out for many years in different regions of its distribution. The aim of this paper is to gather knowledge regarding the effects of thinning on Scots pine stands, from the effects on growth and yield to the provision of ecosystem services in the context of climate change.Area of study: The review covers studies from different regions of the distribution area of Scots pineMaterial and methods: We reviewed the effect of thinning on four aspects: growth and yield, stability against snow and wind, response to drought, and ecosystem services.Main results: Heavy thinning involves a loss in volume yield, although the magnitude depends on the region, site and stand age. Thinning generally does not affect dominant height while the positive effect on tree diameter depends on the thinning regime. The stability of the stand against snow and wind is lower after the first thinning and increases in the long term. The impact of extreme droughts on tree growth is lower in thinned stands, which is linked to a better capacity to recover after the drought. Thinning generally reduces the wood quality, litter mass, and stand structural diversity, while having neutral or positive effects on other ecosystem services, although these effects can vary depending on the thinning regime. However, scarce information is available for most of the ecosystem services.Research highlight: Existing thinning experiments in Scots pine stands provided valuable information about thinning effects, but new experiments which cover a broad range of ecosystem services under different site conditions are still needed.
Forests around the world are experiencing increasingly severe droughts and elevated competitive intensity due to increased tree density. However, the influence of interactions between drought and competition on forest growth remains poorly understood. Using a unique dataset of stand-scale dendrochronology sampled from 6405 trees, we quantified how annual growth of entire tree populations responds to drought and competition in eight, long-term (multi-decadal), experiments with replicated levels of density (e.g., competitive intensity) arrayed across a broad climatic and compositional gradient. Forest growth (cumulative individual tree growth within a stand) declined during drought, especially during more severe drought in drier climates. Forest growth declines were exacerbated by high density at all sites but one, particularly during periods of more severe drought. Surprisingly, the influence of forest density was persistent overall, but these density impacts were greater in the humid sites than in more arid sites. Significant density impacts occurred during periods of more extreme drought, and during warmer temperatures in the semi-arid sites but during periods of cooler temperatures in the humid sites. Because competition has a consistent influence over growth response to drought, maintaining forests at lower density may enhance resilience to drought in all climates.
Drought indices are essential metrics for quantifying drought severity and identifying possible changes in the frequency and duration of drought hazards. In this study, we developed a new high spatial resolution dataset of drought indices covering all of Spain. The dataset includes seven drought indices, spans the period 1961–2014, and has a spatial resolution of 1.1 km and a weekly temporal resolution. A web portal has been created to enable download and visualization of the data. The data can be downloaded as single gridded points for each drought index, but the entire drought index dataset can also be downloaded in netCDF4 format. The dataset will be updated for complete years as the raw meteorological data become available.
Aim
The intensity and frequency of drought have increased considerably during recent decades in some Northern Hemisphere forested areas, and future climate warming could further magnify drought stress. We quantify how forests resist drought events and recover after them, i.e. we determine their growth resilience.
Location
North America and Europe.
Methods
We use a large tree‐ring database to study how drought influences forest growth resilience. We selected 775 tree‐ring width chronologies and studied the occurrence of years with extremely dry conditions (low soil moisture and/or high evaporative stress; hereafter ‘drought’) in these forests. For each drought in each forest we calculated three indices that represent different components of growth resilience to drought: resistance ( Rt ), recovery ( Rc ) and resilience ( Rs ). We related the variation in these indices with geographical, topographic, climatic and ecological conditions from each region.
Results
The three components of forest growth resilience were interrelated. Resistance and recovery were negatively related, and both were positively and nonlinearly related to resilience. Drought resistance increased with latitude, soil moisture and slope, whereas drought recovery decreased with latitude, soil moisture and summer normalized difference vegetation index. Drought resilience increased with elevation and decreased with the variation in soil moisture. Temperate broadleaf forests from wet regions showed a greater growth resistance (e.g. north‐eastern USA, central Europe) while conifer forests from dry to semi‐arid regions (e.g. south‐western USA, southern Europe) presented a greater growth recovery.
Main conclusions
The geographical patterns of growth resilience indices confirm the existence of different strategies among forests to cope with droughts, depending on the biome, the tree species and the prevailing climatic conditions. Geographical patterns in soil moisture availability tend to override species‐specific responses to drought.
