Wood anatomy and carbon-isotope discrimination support long-term hydraulic deterioration as a major cause of drought-induced dieback

Earlywood vessel characteristics are early indicators of drought-induced decline in ring-porous oak species within the Mediterranean Basin

Heat and drought stress have triggered forest dieback episodes worldwide, affecting oak forests, particularly in hotspots of climate change such as the Mediterranean Basin. However, forecasting dieback is not straightforward. In this study, we used the earlywood anatomy to improve dieback forecasts in five oak species characterized by different drought sensitivity (i.e. from high to low Quercus robur, Q. cerris, Q. frainetto and Q. canariensis, Q. humilis, Q. pubescens) across Italy and Spain. We measured radial growth, expressed as basal area increment (BAI), earlywood hydraulic diameter (Dh) and vessel area of coexisting non-declining (ND) and declining (D) trees in each stand. Then, we calculated the product between the coefficient of variation (CV) of vessel area and a spatial aggregation index (AI). High CV × AI values indicate regularly spaced vessels with variable area of vessels, while low values correspond to clustered vessels with similar area. ND trees showed higher BAI values than D trees from 10 to 40 years before the dieback onset, when ND trees grew 20-50 % more than the D trees. We observed a decline in the vessel area CV several decades prior to dieback in D trees, with the exception of Q. cerris. The AI showed higher values in ND than in D trees. Consequently, the CV × AI product was consistently higher in ND than in D trees. The CV × AI divergence between ND and D trees was pronounced in the wettest sites, specifically for Q. robur and Q. humilis. Time series of CV × AI effectively differentiated trees based on their vigor. Wood anatomy variables could be used to enhance predictions of vulnerability to drought-induced dieback. This study can help identify vulnerable trees before the onset of dieback symptoms, serving as a tool to support the management of forests prone to drought.

Loss of Stomatal Regulation Sensitivity to CO2 and Reduced Xylem Hydraulic Conductivity Contribute to Long-Term Tree Decline and Mortality

Increasing aridity is a major threat to forests worldwide. Understanding tree functional constraints under drought and their impacts on resilience and mortality among species is crucial to assess the impacts of global change on forests. We analyzed the long-term drought and atmospheric CO2 responses in three Mediterranean co-occurring species with differing drought tolerances (Pinus pinaster < Pinus pinea < Juniperus oxycedrus). In this mixed forest, P. pinaster exhibited widespread mortality and mistletoe infection, P. pinea showed scattered mortality, and J. oxycedrus showed no decline. Using tree-ring data (1978-2016), we compared intrinsic water-use efficiency (iWUE) and xylem hydraulic traits in healthy and non-healthy individuals of both pine species and healthy junipers. Healthy P. pinaster trees produced a more hydraulically efficient xylem, with wider lumen tracheids, than non-healthy trees, whereas P. pinea showed no anatomical differences between health statuses. Healthy P. pinaster displayed greater anatomical plasticity, adjusting hydraulic conductivity and cell-wall thickness to water availability. Despite small differences in average iWUE, the response of iWUE to rising CO2 and drought differed between species and health statuses. J. oxycedrus and P. pinea showed steady iWUE increases, but P. pinea experienced periods of stagnation following an extreme drought, later recovering regardless of health status. In contrast, iWUE in P. pinaster plateaued for over 20 years after a decline-inducing drought, particularly in non-healthy, mistletoe-infected trees. Differences in iWUE response to CO2 and anatomical plasticity to drought may explain the contrasting mortality patterns among these coniferous species. Our results suggest a long-term decline spiral in P. pinaster induced by low hydraulic efficiency in drought-induced defoliated trees and limited physiological responses to rising CO2 and drought. Increasing drought stress makes pine recovery increasingly unlikely.

Tree post-drought recovery: scenarios, regulatory mechanisms and ways to improve

Efficient post-drought recovery of growth and assimilation enables a plant to return to its undisturbed state and functioning. Unlike annual plants, trees suffer not only from the current drought, but also from cumulative impacts of consecutive water stresses which cause adverse legacy effects on survival and performance. This review provides an integrated assessment of ecological, physiological and molecular evidence on the recovery of growth and photosynthesis in trees, with a view to informing the breeding of trees with a better ability to recover from water stress. Suppression of recovery processes can result not only from stress damage but also from a controlled downshift of recovery as part of tree acclimation to water-limited conditions. In the latter case, recovery processes could potentially be activated by turning off the controlling mechanisms, but several obstacles make this unlikely. Tree phenology, and specifically photoperiodic constraints, can limit post-drought recovery of growth and photosynthesis, and targeting these constraints may represent a promising way to breed trees with an enhanced ability to recover post-drought. The mechanisms of photoperiod-dependent regulation of shoot, secondary and root growth and of assimilation processes are reviewed. Finally, the limitations and trade-offs of altering the photoperiodic regulation of growth and assimilation processes are discussed.

Seabuckthorn (Hippophae rhamnoides L.) plantation degradation aggravates microbial metabolic C and P limitations on the Northern Loess Plateau in China

Vegetation degradation in arid and semi-arid regions reduces plant C inputs to the soil, which can impede soil nutrient cycling because of the limited C source for microbial metabolism. However, whether vegetation degradation aggravates microbial nutrient limitation in degraded ecosystems in arid and semi-arid regions is not fully understood. Here, we investigated changes in soil enzyme activity and microbial nutrient limitation along a well-documented gradient of degraded seabuckthorn (Hippophae rhamnoides L.) (slightly degraded, canopy dieback <25 %, moderately degraded, canopy dieback 25 %-75 %, and severely degraded, canopy dieback >75 %) in Liang (long ridge) and gully channel locations in the Pisha Sandstone region of the Loess Plateau, China. We found that as the magnitude of seabuckthorn degradation increased, activities of C-acquiring enzymes and ratios of C:N and C:P enzymes (0.54-0.80 and 0.52-0.77, respectively) increased whereas the N:P enzyme ratio (0.93-0.99) decreased. Stoichiometric modelling further indicated that microorganisms were limited by soil C and P (vector angle >45°) in the seabuckthorn plantation region, and the degradation of seabuckthorn plantation aggravated microbial C and P limitations. Partial least squares path modelling revealed that seabuckthorn degradation (canopy dieback) was the main factor explaining microbial C limitation variations, while soil physicochemical properties (pH and soil moisture content) and understory plant parameters (litter biomass) were the major factors underlying microbial P limitation of long ridge and gully channel formations, respectively. Our findings highlight synergistic changes between aboveground and belowground processes, suggesting an unexpected negative effect of vegetation degradation on soil microbial community and nutrient cycling. These insights offer a direction for the development of plantation nutrients management strategies in semi-arid and arid areas.

Contrasting physiological strategies explain heterogeneous responses to severe drought conditions within local populations of a widespread conifer

Understanding how trees prioritize carbon gain at the cost of drought vulnerability under severe drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of severe drought in 29 mature white spruce (Picea glauca [Moench] Voss) families grown in a common garden trial. During the drought episode, the majority of families displayed decreased growth and exhibited either sustained or increased intrinsic water-use efficiency (iWUE), which was largely influenced by reduced stomatal conductance as revealed by the dual carbon‑oxygen isotope approach. Different water-use strategies were detected within white spruce populations in response to drought conditions. Our results revealed intraspecific variation in the prevailing physiological mechanisms underlying drought response within and among populations of Picea glauca. The presence of different genetic groups reflecting diverse water-use strategies within this largely-distributed conifer is likely to lessen the negative effects of drought and decrease the overall forest ecosystems' sensitivity to it.

Higher latewood to earlywood ratio increases resistance of radial growth to severe droughts in larch

As drought has caused great losses of tree growth across the world, the mechanism of how trees adapt to drought has been extensively investigated. However, how trees change their late- to earlywood ratio (LER) to adapt to severe drought events remains poorly understood. We used a network of Larix principis-rupprechtii earlywood and latewood width data from 1979 to 2018, covering most of the distribution of planted larch across North China, to investigate how latewood proportion affected trees' resistance to drought. The interactions among LER, minimum temperature, vapor pressure deficit (VPD), growing season length, and their contributions to drought resistant (Rt) were estimated using structural equation models. The results show a significant increase in LER of the juvenile wood throughout the first 15 growth rings after which it stabilizes. The LER decreased significantly with elevation for the juvenile wood. March-May temperature and VPD were the main determinant in the LER of mature wood. The sensitivity of radial growth to droughts was positively changed with LER when LER was below 0.50, but negatively changed with LER when LER is above 0.50. We confirmed that high LER increases resistance of tree growth to severe droughts in L. principis-rupprechtii. Our results highlight that a higher proportion of latewood is formed in dry years, and this high drought sensitivity of LER in turn led to an increased resistance to drought. This combination of reduced radial growth during dry years, while the latewood proportion remains increases maybe an adaptive strategy of larch trees to cope with severe droughts.

