• Energy Research

Abstract Forest decline under environmental stress is expressed by regeneration failure and accelerated mortality in all ontogenic stages at the population level. Characterizing functional traits and mechanisms that best capture species decline and mortality is essential to assess forest dynamics. We analysed sensitivity to increasing water stress in two species with different water‐use strategies on a mixed Quercus pyrenaica–Pinus sylvestris forest where adult pines express vulnerability to climate change but oaks do not. We compared the dynamics of radial growth, wood δ13C and sapwood non‐structural carbohydrates (NSCs) in response to drought at different time‐scales in both species and two age cohorts in pine. Both species were very sensitive to water stress, which influenced trait phenotypic plasticity at short‐ and long time‐scales. Water‐use strategy in pines of both ages was more conservative than in the more drought‐tolerant oak. Both species showed negative growth trends despite increasing intrinsic water‐use efficiency. Recent growth of pines is slower than it was in the past. Carbon isotope discrimination trends in young pines suggested increasing leaf gas exchange constraints. NSCs were far from depletion in both species and all pine ages. Intra‐ and inter‐annual NSC variability was higher in oaks than in pines and in soluble sugars (SS) than in starch. SS were lowest in young pines. Sensitivity of NSCs to contrasting climatic years was low in pines, and NSC levels mostly remained homeostatic for this species. The sensitivity to climate expressed suggests different C allocation strategies, with less coupling between radial growth and current‐year photosynthesis in young pines. Synthesis. Pines expressed negative responses to increased water stress regardless of age, showing rising gas exchange constraints through tighter stomatal control of water losses than oaks. Young pines showed similar functional responses to water stress than old pines in decline, which suggests species‐level vulnerability and could be regarded as early warning signals anticipating mortality in pines. Yet, given the high sensitivity to drought also expressed by the non‐declining oak, it would have been difficult to unequivocally disentangle species decline based only on the functional traits analysed.

AbstractClimate warming is driving an advance of leaf unfolding date in temperate deciduous forests, promoting longer growing seasons and higher carbon gains. However, an earlier leaf phenology also increases the risk of late frost defoliation (LFD) events. Compiling the spatiotemporal patterns of defoliations caused by spring frost events is critical to unveil whether the balance between the current advance in leaf unfolding dates and the frequency of LFD occurrence is changing and represents a threaten for the future viability and persistence of deciduous forests. We combined satellite imagery with machine learning techniques to reconstruct the spatiotemporal patterns of LFD events for the 2003–2018 period in the Iberian range of European beech (Fagus sylvatica), at the drier distribution edge of the species. We used MODIS Vegetation Index Products to generate a Normalized Difference Vegetation Index (NDVI) time series for each 250 × 250 m pixel in a total area of 1,013 km2 (16,218 pixels). A semi‐supervised approach was used to train a machine learning model, in which a binary classifier called Support Vector Machine with Global Alignment Kernel was used to differentiate between late frost and non‐late frost pixels. We verified the obtained estimates with photointerpretation and existing beech tree‐ring chronologies to iteratively improve the model. Then, we used the model output to identify topographical and climatic factors that determined the spatial incidence of LFD. During the study period, LFD was a low recurrence phenomenon that occurred every 15.2 yr on average and showed high spatiotemporal heterogeneity. Most LFD events were condensed in 5 yr and clustered in western forests (86.5% in one‐fifth of the pixels) located at high elevation with lower than average precipitation. Elevation and longitude were the major LFD risk factors, followed by annual precipitation. The synergistic effects of increasing drought intensity and rising temperature combined with more frequent late frost events may determine the future performance and distribution of beech forests. This interaction might be critical at the beech drier range edge, where the concentration of LFD at high elevations could constrain beech altitudinal shifts and/or favor species with higher resistance to late frosts.