Pinus halepensis plantations are widespread throughout semiarid–subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year−1). Declining stands (stand basal area increment, BAIstand ≈ −3 % year−1) were treated by thinning: (1) intense thinning—tree density, BAstand, and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate—56, 48, and 26 %, respectively; (3) control (~560 tree ha−1). Plots of 70 × 70 m were used in four replicates. Individual tree- to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential (ψ
PD) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth (ψ
PD = −1.7, −1.8, and −2.0 MPa; mortality = 0.2, 2, and 5 % year−1; BAItree = 3.4, 2.0, and 1.4 % year−1 in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LAtree) fluctuated with association to annual rainfall. Higher LAtree caused by thinning reflected a “selection effect” while increased leaf area efficiency (BAI per unit LAtree) was attributed to a “release effect” of thinning.
Foresters frequently lack sufficient information about thinning intensity effects to optimize semi-natural forest management and their effects and interaction with climate are still poorly understood. In an Abies pinsapo-Pinus pinaster-Pinus sylvestris forest with three thinning intensities, a dendrochronologial approach was used to evaluate the short-term responses of basal area increment (BAI), carbon isotope (δ13C) and water use efficiency (iWUE) to thinning intensity and climate. Thinning generally increased BAI in all species, except for the heavy thinning in P. sylvestris. Across all the plots, thinning increased 13C-derived water-use efficiency on average by 14.49% for A. pinsapo, 9.78% for P. sylvestris and 6.68% for P. pinaster, but through different ecophysiological mechanisms. Our findings provide a robust mean of predicting water use efficiency responses from three coniferous species exposed to different thinning strategies which have been modulated by climatic conditions over time.
Rear (warm) edge populations are often considered more susceptible to warming than central (cool) populations because of the warmer ambient temperatures they experience, but this overlooks the potential for local variation in thermal tolerances. Here we provide conceptual models illustrating how sensitivity to warming is affected throughout a species' geographical range for locally adapted and non-adapted populations. We test these models for a range-contracting seaweed using observations from a marine heatwave and a 12-month experiment, translocating seaweeds among central, present and historic range edge locations. Growth, reproductive development and survivorship display different temperature thresholds among central and rear-edge populations, but share a 2.5 degrees C anomaly threshold. Range contraction, therefore, reflects variation in local anomalies rather than differences in absolute temperatures. This demonstrates that warming sensitivity can be similar throughout a species geographical range and highlights the importance of incorporating local adaptation and acclimatization into climate change vulnerability assessments.
Global temperature rise and extremes accompanying drought
threaten forests1,2 and their associated climatic feedbacks3,4.
Our ability to accurately simulate drought-induced forest
impacts remains highly uncertain5,6 in part owing to our
failure to integrate physiological measurements, regionalscale
models, and dynamic global vegetation models(DGVMs).
Here we show consistent predictions of widespread mortality
of needleleaf evergreen trees (NET) within Southwest USA
by 2100 using state-of-the-art models evaluated against
empirical data sets. Experimentally, dominant Southwest
USA NET species died when they fell below predawn water
potential ( pd) thresholds (April–August mean) beyond which
photosynthesis, hydraulic and stomatal conductance, and
carbohydrate availability approached zero. The evaluated
regional models accurately predicted NET pd, and 91% of
predictions (10 out of 11) exceeded mortality thresholds
within the twenty-first century due to temperature rise.
The independent DGVMs predicted �50% loss of Northern
Hemisphere NET by 2100, consistent with the NET findings
for SouthwestUSA. Notably, the global models underestimated
future mortality within Southwest USA, highlighting that
predictions of future mortality within global models may
be underestimates. Taken together, the validated regional
predictions and the global simulations predict widespread
conifer loss in coming decades under projected globalwarming.
Thinning fosters individual tree growth by increasing the availability of water, light and nutrients. At sites where water rather than light is limiting, thinning also enhances soil evaporation and might not be beneficial. Detailed knowledge of the short‐ to long‐term physiological response underlying the growth responses to thinning is crucial for the management of forests already suffering from recurrent drought‐induced dieback.