Climate response of a glacial relict conifer across its distribution range is invariant in space but not in time

Climate change impacts on forest trees will be particularly severe for relict species endemic to the subalpine forest, such as Pinus cembra in the Alps and Carpathians. Most current knowledge about the response of this species to climate comes from tree-ring width analysis. However, this approach cannot perform in-depth and highly time-resolved analysis on the climate influence on specific growth processes and xylem functions. We analyzed xylem anatomical traits from six sites covering most of the longitudinal range of this species. Associations between climate and cell number, lumen area and cell wall thickness were computed for the 1920-2010 period using climate records aligned to degree-day temperature sum thresholds. The anatomical chronologies were clearly distinct between the Alps and Carpathians. However, climate responses were similar for all sites, suggesting common species-specific response mechanisms. Temperature showed a positive correlation with both cell number and cell wall thickness. Cell lumen size exhibited an early positive association, followed by strong negative association with temperature and a positive one with precipitation. This highlights that the cell enlargement process was negatively related to high temperature at high elevation, where meristematic processes are rather supposed to be constrained by low temperatures. Therefore, long-term climate warming can have negative consequences on the xylem potential to transport water at all investigated sites. Moreover, in the last 30 years, we observed a slight anticipation of some responses and a decrease in climate sensitivity of some xylem parameters. Our findings provide evidence of temporally unstable but spatially consistent climate response of Pinus cembra from the Alps to the Carpathians. The low diversity in xylem phenotypic responses to climate suggests that future warming could extensively and evenly affect the species throughout its entire distribution.

The admixture of Quercus sp. in Pinus sylvestris stands influences wood anatomical trait responses to climatic variability and drought events

Introduction

Forests are threatened by increasingly severe and more frequent drought events worldwide. Mono-specific forests, developed as a consequence of widespread management practices established early last century, seem particularly susceptible to global warming and drought compared with mixed-species forests. Although, in several contexts, mixed-species forests display higher species diversity, higher productivity, and higher resilience, previous studies highlighted contrasting findings, with not only many positive but also neutral or negative effects on tree performance that could be related to tree species diversity. Processes underlying this relationship need to be investigated. Wood anatomical traits are informative proxies of tree functioning, and they can potentially provide novel long-term insights in this regard. However, wood anatomical traits are critically understudied in such a context. Here, we assess the role of tree admixture on Pinus sylvestris L. xylem traits such as mean hydraulic diameter, cell wall thickness, and anatomical wood density, and we test the variability of these traits in response to climatic parameters such as temperature, precipitation, and drought event frequency and intensity.

Methods

Three monocultural plots of P. sylvestris and three mixed-stand plots of P. sylvestris and Quercus sp. were identified in Poland and Spain, representing Continental and Mediterranean climate types, respectively. In each plot, we analyzed xylem traits from three P. sylvestris trees, for a total of nine trees in monocultures and nine in mixed stands per study location.

Results

The results highlighted that anatomical wood density was one of the most sensitive traits to detect tree responses to climatic conditions and drought under different climate and forest types. Inter-specific facilitation mechanisms were detected in the admixture between P. sylvestris and Quercus sp., especially during the early growing season and during stressful events such as spring droughts, although they had negligible effects in the late growing season.

Discussion

Our findings suggest that the admixture between P. sylvestris and Quercus sp. increases the resilience of P. sylvestris to extreme droughts. In a global warming scenario, this admixture could represent a useful adaptive management option.

Optimal balancing of xylem efficiency and safety explains plant vulnerability to drought

In vast areas of the world, forests and vegetation are water limited and plant survival depends on the ability to avoid catastrophic hydraulic failure. Therefore, it is remarkable that plants take hydraulic risks by operating at water potentials (ψ) that induce partial failure of the water conduits (xylem). Here we present an eco-evolutionary optimality principle for xylem conduit design that explains this phenomenon based on the hypothesis that conductive efficiency and safety are optimally co-adapted to the environment. The model explains the relationship between the tolerance to negative water potential (ψ50 ) and the environmentally dependent minimum ψ (ψmin ) across a large number of species, and along the xylem pathway within individuals of two species studied. The wider hydraulic safety margin in gymnosperms compared to angiosperms can be explained as an adaptation to a higher susceptibility to accumulation of embolism. The model provides a novel optimality-based perspective on the relationship between xylem safety and efficiency.

The effects of intrinsic water-use efficiency and climate on wood anatomy

Climate warming may induce growth decline in warm-temperate areas subjected to seasonal soil moisture deficit, whereas increasing atmospheric CO₂ concentration (C_a) is expected to enhance tree growth. An accurate understanding of tree growth and physiological processes responding to climate warming and increasing C_a is critical. Here, we analyzed tree-ring stable carbon isotope and wood anatomical traits of Pinus tabuliformis from Qinling Mountains in China to understand how lumen diameter (LD) determining potential hydraulic conductivity and cell-wall thickness (CWT) determining carbon storage responded to climate and C_a. The effects of climate and C_a on intrinsic water-use efficiency (iWUE) were isolated, and iWUE values due to only-climate (iWUE_Clim) and only-CO₂ effects (iWUE_CO2) were obtained. During a low-iWUE period, the influences of climate on earlywood (EW) LD and latewood (LW) CWT prevailed. During a high-iWUE period, CO₂ fertilization promoted cell enlargement and carbon storage but this was counteracted by a negative influence of climate warming. The limiting direct effects of iWUE_Clim and indirect effects of climate on EW LD were greater than on LW CWT. P. tabuliformis in temperate forests will face a decline of growth and carbon fixation, but will produce embolism-resistant tracheids with narrow lumen responding to future hotter droughts.

Blue is the fashion in Mediterranean pines: New drought signals from tree-ring density in southern Europe

Long-term records of tree-ring width (TRW), latewood maximum density (MXD) and blue intensity (BI) measurements on conifers have been largely used to develop high-resolution temperature reconstructions in cool temperate forests. However, the potential of latewood blue intensity (LWBI), less commonly used earlywood blue intensity (EWBI), and delta (difference between EWBI and LWBI, dBI) blue intensity in Mediterranean tree species is still unexplored. Here we developed BI chronologies in moist-elevation limits of the most southwestern European distribution of Pinus nigra subsp. salzmanii Arnold. We tested whether BI variables derived from tree rings of black pine are better proxies than ring-width variables to reconstruct long-term changes in climatic factors and water availability. For this we applied correlations and regression analyses with daily and monthly climate data, a spatial and temporal drought index (Standardized Precipitation-Evapotranspiration Index-SPEI) and Vapour Pressure Deficit (VPD), as well as atmospheric circulation patterns: North Atlantic Oscillation (NAO), Southern Oscillation Index (SOI) and Western Mediterranean Oscillation (WeMO). We found a positive relation between black pine growth (RW) and temperature during the winter preceding the growing season. Among all variables LWBI and dBI were found to be more sensitive than TRW to SPEI at low-elevation site, with EWBI series containing an opposite climatic signal. LWBI and dBI were significantly related to June and September precipitation at high-elevation site. Winter VPD was related with higher EWI and LWI series, whereas dBI and EWBI were related with January SOI and February NAO. We confirm the potential of long-term dBI series to reconstruct climate in drought-prone regions. This novel study in combination with other wood anatomical measurements has wide implications for further use of BI to understand and reconstruct environmental changes in Mediterranean conifer forests.