Les événements climatiques extrêmes, tels que les gelées tardives au printemps pendant le rinçage des feuilles, ont des impacts considérables sur la croissance radiale des feuillus tempérés. Bien que toutes les espèces de feuillus soient potentiellement vulnérables, les dommages dépendent des particularités du climat local, de l'espèce et de sa phénologie. L'impact des gelées de fin de printemps a été largement étudié dans l'hémisphère Nord, mais l'incidence potentielle dans les espèces d'arbres de l'hémisphère Sud est encore mal connue. Ici, nous reconstruisons l'occurrence du gel printanier dans 30 peuplements de l'arbre à feuilles caduques Nothofagus pumilio dans son aire de répartition septentrionale dans les Andes patagoniennes. Nous avons identifié des réductions de la largeur des cernes des arbres au niveau du peuplement non associées à des épisodes de sécheresse régionaux ou locaux, correspondant à des températures printanières minimales inhabituelles pendant le dépliage des feuilles. Plusieurs gelées printanières ont été identifiées le long de la distribution septentrionale de N. pumilio, étant plus fréquentes dans les forêts argentines plus continentales. Le gel printanier en 1980 avait la plus grande étendue spatiale. Les gelées printanières de 1980 et 1992 ont également provoqué des dommages dans les vergers régionaux. Les dommages causés par le gel printanier ont été associés à (i) une période de températures anormalement chaudes au début du dépliage des feuilles, suivie (ii) de températures glaciales. Cette étude nous aide à mieux comprendre les contraintes climatiques qui pourraient déterminer la croissance et la dynamique futures des forêts de feuillus andines et l'utilisation potentielle des cernes d'arbres comme archives des événements extrêmes des gelées printanières dans le nord de la Patagonie. Los eventos climáticos extremos, como las heladas tardías en primavera durante el lavado de hojas, tienen un impacto considerable en el crecimiento radial de los árboles de hoja ancha templados. Aunque todas las especies de hoja ancha son potencialmente vulnerables, el daño depende de las particularidades del clima local, la especie y su fenología. El impacto de las heladas de finales de la primavera ha sido ampliamente investigado en el hemisferio norte, pero la incidencia potencial en las especies de árboles del hemisferio sur aún es poco conocida. Aquí, reconstruimos la ocurrencia de heladas primaverales en 30 rodales del árbol caducifolio Nothofagus pumilio en su rango de distribución norte en los Andes patagónicos. Identificamos reducciones del ancho de los anillos de los árboles a nivel del rodal no asociadas con eventos de sequía regionales o locales, que coinciden con temperaturas mínimas inusuales de primavera durante el despliegue de las hojas. Se identificaron varias heladas primaverales a lo largo de la distribución norte de N. pumilio, siendo más frecuentes en los bosques argentinos más continentales. Las heladas de primavera en 1980 tuvieron la mayor extensión espacial. Las heladas de primavera de 1980 y 1992 también provocaron daños en los huertos regionales. El daño por heladas primaverales se asoció con (i) un período de temperaturas inusualmente cálidas al comienzo