We applied a dual isotope approach together with mechanistic isotope models to study the physiological processes underlying long‐term growth enhancement of heavily thinned Pinus sylvestris in a xeric forest in Switzerland. This approach allowed us to identify and disentangle thinning‐induced changes in stomatal conductance and assimilation rate.
At our xeric study site, the increase in stomatal conductance far outweighed the increase in assimilation, implying that growth release in heavily thinned trees is primarily driven by enhanced water availability rather than increased light availability.
We conclude that in forests with relatively isohydric species (drought avoiders) that are growing close to their physiological limits, thinning is recommended to maintain a less negative water balance and thus foster tree growth, and ultimately the survival of forest trees under drought.
La variabilidad climática actual se caracteriza por provocar cambios bruscos en los cursos fitoclimáticos, lo que está desencadenando procesos de mortalidad de especies forestales. El objetivo de este trabajo es analizar la adecuación fitoclimática de las masas artificiales de Pinus sylvestris L. en la Sierra de los Filabres (Almería), para lo cual se analizaron 35 variables fitoclimáticas que determinan la estabilidad climática de la especie. El estudio de los valores interanuales de las variables climáticas indica que entre 1970 y 1980 comenzó una perturbación climática amplia, modificando la estacionalidad y la variabilidad de las precipitaciones y de las temperaturas. Como rasgos más significativos tenemos el aumento de las temperaturas, sobre todo de las máximas, el aumento de la oscilación térmica y de los extremos diarios, la disminución global de precipitación, el aumento grave de la aridez primaveral y el deterioro de todas las variables ómbricas. El estudio de las estaciones con masas repobladas de pino silvestre en Andalucía oriental indica que las estaciones están fuera de la nube de puntos de las masas de pino silvestre establecidas en España. Las masas de pino silvestre de la Sierra de los Filabres están notablemente fuera de estación en todas sus características fitoclimáticas y pueden desaparecer progresiva o bruscamente, sobre todo si se implanta un cambio climático.
Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought‐induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought‐related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir ( A bies alba ), S cots pine ( P inus sylvestris ) and Aleppo pine ( P inus halepensis ), to a severe drought event.
We compared basal area increment ( BAI ) trends and responses to climate and drought in declining (very defoliated and dying) vs. non‐declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non‐structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood‐inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death.
Declining silver fir and Scots pine trees showed less growth than non‐declining trees one to three decades, respectively, before the drought event, whereas A leppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood‐inhabiting fungi were found in post‐drought declining trees apart from blue‐stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death.
Synthesis . Early warning signals of drought‐triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for S cots and A leppo pines. Longer growth and defoliation series, additional vigour parameters and multi‐species comparisons are required to understand and predict drought‐induced tree death.
Pruning allows knot-free timber to be obtained, thereby increasing the value of the highest-value wood products. However, the effect of pruning on growth is under discussion, and knowledge about the tree response to the simultaneous development of thinning and pruning is scarce. The objective of this study was to analyze the effect of the interaction of thinning and pruning on tree and stand level and the annual radial growth of two pine species native to Mediterranean mountains. We used long-term data of three trials installed in pine stands where several combinations of pruning and thinning were developed. Five inventories were carried out for each trial, and the mean dasometric features of the different treatments were compared using linear mixed models including a compe-tition index. In addition, we collected cores from ten trees per plot in order to evaluate the annual response of trees to the thinning and pruning. We analyzed the annual radial growth using a semiparametric approach through a smooth penalized spline including rainfall and temperature covar-iates. Pruning did not show any effect on growth. However, larger diameter and increased annual radial growth were found in thinned plots, both with and without pruning, as compared to unthinned plots. Also, we found significant effects of climate on annual radial growth. We recommend the application of thinning and pruning in stands of Med-iterranean mountains in order to get knot-free timber since growth reduction was not found in thinned stands.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