The Interplay of the Tree and Stand-Level Processes Mediate Drought-Induced Forest Dieback: Evidence from Complementary Remote Sensing and Tree-Ring Approaches

Drought-induced forest dieback can lead to a tipping point in community dominance, but the coupled response at the tree and stand-level response has not been properly addressed. New spatially and temporally integrated monitoring approaches that target different biological organization levels are needed. Here, we compared the temporal responses of dendrochronological and spectral indices from 1984 to 2020 at both tree and stand levels, respectively, of a drought-prone Mediterranean Pinus pinea forest currently suffering strong dieback. We test the influence of climate on temporal patterns of tree radial growth, greenness and wetness spectral indices; and we address the influence of major drought episodes on resilience metrics. Tree-ring data and spectral indices followed different spatio-temporal patterns over the study period (1984–2020). Combined information from tree growth and spectral trajectories suggests that a reduction in tree density during the mid-1990s could have promoted tree growth and reduced dieback risk. Additionally, over the last decade, extreme and recurrent droughts have resulted in crown defoliation greater than 40% in most plots since 2019. We found that tree growth and the greenness spectral index were positively related to annual precipitation, while the wetness index was positively related to mean annual temperature. The response to drought, however, was stronger for tree growth than for spectral indices. Our study demonstrates the value of long-term retrospective multiscale analyses including tree and stand-level scales to disentangle mechanisms triggering and driving forest dieback.

Aridity and cold temperatures drive divergent adjustments of European beech xylem anatomy, hydraulics and leaf physiological traits

Understanding plant trait coordination and variance across climatic gradients is critical for assessing forests' adaptive potential to climate change. We measured 11 hydraulic, anatomical and leaf-level physiological traits in European beech (Fagus sylvatica L.) along a moisture and temperature gradient in the French Alps. We assessed how traits covaried, and how their population-level variances shifted along the gradient. The intrapopulation variances of vessel size and xylem-specific conductivity reduced in colder locations as narrow vessels were observed in response to low temperature. This decreased individual-level water transport capacity compared with the warmer and more xeric sites. Conversely, the maximum stomatal conductance and Huber value variances were constrained in the arid and warm locations, where trees showed restricted gas exchange and higher xylem-specific conductivity. The populations growing under drier and warmer conditions presented wide variance for the xylem anatomical and hydraulic traits. Our results suggest that short-term physiological acclimation to raising aridity and heat in southern beech populations may occur mainly at the leaf level. Furthermore, the wide variance of the xylem anatomical and hydraulic traits at these sites may be advantageous since more heterogeneous hydraulic conductivity could imply populations' greater tree-tree complementarity and resilience against climatic variability. Our study highlights that both intrapopulation trait variance and trait network analysis are key approaches for understanding species adaptation and the acclimation potential to a shifting environment.

No xylem phenotypic plasticity in mature Picea abies and Fagus sylvatica trees after 5 years of throughfall precipitation exclusion

Forest trees are experiencing increasing frequency and intensity of drought events with climate change. We investigated xylem and phloem traits from mature Fagus sylvatica and Picea abies trees after 5 years of complete exclusion of throughfall precipitation during the growing season. Xylem and phloem anatomy, leaf and branch biomass were analysed along top branches of ~1.5 m lenght in 5 throughfall precipitation excluded (TE) and 5 control (CO) trees of both beech and spruce. Xylem traits were analysed on wood cores extracted from the stem at breast height. In the top branches of both species, the lumen diameter (or area) of xylem and phloem conduits did not differ between TE and CO trees. At breast height, TE trees of both species produced narrower xylem rings and conduits. While allocation to branch (BM) and needle biomass (LM) did not change between TE and CO in P. abies, TE F. sylvatica trees allocated proportionally more biomass to leaves (LM) than BM compared with CO. Despite artificial drought increased the mortality in the TE plots, our results revealed no changes in both xylem and phloem anatomies, undermining the hypothesis that successful acclimation to drought would primarily involve increased resistance against air embolism.

Linking the growth patterns of coniferous species with their performance under climate aridization

Tree growth is highly sensitive to water deficit. At the same time, growth processes substantially influence tree performance under water stress by changing the root-absorbing surface, leaf-transpiring surface, amount of conducting xylem, etc. Drought-induced growth suppression is often higher in conifers than in broadleaf species. This review is devoted to the relations between the growth of coniferous plants and their performance under increasing climate aridization in the temperate and boreal zones of the Northern Hemisphere. For adult trees, available evidence suggests that increasing the frequency and severity of water deficit would be more detrimental to those plants that have higher growth in favorable conditions but decrease growth more prominently under water shortage, compared to trees whose growth is less sensitive to moisture availability. Not only the overall sensitivity of growth processes to water supply but also the asymmetry in response to lower-than-average and higher-than-average moisture conditions can be important for the performance of coniferous trees under upcoming adverse climate change. To fully understand the tree response under future climate change, the responses to both drier and wetter years need to be analyzed separately. In coniferous seedlings, more active growth is usually linked with better drought survival, although physiological reasons for such a link can be different. Growth stability under exacerbating summer water deficit in coniferous plants can be maintained by more active spring growth and/or by a bimodal growth pattern; each strategy has specific advantages and drawbacks. The optimal choice of growth strategy would be critical for future reforestation programs.

The role of nutritional impairment in carbon-water balance of silver fir drought-induced dieback

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ¹⁸ O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower δ¹⁸ O than ND trees. The uncoupling between iWUE and δ¹⁸ O, together with the positive correlations between P and K leaf concentrations and δ¹⁸ O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies.

Wood Anatomical Traits Respond to Climate but More Individualistically as Compared to Radial Growth: Analyze Trees, Not Means

Wood encodes environmental information that can be recovered through the study of tree-ring width and wood anatomical variables such as lumen area or cell-wall thickness. Anatomical variables often provide a stronger hydroclimate signal than tree-ring width, but they show a low tree-to-tree coherence. We investigate the sources of variation in tree-ring width, lumen area, and cell-wall thickness in three pine species inhabiting sites with contrasting climate conditions: Pinus lumholtzii in wet-summer northern Mexico, and Pinus halepensis and Pinus sylvestris in dry-summer north-eastern Spain. We quantified the amount of variance of these three variables explained by spring and summer water balance and how it varied among trees. Wood anatomical variables accounted for a larger inter-individual variability than tree-ring width data. Anatomical traits responded to hydroclimate more individualistically than tree-ring width. This individualistic response represents an important issue in long-term studies on wood anatomical characteristics. We emphasized the degree of variation among individuals of the same population, which has far-reaching implications for understanding tree species’ responses to climate change. Dendroclimatic and wood anatomical studies should focus on trees rather than on the mean population series.

Annual Carbon Sequestration Patterns in Trees: A Case Study from Scots Pine Monospecific Stands and Mixed Stands with Sessile Oak in Central Poland

The need to understand the carbon sequestration ability of trees under current and future climatic scenarios is fundamental to predict the role of forest in counterbalancing the global warming. In this study, we investigated the carbon sequestration ability of Pinus sylvestris L. in a setting of pure and mixed forests with Quercus petraea (Matt.) Liebl. in Central Poland. Beside the traditional growth measures, i.e., Ring Width, Basal Area Increment, and wood density, we utilized also a new Index called BAIden, which combines Basal Area Increment and mean ring wood density to depict the carbon sequestration ability of trees. Pinus sylvestris showed different sensitivity to climatic variability depending on tree admixture, while the Basal Area Increment and wood density presented few differences between pure and mixed forests. According to the BAIden index, carbon accumulation in P. sylvestris showed similar sensitivity to climatic variability in pure and mixed forests. The new index was also informative on the main climatic drivers of carbon sequestration. Considering future climatic scenarios, the carbon sequestration ability of P. sylvestris will be facilitated by rising temperatures in late winter-early spring and reduced by decreasing precipitation and rising temperatures during summer. Finally, we discussed the perspective and applicability of BAIden for further studies on carbon sequestration ability under climate change.