del despliegue de la hoja, seguido de (ii) temperaturas de congelación. Este estudio ayuda a ampliar nuestra comprensión de las limitaciones climáticas que podrían determinar el crecimiento futuro y la dinámica de los bosques caducifolios andinos y el uso potencial de los anillos de árboles como archivos de eventos extremos de heladas primaverales en el norte de la Patagonia. Extreme climatic events, such as late frosts in spring during leaf flush, have considerable impacts on the radial growth of temperate broadleaf trees. Albeit, all broadleaved species are potentially vulnerable, damage depends on the particularities of the local climate, the species, and its phenology. The impact of late spring frosts has been widely investigated in the Northern Hemisphere, but the potential incidence in Southern Hemisphere tree species is still poorly known. Here, we reconstruct spring frost occurrence at 30 stands of the deciduous tree Nothofagus pumilio in its northern range of distribution in the Patagonian Andes. We identified tree ring-width reductions at stand level not associated with regional or local drought events, matching unusual minimum spring temperatures during leaf unfolding. Several spring frosts were identified along the northern distribution of N. pumilio, being more frequent in the more continental Argentinean forests. Spring frost in 1980 had the largest spatial extent. The spring frosts in 1980 and 1992 also induced damages in regional orchards. Spring frost damage was associated with (i) a period of unusually warm temperatures at the beginning of leaf unfolding, followed by (ii) freezing temperatures. This study helps expand our understanding of the climatic constraints that could determine the future growth and dynamics of Andean deciduous forests and the potential use of tree-rings as archives of extreme events of spring frosts in northern Patagonia. الأحداث المناخية المتطرفة، مثل الصقيع المتأخر في الربيع أثناء تدفق الأوراق، لها تأثيرات كبيرة على النمو الشعاعي لأشجار الأوراق العريضة المعتدلة. على الرغم من أن جميع الأنواع ذات الأوراق العريضة معرضة للخطر، إلا أن الضرر يعتمد على خصائص المناخ المحلي والأنواع والظواهر الخاصة بها. تم التحقيق على نطاق واسع في تأثير الصقيع في أواخر الربيع في نصف الكرة الشمالي، ولكن لا يزال الحدوث المحتمل في أنواع الأشجار في نصف الكرة الجنوبي غير معروف بشكل كافٍ. هنا، نعيد بناء حدوث الصقيع الربيعي في 30 منصة للشجرة المتساقطة Nothofagus pumilio في نطاق توزيعها الشمالي في جبال الأنديز الباتاغونية. حددنا انخفاضات في عرض حلقة الشجرة على مستوى الحامل لا ترتبط بأحداث الجفاف الإقليمية أو المحلية، مما يطابق الحد الأدنى غير العادي من درجات حرارة الربيع أثناء كشف الأوراق. تم تحديد العديد من الصقيع الربيعي على طول التوزيع الشمالي لـ N. pumilio، كونها أكثر تكرارًا في الغابات الأرجنتينية القارية. كان للصقيع الربيعي في عام 1980 أكبر امتداد مكاني. كما تسبب الصقيع الربيعي في عامي 1980 و 1992 في أضرار في البساتين الإقليمية. ارتبط تلف الصقيع الربيعي بـ (1) فترة من درجات الحرارة الدافئة بشكل غير عادي في بداية ظهور الأوراق، تليها (2) درجات الحرارة المتجمدة. تساعد هذه الدراسة على توسيع فهمنا للقيود المناخية التي يمكن أن تحدد النمو والديناميكيات المستقبلية لغابات الأنديز المتساقطة والاستخدام المحتمل لحلقات الأشجار كأرشيف للأحداث المتطرفة لصقيع الربيع في شمال باتاغونيا.