There is great interest in determining the effects of forest thinning as a tool to improve growth recovery from drought in different tree species and climatic conditions. However, we lack a robust framework to determine how transient are post-drought growth resilience and enhancement, and if such growth improvement involves an uncoupling with climate conditions. We used regression analysis to determine differences in growth, sensitivity to drought and previous-year growth, and long-term growth in five plantations of three pine species (Pinus halepensis Mill., Pinus nigra Arn. and Pinus sylvestris L.) under different thinning intensities. Then, we simulated post-drought and post-thinning growth trajectories based on fitted models, and we computed drought resistance, resilience and recovery indices based on these trajectories. Moreover, the simulation allowed us to calculate the time to recovery after a drought. Using this analytical framework, we found that thinning enhanced radial growth (between 85 and 150%, significant in all sites with p < 0.05), and reduced previous-year growth dependence (between −13 and − 26%, significant in two out of five sites) and climatic dependence of growth (−23 to −49%, significant in two sites). We interpret these effects as a result of competition reduction by thinning and a transitory alleviation of growth climatic constraints. Thinning consistently improved drought resistance (+4–20%) and resilience (+1–4%). Recovery, on the contrary, was reduced (−1 to −15%). Since the growth loss during the drought was reduced due to higher drought resistance, the recovery was proportionally lower. Thinning reduced the time to recovery by one to two years. The thinning legacy effect persisted up to 15 to 20 years after thinning. Taken together, these findings enhance the benefits of adaptive silviculture in making pine plantations less vulnerable to unfavourable extreme climate events such as droughts. We present a novel and robust analytical framework to assess drought-thinning interactive effects on tree growth.
Drought-induced tree decline and accelerated mortality under global change threaten forests worldwide. Identifying environmental and structural factors driving tree mortality of species with different ecology is essential to understand forest dynamics. We compared three coniferous species and trees with different health status (i.e. healthy, declining and dead) on a xeric Mediterranean ecotone where Pinus pinaster shows widespread decline (defoliation, mistletoe, high mortality) while more drought-tolerant Pinus pinea and Juniperus oxycedrus do not. We analysed individual tree and stand characteristics and regeneration of all species to understand how different abiotic factors at different time scales affected stand species dynamics in relation to tree decline and mortality. Only dead and declining pine trees showed recent multiannual negative radial growth trends. The growth decline started after two extreme droughts, being 10 years longer in the declining species, P. pinaster, than in non-declining P. pinea. We observed no signs of decline or mortality in the most drought-tolerant species, J. oxycedrus. Variables related to moisture availability, like stand dominant height and the tree response to precipitation, increased survival probability in P. pinaster but not in P. pinea. Despite P. pinaster dominating the canopy, its regeneration was very scarce and less abundant than that of co-occurring more drought-tolerant species. Our results suggest that drought-induced mortality was related to species-specific sensitivity to water stress and may be inducing compositional changes towards dominance of more drought-tolerant species. Consequently, at its lower elevation limit P. pinaster might be able to persist only in microsites with higher moisture availability.
Carbon pricing can play an essential role in promoting adaptive silviculture (including thinning) in Mediterranean pine forests. Commercial thinning can improve carbon sequestration and stocks, thereby contributing to climate change. In this paper, we present a mixed-integer programming model that optimizes the thinning schedule of stands and maximizes Net Present Value (NPV) over a 10-year planning period. In addition to harvesting and transport costs, the model included revenue from two sources: sawlogs and biomass extracted during the thinning, and credits from carbon stocks at the end of the planning period. The study area comprised 631 pine stands located in Andalusia, southern Spain, totaling a forest area of nearly 30,000 ha. Airborne Lidar data and allometric biomass models were used to estimate above ground forest biomass, including commercial timber, and carbon stocks. The analysis focused on the impact of carbon prices on NPV, optimal thinning schedules, flows of sawlogs and biomass products to customers, and carbon stocks at the end of the planning period. Increased NPV values ranging between €4.8 M and €24.9 M were obtained in scenarios that included a price for carbon stocks in addition to revenue from sawlogs and biomass. Carbon prices also impacted the flow of sawlogs and biomass delivered to end customers. On average, in scenarios with carbon prices, the flow of sawlogs increased from about 29,700 tonnes in year 1 to about 38,200 tonnes in year 10. These results reveal that thinning schedules and NPV are very sensitive to carbon prices and that the best economic returns are obtained when carbon stocks are added to sawlogs and biomass as a source of revenue.