Declines in canopy greenness and tree growth are caused by combined climate extremes during drought-induced dieback

Several dieback episodes triggered by droughts are revealing the high vulnerability of Mediterranean forests, manifested as declines in growth, increased defoliation, and rising mortality rates. Understanding forest responses to such climate extreme events is of high priority for predicting their future vegetation dynamics. We examined how remotely sensed measures of vegetation activity (NDVI, Normalized Difference Vegetation Index) and radial growth (BAI, basal area increment) responded to climate extreme events. We considered tree (Pinus sylvestris, Quercus pubescens, Quercus frainetto) and shrub (Juniperus phoenicea) populations from Italy and Spain showing recent dieback phenomena. Two components of drought, namely elevated atmospheric demand (VPD, vapor pressure deficit) and low soil moisture were analyzed in nearby stands showing or not showing dieback symptoms. Dieback stands exhibited lower NDVI values than non-dieback stands. NDVI and BAI were positively related in all sites except for the dieback stand of Q. frainetto that was negatively related. Such NDVI-BAI linkages were related to specific time windows, which could be useful for identifying when climatic conditions have the greatest influence on vegetation. Growth decline occurred in response to increasing VPD, but responses differed among species. J. phoenicea was the most negatively impacted by higher VPD, whereas oaks responded to soil moisture. A high VPD was related to stronger growth reduction in dieback P. sylvestris trees regardless of soil moisture changes. We highlighted that coupling between proxies of forest productivity (NDVI, BAI) allows better understanding and forecasting of drought-induced dieback phenomena in forests and shrublands. Scaling up from tree to stand levels might be feasible when using the maximum growing season NDVI, which can be applied for retrospective modeling of the impact of drought stress on forest productivity and tree growth.

Influences of summer warming and nutrient availability on Salix glauca L. growth in Greenland along an ice to sea gradient

The combined effects of climate change and nutrient availability on Arctic vegetation growth are poorly understood. Archaeological sites in the Arctic could represent unique nutrient hotspots for studying the long-term effect of nutrient enrichment. In this study, we analysed a time-series of ring widths of Salix glauca L. collected at nine archaeological sites and in their natural surroundings along a climate gradient in the Nuuk fjord region, Southwest Greenland, stretching from the edge of the Greenlandic Ice Sheet in the east to the open sea in the west. We assessed the temperature-growth relationship for the last four decades distinguishing between soils with past anthropogenic nutrient enrichment (PANE) and without (controls). Along the East-West gradient, the inner fjord sites showed a stronger temperature signal compared to the outermost ones. Individuals growing in PANE soils had wider ring widths than individuals growing in the control soils and a stronger climate-growth relation, especially in the inner fjord sites. Thereby, the individuals growing on the archaeological sites seem to have benefited more from the climate warming in recent decades. Our results suggest that higher nutrient availability due to past human activities plays a role in Arctic vegetation growth and should be considered when assessing both the future impact of plants on archaeological sites and the general greening in landscapes with contrasting nutrient availability.

Wood anatomy and tree-ring stable isotopes indicate a recent decline in water-use efficiency in the desert tree Moringa peregrina

The ability of desert plants to adapt to future climate changes and maximize their water-use efficiency will determine their survival. This study uses wood anatomy and δ¹³C and δ¹⁸O isotope analyses to investigate how Moringa peregrina trees in the Egyptian desert have responded to the environment over the last 10 years. Our results show that M. peregrina tree-ring widths (TRWs) have generally declined over the last decade, although individual series are characterized by high variability and low Rbars. Vessel lumen area percentages (VLA%) are low in wet years but increase significantly in dry years, such as the period 2017-2020. Stable δ¹³C isotope values decrease between 2010 (- 23.4‰) and 2020 (- 24.9‰), reflecting an unexpected response to an increase in drought conditions. The mean δ¹⁸O value (± standard error, SE) for the first ten rings of each tree from bark to pith (2020-2010) is 33.0 ‰ ± 0.85 with a range of 29.2-36.3‰, which indicates a common drought signal. The intrinsic water-use efficiency (iWUE) declines gradually with time, from 130.0 µmol mol^-1 in 2010 to 119.4 µmol mol^-1 in 2020. The intercellular carbon concentration (C_i) and C_i/C_a ratio increase over the same period, likely as a result of decreasing iWUE. The results show that M. peregrina trees seem to cool their leaves and the boundary air at the cost of saving water.

Retrospective analysis of wood anatomical traits and tree-ring isotopes suggests site-specific mechanisms triggering Araucaria araucana drought-induced dieback

In 2010-2018, Northern Patagonia featured the longest severe drought of the last millennium. This extreme dry spell triggered widespread growth decline and forest dieback. Nonetheless, the roles played by the two major mechanisms driving dieback, hydraulic failure and carbon starvation, are still not clear and understudied in this seasonally dry region. Here, for the 1800-2017 period, we apply a retrospective analysis of radial growth, wood anatomical traits (lumen area, cell-wall thickness) and δ¹³ C and δ¹⁸ O stable isotopes to assess dieback causes of the iconic conifer Araucaria araucana. We selected three stands where declining (defoliated) and nondeclining (not defoliated) trees coexisted along a precipitation gradient from the warm-dry Coastal Range to the cool-wet Andes. At all sites declining trees showed lower radial growth and lower theoretical hydraulic conductivity, suggesting a long-lasting process of hydraulic deterioration in their water transport system compared to nondeclining, coexisting trees. Wood anatomical traits evidenced that this divergence between declining and nondeclining trees started at least seven decades before canopy dieback. In the drier stands, declining trees showed higher water-use efficiency (WUE) throughout the whole period, which we attributed to early stomatal closure, suggesting a greater carbon starvation risk consistent with thinner cell walls. In the wettest stand, we found the opposite pattern. Here, a reduction in WUE coupled with thicker cell walls suggested increased carbon assimilation rates and exposure to drought-induced hydraulic failure. The δ¹⁸ O values indicated different strategies of gas exchange between sites, which are likely a consequence of microsite conditions and water sources. Multiproxy, retrospective quantifications of xylem anatomical traits and tree-ring isotopes provide a robust tool to identify and forecast, which stands or trees will show dieback or, on the contrary, which will likely withstand and be more resilient to future hotter droughts.

Using spectral indices as early warning signals of forest dieback: The case of drought-prone Pinus pinaster forests

Forest dieback processes linked to drought are expected to increase due to climate warming. Remotely sensed data offer several advantages over common field monitoring methods such as the ability to observe large areas on a systematic basis and monitoring their changes, making them increasingly used to assess changes in forest health. Here we aim to use a combined approximation of fieldwork and remote sensing to explore possible links between forest dieback and land surface phenological and trend variables derived from long Landsat time series. Forest dieback was evaluated in the field over 31 plots in a Mediterranean, xeric Pinus pinaster forest. Landsat 31-year time series of three greenness (EVI, NDVI, SAVI) and two wetness spectral indices (NMDI and TCW) were derived covering the period 1990-2020. Spectral indices from time series were decomposed into trend and seasonality using a Bayesian estimator while the relationships of the phenological and trend variables among levels of damage were assessed using linear and additive mixed models. We have not found any statistical pieces of evidence of extension or shortening patterns for the length of the phenological season over the examined 31-year period. Our results indicate that the dieback process was mainly related to the trend component of the spectral indices series whereas the phenological metrics were not related to forest dieback. We also found that plots with more dying or damaged trees displayed lower spectral indices trends after a severe drought event in the middle of the 1990s, which confirms the Landsat-derived spectral indices as indicators of early-warning signals. Drops in trends occurred earlier for wetness indices rather than for greenness indices which suggests that the former could be more appropriate for dieback detection, i.e. they could be used as early warning signals of impending loss of tree vigor.

Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback

Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two Pinus pinaster stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.

Allometry and Post-Drought Growth Resilience of Pedunculate Oak (Quercus robur L.) Varieties

This paper presents an analysis of the radial growth, tree dimensions, and allometry of three phenological pedunculate oak (Quercus robur L.; QURO) varieties (early (E-QURO), typical (T-QURO), and late (L-QURO)), from a common garden experiment. We focused on the resistance and resilience of each variety to drought events, which occurred in 2012 and 2017, as well as their recovery potential during juvenile and mature growth phases, with the goal of clarifying how QURO drought sensitivity is influenced by tree phenology and growth stage. Our results indicate that E-QURO is more drought resistant, while T-QURO and L-QURO exhibit greater recovery potential after a drought event. Hence, typical and late QURO varieties are better prepared to withstand climate change. We also noted differences in the physical dimensions and the allometry of the studied QURO varieties. On average, 21-year-old QURO specimens from the analyzed stand are 9.35 m tall, have a crown width (CW) of 8.05 m, and a diameter at breast height (DBH) of 23.71 cm. Although T-QURO varieties had the greatest DBH and CW, they were shorter than E- and L-QURO, which are similar in height. T-QURO is also shorter relative to DBH, while L-QURO has a wider crown relative to tree height (TH). Intra-variety variations are higher than variations among half-sib (open-pollinated) families of each variety. Moreover, the adopted regression model provided a better fit to the CW/DBH ratio than to TH/DBH and CW/TH.