AbstractWild fungi play a critical role in forest ecosystems, and its recollection is a relevant economic activity. Understanding fungal response to climate is necessary in order to predict future fungal production in Mediterranean forests under climate change scenarios. We used a 15‐year data set to model the relationship between climate and epigeous fungal abundance and productivity, for mycorrhizal and saprotrophic guilds in a Mediterranean pine forest. The obtained models were used to predict fungal productivity for the 2021–2080 period by means of regional climate change models. Simple models based on early spring temperature and summer–autumn rainfall could provide accurate estimates for fungal abundance and productivity. Models including rainfall and climatic water balance showed similar results and explanatory power for the analyzed 15‐year period. However, their predictions for the 2021–2080 period diverged. Rainfall‐based models predicted a maintenance of fungal yield, whereas water balance‐based models predicted a steady decrease of fungal productivity under a global warming scenario. Under Mediterranean conditions fungi responded to weather conditions in two distinct periods: early spring and late summer–autumn, suggesting a bimodal pattern of growth. Saprotrophic and mycorrhizal fungi showed differences in the climatic control. Increased atmospheric evaporative demand due to global warming might lead to a drop in fungal yields during the 21st century.

Increased drought frequency and severity may reshape tree species distribution in arid environments. Dioecious tree species may be more sensitive to climate warming if sex-related vulnerability to drought occurs, since lower performance of one sex may drive differential stress tolerance, sex-related mortality rates and biased sex ratios. We explored the effect of sex and environment on branch hydraulic (hydraulic conductivity and vulnerability to embolism) and trunk anatomical traits in both sexes of the dioecious conifer Juniperus thurifera L. at two sites with contrasting water availability. Additionally, we tested for a trade-off between hydraulic safety (vulnerability to embolism) and efficiency (hydraulic conductivity). Vulnerability to embolism and hydraulic conductivity were unaffected by sex or site at branch level. In contrast, sex played a significant role in xylem anatomy. We found a trade-off between hydraulic safety and efficiency, with larger conductivities related to higher vulnerabilities to embolism. At the anatomical level, females' trunk showed xylem anatomical traits related to greater hydraulic efficiency (higher theoretical hydraulic conductivity) over safety (thinner tracheid walls, lower Mork's Index), whereas males' trunk anatomy followed a more conservative strategy, especially in the drier site. Reconciling the discrepancy between branch hydraulic function and trunk xylem anatomy would require a thorough and integrated understanding of the tree structure-function relationship at the whole-plant level. Nevertheless, lower construction costs and higher efficiency in females' xylem anatomy at trunk level might explain the previously observed higher growth rates in mesic habitats. However, prioritizing efficiency over safety in trunk construction might make females more sensitive to drought, endangering the species' persistence in a drier world.

Background: Classifying species from the high tropical Andes into plant life forms (PLF) can provide a more functional understanding of tropical treeline dynamics. However, little is known about the potential response of PLFs to climate warming. Aims: The objective of this work was to evaluate the response of PLFs to environmental conditions above the upper montane forest in the Venezuelan Andes and forecast their potential distribution under warming scenarios using habitat suitability models. Methods: We classified species into nine PLFs, analysed their current distribution using CCA and non-linear multiple regressions and forecasted their potential distribution under warming scenarios. We used anatomical leaf traits for shrubs and caulescent rosettes to refine their potential responses to climate. Results: Tree cover sharply decreased with increasing elevation, while shrub and caulescent rosette distribution differed depending on their leaf traits. Projections suggested an upslope shift of all PLFs with warming and an increase in tree cover. Conclusions: The analysis of the distribution of PLFs, by linking plant adaptive strategies to topography/climate, allowed projecting changes in vegetation physiognomy in response to warming. Projections of a moderate increase in tree cover in the grassland páramo did not indicate the formation of closed forests above the forest line.

AbstractTree‐ring data has been widely used to inform about tree growth responses to drought at the individual scale, but less is known about how tree growth sensitivity to drought scales up driving changes in forest dynamics. Here, we related tree‐ring growth chronologies and stand‐level forest changes in basal area from two independent data sets to test if tree‐ring responses to drought match stand forest dynamics (stand basal area growth, ingrowth, and mortality). We assessed if tree growth and changes in forest basal area covary as a function of spatial scale and tree taxa (gymnosperm or angiosperm). To this end, we compared a tree‐ring network with stand data from the Spanish National Forest Inventory. We focused on the cumulative impact of drought on tree growth and demography in the period 1981–2005. Drought years were identified by the Standardized Precipitation Evapotranspiration Index, and their impacts on tree growth by quantifying tree‐ring width reductions. We hypothesized that forests with greater drought impacts on tree growth will also show reduced stand basal area growth and ingrowth and enhanced mortality. This is expected to occur in forests dominated by gymnosperms on drought‐prone regions. Cumulative growth reductions during dry years were higher in forests dominated by gymnosperms and presented a greater magnitude and spatial autocorrelation than for angiosperms. Cumulative drought‐induced tree growth reductions and changes in forest basal area were related, but initial stand density and basal area were the main factors driving changes in basal area. In drought‐prone gymnosperm forests, we observed that sites with greater growth reductions had lower stand basal area growth and greater mortality. Consequently, stand basal area, forest growth, and ingrowth in regions with large drought impacts was significantly lower than in regions less impacted by drought. Tree growth sensitivity to drought can be used as a predictor of gymnosperm demographic rates in terms of stand basal area growth and ingrowth at regional scales, but further studies may try to disentangle how initial stand density modulates such relationships. Drought‐induced growth reductions and their cumulative impacts have strong potential to be used as early‐warning indicators of regional forest vulnerability.