In Mediterranean pine plantations forest dieback and tree mortality are not only related to increased drought, but also to a lack of management, which intensifies inter-tree competition for available soil water. In this complex context simple but also difficult questions such as why, how and when manage forests should be directly responded and quantified by applied science. In this study we specifically analysed the forest-water relationships of an Aleppo pine plantation where experimental thinning was carried out ten years ago at three different intensities (H: high-, M: moderate- and L: low-thinning plots plus a control one, C). To this end, we again measured tree sap flow, soil water content and meteorological conditions. In addition, the relative importance (RI) of thinning intensity and environmental drivers when explaining tree/stand-water at the short-term were compared with those obtained in this study in order to elucidate how the role of thinning intensity may change on time. The impact of thinning on soil water content showed that significant differences were maintained after ten years (H > M > L > C), but that values between the different thinning intensities were closer than those observed at the short-term. In contrast, tree transpiration from the high-thinning plot was very similar to that from the moderate-thinning one (means of 13 and 14.7 l·day−1, respectively). These results support the idea that an excessive forest opening makes the understorey compete more strongly for water, thus counterbalancing the higher tree transpiration observed in the short-term. The combined analyses of thinning intensity and environmental drivers highlight how the role of thinning intensity in controlling tree and stand transpiration in the short-term was clearly replaced by soil water availability ten years after the thinning intervention (RI means from 13.1 to 39.5% for soil water availability and from 26.8 to 19.0% for thinning intensity). Our results support the need to study how the transpiration-soil water relationships progressively change over the distance in time from thinning in order to assess the impact of understorey properly and thus systematically calculate the ecohydrological turnover at every thinning intensity tested.
Plantations of Mediterranean pines will face warmer and drier climate conditions during the 21st century. However, we lack field assessments of how severe droughts could threaten the viability of many of these plantations, often located at xeric sites and lacking adaptive management measures such as thinning. In this work, we related radial-growth variability to recent defoliation, to evaluate the resilience and vulnerability of Maritime pine (Pinus pinaster) plantations from xeric sites in south-eastern Spain to three recent droughts (1995, 2005, and 2012). Tree diameter and growth resistance after the most severe drought, which occurred in 1995, were positively related to growth. In addition, the current defoliation level, a proxy for tree vigor, was related to growth, decreasing as tree-to-tree competition increased. These findings indicate that increasing drought will reduce growth and increase the needle loss and mortality rate across many dense pine plantations in the Mediterranean Basin, suggesting that they are more prone to die. Thus, limited resilience of plantations may explain their higher defoliation and indicate that future severe droughts could lead to widespread dieback and high mortality at marginal sites. Therefore, if effective management is not implemented, warmer and drier conditions will likely threaten the viability of many of these drought-prone plantations and their ability to deliver key ecosystem services.
In Mediterranean areas where drought-induced forest dieback and tree mortality have been widely reported, it is still under debate how the likely risks of climate change will affect tree growth and consequently forest productivity. Increasing tree mortality has been associated not only to increased drought, but also to a lack of management in many dense pine forests and plantations, where warming may intensify tree-to-tree competition for soil water. This emphasizes the need of using silviculture to adapt dense stands of Mediterranean pine reforestations to warmer and drier conditions. Here we combined dendrochronology and C and O isotope analyses of wood in two Aleppo pine (Pinus halepensis) plantations, growing under semiarid conditions and experimentally thinned at high and moderate intensities along with control. The main aim was to understand the responses of radial growth and water use efficiency (WUEi) to different thinning intensities, and to analyze the effectiveness of thinning to enhance post-drought growth resilience. Thinning had a positive effect on growth, produced an increase of δ¹⁸O, reduced growth sensitivity to drought and decreased WUEi, suggesting a reduction of drought stress. These results were consistent across sites, and were significant even 20 years after the intervention took place. Considering the climate effects on growth through the SPEI drought index to calculate resistance and recovery indices, an increase of resistance after thinning was observed. We conclude that high thinning intensity (50% of basal area removed) is a useful silviculture intervention on Mediterranean Aleppo pine plantations that enhances their growth, and makes them less dependent on harsh climatic conditions, improving their resilience against drought and consequently making them better adapted to more unfavourable conditions.