Disentangling the Legacies of Climate and Management on Tree Growth

Legacies of past climate conditions and historical management govern forest productivity and tree growth. Understanding how these processes interact and the timescales over which they influence tree growth is critical to assess forest vulnerability to climate change. Yet, few studies address this issue, likely because integrated long-term records of both growth and forest management are uncommon. We applied the stochastic antecedent modelling (SAM) framework to annual tree-ring widths from mixed forests to recover the ecological memory of tree growth. We quantified the effects of antecedent temperature and precipitation up to 4 years preceding the year of ring formation and integrated management effects with records of harvesting intensity from historical forest management archives. The SAM approach uncovered important time periods most influential to growth, typically the warmer and drier months or seasons, but variation among species and sites emerged. Silver fir responded primarily to past climate conditions (25–50 months prior to the year of ring formation), while European beech and Scots pine responded mostly to climate conditions during the year of ring formation and the previous year, although these responses varied among sites. Past management and climate interacted in such a way that harvesting promoted growth in young silver fir under wet and warm conditions and in old European beech under drier and cooler conditions. Our study shows that the ecological memory associated with climate legacies and historical forest management is species-specific and context-dependent, suggesting that both aspects are needed to properly evaluate forest functioning under climate change.

Amplifying feedback loop between growth and wood anatomical characteristics of Fraxinus excelsior explains size-related susceptibility to ash dieback

Since the 1990s the invasive fungus Hymenoscyphus fraxineus has caused severe crown dieback and high mortality rates in Fraxinus excelsior in Europe. In addition to a strong genetic control of tolerance to the fungus, previous studies have found landscape heterogeneity to be an additional driver of variability in the severity of dieback symptoms. However, apart from climatic conditions related to heat and humidity influencing fungal infection success, the mechanistic understanding of why smaller or slower-growing trees are more susceptible to dieback remains less well understood. Here, we analyzed three stands in Switzerland with a unique setting of 8 years of data availability of intra-annual diameter growth and annual crown health assessments. We complemented this by ring width and quantitative wood anatomical measurements extending back before the monitoring started to investigate if wood anatomical adjustments can help better explain the size-related dieback phenomenon. We found that slower-growing trees or trees with smaller crowns already before the arrival of the fungus were more susceptible to dieback and mortality. Defoliation directly reduced growth as well as maximum earlywood vessel size, and the positive relationship between vessel size and growth rate caused a positive feedback amplifying and accelerating crown dieback. Measured non-structural carbohydrate (NSC) concentrations in the outermost five rings did not significantly vary between healthy and weakened trees, which translate into large differences in absolute available amount of NSCs. Thus, we hypothesize that a lack of NSCs (mainly sugars) leads to lower turgor pressure and smaller earlywood vessels in the following year. This might impede efficient water transport and photosynthesis, and be responsible for stronger symptoms of dieback and higher mortality rates in smaller and slower-growing trees.

Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Climate change is expected to pose major direct and indirect threats to groundwater-dependent forest ecosystems. Forests that concurrently experience increased rates of water extraction may face unprecedented exposure to droughts. Here, we examined differences in stem growth and xylem hydraulic architecture of 216 oak trees from sites with contrasting groundwater availability, including sites where groundwater extraction has led to reduced water availability for trees over several decades. We expected reduced growth and xylem hydraulic capacity for trees at groundwater extraction sites both under normal and unfavourable growing conditions. Compared to sites without extraction, trees at sites with groundwater extraction showed reduced growth and hydraulic conductivity both during periods of moderate and extremely low soil water availability. Trees of low vigour, which were more frequent at sites with groundwater extraction, were not able to recover growth and hydraulic capacity following drought, pointing to prolonged drought effects. Long-term water deficit resulting in reduced CO2 assimilation and hydraulic capacity after drought are very likely responsible for observed reductions in tree vitality at extraction sites. Our results demonstrate that groundwater access maintains tree function and resilience to drought and is therefore important for tree health in the context of climate change.

The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline

Forest decline, in course of climate change, has become a frequently observed phenomenon. Much of the observed decline has been associated with an increasing frequency of climate change induced hotter droughts while decline induced by flooding, late-frost, and storms also play an important role. As a consequence, tree mortality rates have increased across the globe. Despite numerous studies that have assessed forest decline and predisposing factors for tree mortality, we still lack an in-depth understanding of (I) underlying eco-physiological mechanisms, (II) the influence of varying environmental conditions related to soil, competition, and micro-climate, and (III) species-specific strategies to cope with prolonged environmental stress. To deepen our knowledge within this context, studying tree performance within larger networks seems a promising research avenue. Ideally such networks are already established during the actual period of environmental stress. One approach for identifying stressed forests suitable for such monitoring networks is to assess measures related to tree vitality in near real-time across large regions by means of satellite-borne remote sensing. Within this context, we introduce the European Forest Condition monitor (EFCM)-a remote-sensing based, freely available, interactive web information tool. The EFCM depicts forest greenness (as approximated using NDVI from MODIS at a spatial resolution of roughly 5.3 hectares) for the pixel-specific growing season across Europe and consequently allows for guiding research within the context of concurrent forest performance. To allow for inter-temporal comparability and account for pixel-specific features, all observations are set in relation to normalized difference vegetation index (NDVI) records over the monitoring period beginning in 2001. The EFCM provides both a quantile-based and a proportion-based product, thereby allowing for both relative and absolute comparison of forest greenness over the observational record. Based on six specific examples related to spring phenology, drought, late-frost, tree die-back on water-logged soils, an ice storm, and windthrow we exemplify how the EFCM may help identifying hotspots of extraordinary forest greenness. We discuss advantages and limitations when monitoring forest condition at large scales on the basis of moderate resolution remote sensing products to guide users toward an appropriate interpretation.

Disentangling Mechanisms of Drought-Induced Dieback in Pinus nigra Arn. from Growth and Wood Isotope Patterns

The increased frequency and intensity of warming-induced droughts have triggered dieback episodes affecting many forest types and tree species worldwide. Tree plantations are not exempt as they can be more vulnerable to drought than natural forests because of their lower structural and genetic diversity. Therefore, disentangling the physiological mechanisms leading to growth decline and tree mortality can provide tools to adapt forest management to climate change. In this study, we investigated a Pinus nigra Arn. plantation situated in northern Spain, in which some trees showed canopy dieback and radial-growth decline. We analyzed how radial growth and its responses to drought events differed between non-declining (ND) and declining (D) trees showing low and high canopy defoliation, respectively, in combination with carbon (δ13C) and oxygen (δ18O) isotope ratios in tree rings. The radial growth of P. nigra was constrained by water availability during the growing season and the previous autumn. The radial growth of D trees showed higher sensitivity to drought than ND trees. This fact is in accordance with the lower drought resilience and negative growth trends observed in D trees. Both tree classes differed in their growth from 2012 onwards, with D trees showing a reduced growth compared to ND trees. The positive δ13C-δ18O relationship together with the uncoupling between growth and intrinsic water-use efficiency suggest that D trees have less tight stomatal regulation than ND trees, which could involve a high risk of xylem embolism in the former class. Our results suggest that different water use strategies between coexisting ND and D trees were behind the differences in growth patterns and point to hydraulic failure as a possible mechanism triggering dieback and growth decline.