Previous research has provided insights into the potential response of growth and water use efficiency to thinning in Mediterranean forests, but little is known about the potential benefits of silviculture for plantations under severe drought stress. We selected two stands of Pinus sylvestris L. and P. nigra Arnold., and used dendrochronology and carbon isotopes (δ¹³C) to understand the growth and functional responses of high-density planted pine forests to thinning in drought-prone areas. Resistance, recovery, and resilience indices were calculated for each species using BAI data. We expected heavy thinning to produce stands that were more resilient to drought, particularly for the more drought-tolerant P. nigra. Differences in the basal area increments (BAI) and intrinsic water-use efficiency (iWUE) were found between the unthinned stand and the thinned stands, for both species. After thinning, BAI decreased as iWUE increased but this relationship was only marginally significant in P. sylvestris. Thinning increased growth resistance, recovery, and resilience following the severe drought of 2012. Our findings suggest that water shortage, linked to recurrent droughts, together with high tree competition, negatively affected tree growth and increased iWUE, which explains the dieback of some of these pine plantations. We evidence the vulnerability of densely planted Mediterranean pines to the forecasted warmer and drier conditions. These results show that a heavy thinning treatment (60% of basal area removed) provides a promising silvicultural framework for the adaptation of these drought-sensitive Mediterranean mountain pine forests to the potential risks of climate change.
Climate change can impair ecosystem functions and services in extensive dry forests worldwide. However, attribution of climate change impacts on tree growth and forest productivity is challenging due to multiple inter-annual patterns of climatic variability associated with atmospheric and oceanic circulations. Moreover, growth responses to rising atmospheric CO2, namely carbon fertilization, as well as size ontogenetic changes can obscure the climate change signature as well. Here we apply Structural Equation Models (SEM) to investigate the relative role of climate change on tree growth in an extreme Mediterranean environment (i.e., extreme in terms of the combination of sandy-unconsolidated soils and climatic aridity). Specifically, we analyzed potential direct and indirect pathways by which different sources of climatic variability (i.e warming and precipitation trends, the North Atlantic Oscillation, [NAO]; the Mediterranean Oscillation, [MOI]; the Atlantic Mediterranean Oscillation, [AMO]) affect aridity through their control on local climate (in terms of mean annual temperature and total annual precipitation), and subsequently tree productivity, in terms of basal area increments (BAI). Our results support the predominant role of Diameter at Breast Height (DHB) as the main growth driver. In terms of climate, NAO and AMO are the most important drivers of tree growth through their control of aridity (via effects of precipitation and temperature, respectively). Furthermore and contrary to current expectations, our findings also support a net positive role of climate warming on growth over the last 50 years and suggest that impacts of climate warming should be evaluated considering multi-annual and multi-decadal periods of local climate defined by atmospheric and oceanic circulation in the North Atlantic.
Forecasted increase drought frequency and severity may drive worldwide declines in forest productivity. Species-level responses to a drier world are likely to be influenced by their functional traits. Here, we analyse forest resilience to drought using an extensive network of tree-ring width data and satellite imagery. We compiled proxies of forest growth and productivity (TRWi, absolutely dated ring-width indices; NDVI, Normalized Difference Vegetation Index) for 11 tree species and 502 forests in Spain corresponding to Mediterranean, temperate, and continental biomes. Four different components of forest resilience to drought were calculated based on TRWi and NDVI data before, during, and after four major droughts (1986, 1994–1995, 1999, and 2005), and pointed out that TRWi data were more sensitive metrics of forest resilience to drought than NDVI data. Resilience was related to both drought severity and forest composition. Evergreen gymnosperms dominating semi-arid Mediterranean forests showed the lowest resistance to drought, but higher recovery than deciduous angiosperms dominating humid temperate forests. Moreover, semi-arid gymnosperm forests presented a negative temporal trend in the resistance to drought, but this pattern was absent in continental and temperate forests. Although gymnosperms in dry Mediterranean forests showed a faster recovery after drought, their recovery potential could be constrained if droughts become more frequent. Conversely, angiosperms and gymnosperms inhabiting temperate and continental sites might have problems to recover after more intense droughts since they resist drought but are less able to recover afterwards.
Drought events are increasing globally, and reports of consequent forest mortality are widespread. However, due to a lack of a quantitative global synthesis, it is still not clear whether drought-induced mortality rates differ among global biomes and whether functional traits influence the risk of drought-induced mortality. To address these uncertainties, we performed a global meta-analysis of 58 studies of drought-induced forest mortality. Mortality rates were modelled as a function of drought, temperature, biomes, phylogenetic and functional groups and functional traits. We identified a consistent global-scale response, where mortality increased with drought severity [log mortality (trees trees(-1) year(-1) ) increased 0.46 (95% CI = 0.2-0.7) with one SPEI unit drought intensity]. We found no significant differences in the magnitude of the response depending on forest biomes or between angiosperms and gymnosperms or evergreen and deciduous tree species. Functional traits explained some of the variation in drought responses between species (i.e. increased from 30 to 37% when wood density and specific leaf area were included). Tree species with denser wood and lower specific leaf area showed lower mortality responses. Our results illustrate the value of functional traits for understanding patterns of drought-induced tree mortality and suggest that mortality could become increasingly widespread in the future.
The study tested the hypothesis that future changes in the composition of tree communities, as predicted by species distribution models, could already be apparent in the current regeneration patterns of three pine species (Pinus pinaster, P. nigra and P. sylvestris)inhabiting the central-eastern mountains of Spain. We carried out both an observational study and a seed-sowing experiment to analyze, along an altitudinal and latitudinal gradient, whether recent recruitment patterns indicate an expansion of P. pinaster forests to the detriment of P. nigra ones in the low-altitude southern sites of these mountains; or whether P. sylvestris is being replaced by P. nigra in the high-altitude sites from the same area. The observational study gathered data from 561 plots of the Spanish National Forest Inventory. The seed-sowing experiment tested the effects of irrigation and stand basal area on seedling emergence and survival. Data were analyzed by means of Generalized Linear Models and Generalized Linear Mixed Models. Regeneration of the three pine species responded similarly to the explicative factors studied, but the density of tree seedlings and saplings exhibited a wide spatial heterogeneity. This result suggested that a mosaic of site- and species-specific responses to climate change might mislead model projections on the future forest occupancy of tree species. Yet, we found no indications of neither an expansion nor a contraction of the near future forest occupancy of the tree species studied.
http://authors.elsevier.com/a/1TjEU1L~Gw8~wT
Increasing frequency of extremely dry and hot summers in some regions emphasise the need for silvicultural
approaches to increase the drought tolerance of existing forests in the short term, before long-term
adaptation through species changes may be possible. The aim of this meta-analysis was to assess the
potential of thinning for improving tree performance during and after drought. We used results from
23 experiments that employed different thinning intensities including an unthinned control and focused
on the response variables: radial growth, carbon- and oxygen-isotopes in tree-rings and pre-dawn leafwater
potential. We found that thinning effects on the growth response to drought differed between
broadleaves and conifers, although these findings are based on few studies only in broadleaved forests.
Thinning helped to mitigate growth reductions during drought in broadleaves, most likely via increases
of soil water availability. In contrast, in conifers, comparable drought-related growth reductions and
increases of water-use efficiency were observed in all treatments but thinning improved the postdrought
recovery and resilience of radial growth. Results of meta-regression analysis indicate that benefits
of both moderate and heavy thinning for growth performance following drought (recovery and resilience)
decrease with time since the last intervention. Further, growth resistance during drought became
smaller with stand age while the rate of growth recovery following drought increased over time irrespective
of treatment. Heavy but not moderate thinning helped to avoid an age-related decline in mediumterm
growth resilience to drought. For both closed and very open stands, growth performance during
drought improved with increasing site aridity but for the same stands growth recovery and resilience following
drought was reduced with increasing site aridity. This synthesis of experiments from a wide geographical
range has demonstrated that thinning, in particular heavy thinning, is a suitable approach to
improve the growth response of remaining trees to drought in both conifers and broadleaves but the
underlying processes differ and need to be considered.
Tools for performing model selection and model averaging. Automated model selection through subsetting the maximum model, with optional constraints for model inclusion. Model parameter and prediction averaging based on model weights derived from information criteria (AICc and alike) or custom model weighting schemes.
[Please do not request the full text - it is an R package. The up-to-date manual is available from CRAN].