Juniperus communis populations exhibit low variability in hydraulic safety and efficiency

The performance and distribution of woody species strongly depend on their adjustment to environmental conditions based on genotypic and phenotypic properties. Since more intense and frequent drought events are expected due to climate change, xylem hydraulic traits will play a key role under future conditions, and thus, knowledge of hydraulic variability is of key importance. In this study, we aimed to investigate the variability in hydraulic safety and efficiency of the conifer shrub Juniperus communis based on analyses along an elevational transect and a common garden approach. We studied (i) juniper plants growing between 700 and 2000 m a.s.l. Innsbruck, Austria, and (ii) plants grown in the Innsbruck botanical garden (Austria) from seeds collected at different sites across Europe (France, Austria, Ireland, Germany and Sweden). Due to contrasting environmental conditions at different elevation and provenance sites and the wide geographical study area, pronounced variation in xylem hydraulics was expected. Vulnerability to drought-induced embolisms (hydraulic safety) was assessed via the Cavitron and ultrasonic acoustic emission techniques, and the specific hydraulic conductivity (hydraulic efficiency) via flow measurements. Contrary to our hypothesis, relevant variability in hydraulic safety and efficiency was neither observed across elevations, indicating a low phenotypic variation, nor between provenances, despite expected genotypic differences. Interestingly, the provenance from the most humid and warmest site (Ireland) and the northernmost provenance (Sweden) showed the highest and the lowest embolism resistance, respectively. The hydraulic conductivity was correlated with plant height, which indicates that observed variation in hydraulic traits was mainly related to morphological differences between plants. We encourage future studies to underlie anatomical traits and the role of hydraulics for the broad ecological amplitude of J. communis.

Short- and long-term responses of riparian cottonwoods (Populus spp.) to flow diversion: Analysis of tree-ring radial growth and stable carbon isotopes

Long duration tree-ring records with annual precision allow for the reconstruction of past growing conditions. Investigations limited to the most common tree-ring proxy of ring width can be difficult to interpret, however, because radial growth is affected by multiple environmental processes. Furthermore, studies of living trees may miss important effects of drought on tree survival and forest changes. Stable carbon isotopes can help distinguish drought from other environmental factors that influence tree-ring width and forest stand condition. We quantified tree-ring radial expansion and stable carbon isotope ratios (δ¹³C) in riparian cottonwoods (Populus angustifolia and P. angustifolia x P.trichocarpa) along Snake Creek in Nevada, USA. We investigated how hydrological drought affected tree growth and death at annual to half-century scales in a partially dewatered reach (DW) compared to reference reaches immediately upstream and downstream. A gradual decline in tree-ring basal area increment (BAI) began at DW concurrent to streamflow diversion in 1961. BAI at DW diverged from one reference reach immediately but not from the other until nearly 50 years later. In contrast, tree-ring δ¹³C had a rapid and sustained increase following diversion at DW only, providing the stronger and clearer drought signal. BAI and δ¹³C were not significantly correlated prior to diversion; after diversion they both reflected drought and were correlated for DW trees only. Cluster analyses distinguished all trees in DW from those in reference reaches based on δ¹³C, but BAI patterns left trees intermixed across reaches. Branch and tree mortality were also highest and canopy vigor was lowest in DW. Results indicate that water scarcity strongly limited cottonwood photosynthesis following flow diversion, thus reducing carbon assimilation, basal growth and survival. The dieback was not sudden, but occurred over decades as carbon deficits mounted and depleted streamflow left trees increasingly vulnerable to local meteorological drought.

Scots pine trees react to drought by increasing xylem and phloem conductivities

Drought limits the long-distance transport of water in the xylem due to the reduced leaf-to-soil water potential difference and possible embolism-related losses of conductance and of sugars in the phloem due to the higher viscosity of the dehydrated sugary solution. This condition can have cascading effects in water and carbon (C) fluxes that may ultimately cause tree death. We hypothesize that the maintenance of xylem and phloem conductances is fundamental for survival also under reduced resource availability, when trees may produce effective and low C cost anatomical adjustments in the xylem and phloem close to the treetop where most of the hydraulic resistance is concentrated. We analyzed the treetop xylem and phloem anatomical characteristics in coexisting Scots pine trees, symptomatic and non-symptomatic of drought-induced dieback. We selected the topmost 55 cm of the main stem and selected several sampling positions at different distances from the stem apex to test for differences in the axial patterns between the two groups of trees. We measured the annual ring area, the tracheid hydraulic diameter (Dh) and cell wall thickness (CWT), the conductive phloem area and the average lumen diameter of the 20 largest phloem sieve cells (Dph). Declining trees grew less than the non-declining ones, and despite the similar axial scaling of anatomical traits, had larger Dh and lower CWT. Moreover, declining trees had wider Dph. Our results demonstrate that even under drought stress, maintenance of xylem and phloem efficiencies is of primary importance for survival, even if producing fewer larger tracheids may lead to a xylem more vulnerable to embolism formation.

Reduced above-ground growth and wood density but increased wood chemical concentrations of Scots pine on relict charcoal hearths

Deciphering the drivers of tree growth is a central aim of dendroecology. In this context, soil conditions may play a crucial role, since they determine the availability of water and nutrients for trees. Yet, effects of systematically differing soil conditions on tree growth render a marginally studied topic. In this context, relict charcoal hearths (RCH) - a widespread legacy of anthropogenic charcoal production - render a valuable 'natural' experiment to study possible effects of artificially altered soil conditions on tree growth. We hypothesize, that the differing physico-chemical properties of RCH result in differing wood properties if compared to trees growing on unmodified soils. To test this hypothesis, we analyzed tree-growth, wood density, and wood elemental concentrations of Scots pine as well as physico-chemical soil properties. We applied a classic control-treatment design to compare RCH with unmodified soils. Our analyses identified significantly lower above-ground wood production but systematically higher wood elemental concentrations in RCH-trees compared to control trees. Since we could not identify treatment-specific growth patterns, we hypothesize the observed lower above-ground productivity of Scots pine to indicate an increased root-shoot ratio to compensate for a potentially lower plant water availability on RCH-sites. The observed higher wood elemental concentrations likely reflect higher soil elemental concentrations of Fe, Ca, K, and Mn in RCH soils. In conclusion, our study highlights diverse effects of RCH on tree growth and wood properties and strengthens the value of dendro-chemistry to use the tree-ring archive as proxy for soil conditions within a dendro-ecological context.

Wood anatomical traits in black spruce reveal latent water constraints on the boreal forest

The effects of climate change on high-latitude forest ecosystems are complex, making forecasts of future scenarios uncertain. The predicted lengthening of the growing season under warming conditions is expected to increase tree growth rates. However, there is evidence of an increasing sensitivity of the boreal forest to drought stress. To assess the influence of temperature and precipitation on the growth of black spruce (Picea mariana), we investigated long-term series of wood anatomical traits on 20 trees from four sites along 600 km, the latitudinal range of the closed boreal forest in Quebec, Canada. We correlated the anatomical traits resolved at intraring level with daily temperature, vapor pressure deficit (VPD), and precipitation during the 1943-2010 period. Tree-ring width, number of cells per ring and cell wall thickness were positively affected by spring and summer daily mean and maximum temperature at the northern sites. These results agree with the well-known positive effect of high temperatures on tree ring formation at high latitudes. However, we captured, for the first time in this region, the latent impact of water availability on xylem traits. Indeed, in all the four sites, cell lumen area showed positive correlations with daily precipitation (mostly at low latitude), and/or negative correlations with daily mean and maximum temperature and VPD (mostly at high latitude). We inferred that drought, due to high temperatures, low precipitations, or both, negatively affects cell enlargement across the closed boreal forest, including the northernmost sites. The production of tracheids with narrower lumen, potentially more resistant to cavitation, could increase xylem hydraulic safety under a warmer and drier climate. However, this would result in lower xylem conductivity, with consequent long-term hydraulic deterioration, growth decline, and possibly lead to tree dieback, as observed in other forest ecosystems at lower latitudes.