In water-limited regions, adaptive management of forest and water relationships has been put forward, to implement hydrology-oriented silviculture to reduce stand evapotranspiration and, at the tree level, to improve growth and water use efficiency (WUE). The main goal of this study was to evaluate the effect of thinning in the short and medium term on tree growth, climate (drought) sensitivity, WUE performed using growth and sap flow measurements and WUEi performed using δ13C and δ18O isotopes, in a typical semiarid forest. This approach also evaluated the reliability of isotopes as indicators of the effects of adaptive forest management. A stagnated Aleppo pine plantation was experimentally thinned at high intensity (H98) in 1998 and at High (H), Medium (M) and Low (L) intensities in 2008, along with a control (C). Substantial limitation of tree growth was observed in C. Thinning not only increased growth, but also changed the tree growth-precipitation relationships, with C trees depending more on precipitation than thinned trees did. WUEi after thinning was significantly affected only in the medium term, with C trees being more efficient (94.4 μmolCO2/molH2O) than H98 trees (88.7), especially in dry spells (100.7). WUEi was found to increase when precipitation decreased, regardless of the treatment. However, WUE increased sharply from C (1.26g biomass/L H2O) to H (3.20g/L), showing a clear difference with WUEi observed in the same years. Thinning caused an increase in δ18O in the short term, but no relationship was found between δ18O and tree water use. It can be concluded that forest management improved WUE in spite of higher tree transpiration, but WUEi remained unchanged, probably due to an underestimate of photosynthetic capacity. The dual isotope (δ13C and δ18O) conceptual model was not consistent with our experimental data. Thus, the question of whether stable isotopes can be used as a tool for addressing the ecophysiological impacts of thinning remains open.
To assess if tree age may modulate the main climatic drivers of radial growth, two relict Pinus
nigra subsp. salzmannii populations (Marı´a, most xeric site; Ma´ gina, least xeric site) were sampled in
southern Spain near the limits of the species range. Tree-ring width residual chronologies for two age
groups (mature trees, age # 100 years (minimum 40 years); old trees, age . 100 years) were built to
evaluate their responses to climate by relating them to monthly precipitation and temperature and a
drought index (DRI) using correlation and response functions. We found that drought is the main
driver of growth of relict P. nigra populations, but differences between sites and age classes were also
observed. First, growth in the most xeric site depends on the drought severity during the previous
autumn and the spring of the year of tree-ring formation, whereas in the relatively more mesic site
growth is mainly enhanced by warm and wet conditions in spring. Second, growth of mature trees
responded more to drought severity than that of old trees. Our findings indicate that drought severity
will mainly affect growth of relict P. nigra populations dominated by mature trees in xeric sites. This
conclusion may also apply to similar mountain Mediterranean conifer relicts.
Hydrology-oriented silviculture might adapt Mediterranean forests to climatic changes, although its implementation demands a better understanding and quantification on the water fluxes. The influence of thinning intensity (high, medium, low and a control) and its effect on the mid-term (thinned plots in 1998 and 2008) on the water cycle (transpiration, soil water and interception) and growth [basal area increment (BAI)] were investigated in 55-year-old Aleppo pine trees. Thinning enhanced a lower dependence of growth on climate fluctuations. The high-intensity treatment showed significant increases in the mean annual BAI (from 4.1 to 17.3 cm2) that was maintained in the mid-term. Thinning intensity progressively increased the sap flow velocity (v
s) in all cases with respect to the control. In the mid-term, an increased functionality of the inner sapwood was also observed. Mean daily tree water use ranged from 5 (control) to 18 (high intensity) l tree−1. However, when expressed on an area basis, daily transpiration ranged from 0.18 (medium) to 0.30 mm (control), meaning that in spite of the higher transpiration rates in the remaining trees, stand transpiration was reduced with thinning. Deep infiltration of water was also enhanced with thinning (about 30 % of rainfall) and did not compete with transpiration, as both presented opposite seasonal patterns. The changes in the stand water relationships after 10 years were well explained by the forest cover metric. The blue to green water ratio changed from 0.15 in the control to 0.72 in the high-intensity treatment, with the remaining treatments in the 0.34–0.48 range.