Moisture-driven shift in the climate sensitivity of white spruce xylem anatomical traits is coupled to large-scale oscillation patterns across northern treeline in northwest North America

Tree growth at northern treelines is generally temperature-limited due to cold and short growing seasons. However, temperature-induced drought stress was repeatedly reported for certain regions of the boreal forest in northwestern North America, provoked by a significant increase in temperature and possibly reinforced by a regime shift of the pacific decadal oscillation (PDO). The aim of this study is to better understand physiological growth reactions of white spruce, a dominant species of the North American boreal forest, to PDO regime shifts using quantitative wood anatomy and traditional tree-ring width (TRW) analysis. We investigated white spruce growth at latitudinal treeline across a >1,000 km gradient in northwestern North America. Functionally important xylem anatomical traits (lumen area, cell-wall thickness, cell number) and TRW were correlated with the drought-sensitive standardized precipitation-evapotranspiration index of the growing season. Correlations were computed separately for complete phases of the PDO in the 20th century, representing alternating warm/dry (1925-1946), cool/wet (1947-1976) and again warm/dry (1977-1998) climate regimes. Xylem anatomical traits revealed water-limiting conditions in both warm/dry PDO regimes, while no or spatially contrasting associations were found for the cool/wet regime, indicating a moisture-driven shift in growth-limiting factors between PDO periods. TRW reflected only the last shift of 1976/1977, suggesting different climate thresholds and a higher sensitivity to moisture availability of xylem anatomical traits compared to TRW. This high sensitivity of xylem anatomical traits permits to identify first signs of moisture-driven growth in treeline white spruce at an early stage, suggesting quantitative wood anatomy being a powerful tool to study climate change effects in the northwestern North American treeline ecotone. Projected temperature increase might challenge growth performance of white spruce as a key component of the North American boreal forest biome in the future, when drier conditions are likely to occur with higher frequency and intensity.

Precipitation variability differently affects radial growth, xylem traits and ring porosity of three Mediterranean oak species at xeric and mesic sites

In the Mediterranean basin, diffuse-porous, semi-ring-porous and ring-porous tree species coexist in the same regions. Climate change might differently affect these types, but a mechanistic understanding of drought effects on their xylem structure is lacking. We investigated tree-ring width and xylem functional traits in ring-porous Quercus boissieri, semi-ring-porous Q. ithaburensis and diffuse-porous Q. calliprinos, at xeric (Galilee) and mesic (Golan) sites in the South-Eastern Mediterranean basin. We quantitatively assessed how dry and wet years affect growth and xylem traits in different porosity type oaks, and evaluated whether porosity type is preserved or altered during these years. We measured, counted or computed tree-ring width, vessel number, maximum lumen area, frequency, tree-ring and xylem theoretical hydraulic conductivity along 40-year ring series of 50 trees in total. We also quantified ring porosity in each year using two indices, the Gini coefficient and the porosity ratio of vessel area, and described vessel area intra-ring variations by distribution profiles. We then compared these parameters in the five driest and five wettest years of the 40-year period. Radial growth and functional trait variations were more similar between species in the same site (strong drought effects in Q. ithaburensis and Q. calliprinos in Galilee, moderate effects in Q. boissieri and Q. calliprinos in Golan) than between sites for the same species (Q. calliprinos was more affected in Galilee than in Golan). Ring porosity indices and distribution profiles showed that diffuse-porous xylem structure of Q. calliprinos was maintained even under dry conditions at both sites. However, Q. boissieri xylem shifted from ring-porous in wet and normal years to semi-ring-porous in dry years, i.e. the porous ring cannot be completely built under water constraint. This suggests that ring porous strategy, typical of temperate regions with strong seasonality, might not be realized under future drier conditions in the Mediterranean basin.

Long-term nutrient imbalances linked to drought-triggered forest dieback

Drought-induced forest dieback is causing reductions in productivity, increasing tree mortality and impairing terrestrial carbon uptake worldwide. However, the role played by long-term nutrient imbalances during drought-induced dieback is still unknown. To improve our knowledge on the relationships between dieback and nutrient imbalances, we analysed wood anatomical traits (tree-ring width and wood density), soil properties and long-term chemical information in tree-ring wood (1900-2010) by non-destructive Micro X-ray fluorescence (μXRF) and destructive (ICP-OES) techniques. We studied two major European conifers with ongoing drought-induced dieback in mesic (Abies alba, silver fir) and xeric (Pinus sylvestris, Scots pine) sites. In each site we compared coexisting declining (D) and non-declining (ND) trees. We used dendrochronology and generalized additive and linear mixed models to analyse trends in tree-ring nutrients and their relationships with wood traits. The D trees presented lower growth and higher minimum wood density than ND trees, corresponding to a smaller lumen area of earlywood tracheids and thus a lower theoretical hydraulic conductivity. These differences in growth and wood-anatomy were more marked in silver fir than in Scots pine. Moreover, most of the chemical elements showed higher concentrations in D than in ND trees during the last two-five decades (e.g., Mn, K and Mg), while Ca and Na increased in the sapwood of ND trees. The Mn concentrations, and related ratios (Ca:Mn, Mn:Al and P:Mn) showed the highest differences between D and ND trees for both tree species. These findings suggest that a reduced hydraulic conductivity, consistent with hydraulic impairment, is affecting the use of P in D trees, making them more prone to drought-induced damage. The retrospective quantifications of Mn ratios may be used as early-warning signals of impending dieback.

Legacies of more frequent drought in ponderosa pine across the western United States

Despite widespread interest in drought legacies-multiyear impacts of drought on tree growth-the key implication of reported drought legacies remains unaddressed: as impaired growth and slow recovery associated with drought legacies are pervasive across forest ecosystems, what is the impact of more frequent drought conditions? We investigated the assumption that either multiple drought years occurring during a short period (multiyear droughts), or droughts occurring during the recovery period from previous drought (compounded droughts), are detrimental to subsequent growth. There is evidence that drought responses may vary among populations of widespread species, leading us to examine regional differences in responses of the conifer Pinus ponderosa to historic drought frequency in the western United States. More frequent drought conditions incurred additional growth declines and shifts in growth-climate sensitivities in the years following drought relative to single-drought events, with 'triple-droughts' being worse than 'double-droughts'. Notably, prediction skill was not strongly reduced when ignoring compounded droughts, a consequence of the temporally comprehensive formulation of our stochastic antecedent model that accounts for the climatic memory of tree growth. We argue that incorporating drought-induced temporal variability in tree growth sensitivities can aid inference gained from statistical models, where more simplistic models could overestimate the severity of drought legacies. We also found regional differences in response to repeated drought, and suggest plastic post-drought sensitivities and climatic memory may represent beneficial physiological adjustments in interior regions. Within-species variability may thus mediate forest responses to increasing drought frequency under future climate change, but experimental approaches using more species are necessary to improve our understanding of the mechanisms that underlie drought legacy effects on tree growth.

Anisohydric behavior linked to persistent hydraulic damage and delayed drought recovery across seven North American tree species

The isohydry-anisohydry spectrum has become a popular way to characterize plant drought responses and recovery processes. Despite the proven utility of this framework for understanding the interconnected physiological changes plants undergo in response to water stress, new challenges have arisen pertaining to the traits and tradeoffs that underlie this concept. To test the utility of this framework for understanding hydraulic traits, drought physiology and recovery, we applied a 6 wk experimental soil moisture reduction to seven tree species followed by a 6 wk recovery period. Throughout, we measured hydraulic traits and monitored changes in gas exchange, leaf water potential, and hydraulic conductivity. Species' hydraulic traits were not coordinated, as some anisohydric species had surprisingly low resistance to embolism (P₅₀ ) and negative safety margins. In addition to widespread hydraulic damage, these species also experienced reductions in photosynthesis and stem water potential during water stress, and delayed recovery time. Given that we observed no benefit of being anisohydric either during or after drought, our results indicate the need to reconsider the traits and tradeoffs that underlie anisohydric behavior, and to consider the environmental, biological and edaphic processes that could allow this strategy to flourish in forests.

Coupled ecohydrology and plant hydraulics modeling predicts ponderosa pine seedling mortality and lower treeline in the US Northern Rocky Mountains

We modeled hydraulic stress in ponderosa pine seedlings at multiple scales to examine its influence on mortality and forest extent at the lower treeline in the northern Rockies. We combined a mechanistic ecohydrologic model with a vegetation dynamic stress index incorporating intensity, duration and frequency of hydraulic stress events, to examine mortality from loss of hydraulic conductivity. We calibrated our model using a glasshouse dry-down experiment and tested it using in situ monitoring data on seedling mortality from reforestation efforts. We then simulated hydraulic stress and mortality in seedlings within the Bitterroot River watershed of Montana. We show that cumulative hydraulic stress, its legacy and its consequences for mortality are predictable and can be modeled at local to landscape scales. We demonstrate that topographic controls on the distribution and availability of water and energy drive spatial patterns of hydraulic stress. Low-elevation, south-facing, nonconvergent locations with limited upslope water subsidies experienced the highest rates of modeled mortality. Simulated mortality in seedlings from 2001 to 2015 correlated with the current distribution of forest cover near the lower treeline, suggesting that hydraulic stress limits recruitment and ultimately constrains the low-elevation extent of conifer forests within the region.

Disentangling seasonal and interannual legacies from inferred patterns of forest water and carbon cycling using tree-ring stable isotopes

Tree-ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross-correlation in the δ¹³ C and δ¹⁸ O isotope ratios in the earlywood (EW) and latewood (LW) fractions of Pinus ponderosa trees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ¹³ C from EW and LW has significant positive cross-correlations at most sites, whereas EW and LW δ¹⁸ O values were cross-correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross-correlations in both δ¹³ C and δ¹⁸ O can be largely explained by a low-frequency (multiple-year) mode that may be associated with long-term climate change. We isolated, and statistically removed, the low-frequency correlation, which resulted in greater geographical differentiation of the EW and LW isotope signals. The remaining higher-frequency (seasonal) cross-correlations between EW and LW isotope ratios were explored using a mechanistic isotope fractionation-climate model. This showed that lower atmospheric vapor pressure deficits associated with monsoon rain increase the EW-LW differentiation for both δ¹³ C and δ¹⁸ O at southern sites, compared to northern sites. Our results support the hypothesis that dominantly unimodal precipitation regimes, such as near the northern boundary of the NAM, are more likely to foster cross-correlations in the isotope signals of EW and LW, potentially due to greater sharing of common carbohydrate and soil water resource pools, compared to southerly sites with bimodal precipitation regimes.

Drought-Affected Populus simonii Carr. Show Lower Growth and Long-Term Increases in Intrinsic Water-Use Efficiency Prior to Tree Mortality

The Three-North Shelter Forest (TNSF) is a critical ecological barrier against sandstorms in northern China, but has shown extensive decline and death in Populus simonii Carr. in the last decade. We investigated the characteristics—tree-ring width, basal area increment (BAI), carbon isotope signature (13Ccor), and intrinsic water-use efficiency (iWUE)—of now-dead, dieback, and non-dieback trees in TNSF shelterbelts of Zhangbei County. Results from the three groups were compared to understand the long-term process of preceding drought-induced death and to identify potential early-warning proxies of drought-triggered damage. The diameter at breast height (DBH) was found to decrease with the severity of dieback, showing an inverse relationship. In all three groups, both tree-ring width and BAI showed quadratic relationships with age, and peaks earlier in the now-dead and dieback groups than in the non-dieback group. The tree-ring width and BAI became significantly lower in the now-dead and dieback groups than in the non-dieback group from 17 to 26 years before death, thus, these parameters can serve as early-warning signals for future drought-induced death. The now-dead and dieback groups had significantly higher δ13Ccor and iWUEs than the non-dieback group at 7–16 years prior to the mortality, indicating a more conservative water-use strategy under drought stress compared with non-dieback trees, possibly at the cost of canopy defoliation and long-term shoot dieback. The iWUE became significantly higher in the now-dead group than in the dieback group at 0–7 years before death, about 10 years later than the divergence of BAI. After the iWUE became significantly different among the groups, the now-dead trees showed lower growth and died over the next few years. This indicates that, for the TNSF shelterbelts studied, an abrupt iWUE increase can be used as a warning signal for acceleration of impending drought-induced tree death. In general, we found that long-term drought decreased growth and increased iWUE of poplar tree. Successive droughts could drive dieback and now-dead trees to their physiological limits of drought tolerance, potentially leading to decline and mortality episodes.

Tree-ring anatomy and carbon isotope ratio show both direct and legacy effects of climate on bimodal xylem formation in Pinus pinea

Understanding how climate affects xylem formation is critical for predicting the impact of future conditions on tree growth and functioning in the Mediterranean region, which is expected to face warmer and drier conditions. However, mechanisms of growth response to climate at different temporal scales are still largely unknown, being complicated by separation between spring and autumn xylogenesis (bimodal temporal pattern) in most species such as Mediterranean pines. We investigated wood anatomical characteristics and carbon stable isotope composition in Mediterranean Pinus pinea L. along tree-ring series at intra-ring resolution to assess xylem formation processes and responses to intra-annual climate variability. Xylem anatomy was strongly related to environmental conditions occurring a few months before and during the growing season, but was not affected by summer drought. In particular, the lumen diameter of the first earlywood tracheids was related to winter precipitation, whereas the size of tracheids produced later was influenced by mid-spring precipitation. Diameter of latewood tracheids was associated with precipitation in mid-autumn. In contrast, tree-ring carbon isotope composition was mostly related to climate of the previous seasons. Earlywood was likely formed using both recently and formerly assimilated carbon, while latewood relied mostly on carbon accumulated many months prior to its formation. Our integrated approach provided new evidence on the short-term and carry-over effects of climate on the bimodal temporal xylem formation in P. pinea. Investigations on different variables and time scales are necessary to disentangle the complex climate influence on tree growth processes under Mediterranean conditions.

Detecting early warning signals of tree mortality in boreal North America using multiscale satellite data

Increasing tree mortality from global change drivers such as drought and biotic infestations is a widespread phenomenon, including in the boreal zone where climate changes and feedbacks to the Earth system are relatively large. Despite the importance for science and management communities, our ability to forecast tree mortality at landscape to continental scales is limited. However, two independent information streams have the potential to inform and improve mortality forecasts: repeat forest inventories and satellite remote sensing. Time series of tree-level growth patterns indicate that productivity declines and related temporal dynamics often precede mortality years to decades before death. Plot-level productivity, in turn, has been related to satellite-based indices such as the Normalized difference vegetation index (NDVI). Here we link these two data sources to show that early warning signals of mortality are evident in several NDVI-based metrics up to 24 years before death. We focus on two repeat forest inventories and three NDVI products across western boreal North America where productivity and mortality dynamics are influenced by periodic drought. These data sources capture a range of forest conditions and spatial resolution to highlight the sensitivity and limitations of our approach. Overall, results indicate potential to use satellite NDVI for early warning signals of mortality. Relationships are broadly consistent across inventories, species, and spatial resolutions, although the utility of coarse-scale imagery in the heterogeneous aspen parkland was limited. Longer-term NDVI data and annually remeasured sites with high mortality levels generate the strongest signals, although we still found robust relationships at sites remeasured at a typical 5 year frequency. The approach and relationships developed here can be used as a basis for improving forest mortality models and monitoring systems.

Disentangling the climate-driven bimodal growth pattern in coastal and continental Mediterranean pine stands

Mediterranean climate promotes two distinct growth peaks separated by summer quiescence in trees. This bimodal pattern has been associated to favourable growing conditions during spring and autumn when mild temperatures and soil-water availability enhance cambial activity. Climatic models predict progressive warming and drying for the Mediterranean Basin, which could shorten or shift the spring and autumn growing seasons. We explored this idea by comparing two sites with different Mediterranean climate types (continental/dry and coastal/wet) and studied how climate drives the bimodal growth pattern in Aleppo pine (Pinus halepensis). Specifically we investigated the intra-annual changes in wood anatomy and the corresponding formation of density fluctuations (IADF). Trees on both sites were analyzed by dendrometer monitoring and by developing chronologies of wood anatomical traits. Radial-increment dynamics followed a similar bimodal pattern in both sites but coastal trees showed higher increments during the spring and autumn growth peaks, especially in autumn. The summer rest of cambium activity occurs almost one month earlier in the coastal than in the inland site. Lumen area and cell-wall thickness were significantly smaller in the continental site, while the increment rate of cell-wall thickness during an IADF event was much higher in the coastal pines. The accumulated soil moisture deficit was the main climatic constraint of tracheid enlargement in continental pines. Intra-annual density fluctuations were more frequent in the coastal trees where wood anatomy features recover to average values after such events, meanwhile inland trees presented a much lower recovery rate. Growth bimodality and the formation of density fluctuations were linked, but mild climate of the coastal site allows a longer growing season, which explains why trees in this area showed higher and more variable growth rates.

Early-Warning Signals of Individual Tree Mortality Based on Annual Radial Growth

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter-annual growth variability and a decrease in growth synchrony in the last ∼20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.