Arboreal and prostrate conifers coexisting in Mediterranean high mountains differ in their climatic responses

In contrast to most high elevation areas, plant growth at Mediterranean mountains is exposed to a summer drought period, which represents an additional climatic constraint to low temperatures. Although arboreal and shrubby conifers coexist at high altitudes, most dendroecological studies have focused on climatic responses of tree species, whereas those of shrubby species remain mostly unexplored. We built tree-ring width chronologies for two conifer species, a shrub (Juniperus sabina) and a tree (Pinus sylvestris), coexisting at three high-altitude localities of the Iberian System mountains, eastern Spain. We analyzed their climate–growth relationships for the period 1950–2009 using correlation analyses and multiple regressions. Coexisting species responded to year-to-year climatic variability in different ways. Radial growth in junipers and pines responded positively to April and May temperatures, respectively. Summer drought constrained growth in both cases, although its impact was stronger on junipers than on pines. Our findings suggest that junipers respond earlier than pines to spring temperatures due to their prostrate morphology which may enhance a fast warming of their cambial meristems after snowmelt. The higher dependence of J. sabina on summer rainfall as compared with co-occurring pines confirms that drought stress negatively impacts secondary growth in Mediterranean mountains. This sensitivity to water availability may be caused by the juniper shallow root systems, which mainly use superficial soil water. The climatic signal registered in J. sabina allows studying the response of other similar shrubby woody species growing in Mediterranean alpine areas to the ongoing climate warming, which could also reduce water availability.     

Abiotic factors modulate post-drought growth resilience of Scots pine plantations and rear-edge Scots pine and oak forests

The proportion of planted forests in the Mediterranean Basin is one of the largest in the world. These plantations are dominated by pine species and present a series of characteristics such as low elevation, high competition or small tree size that make them more vulnerable to droughts. However, quantitative assessments of their post-drought growth resilience in accordance with species, site factors and tree characteristics are lacking. In this study we sampled 164 trees at four forest sites located in the drought-prone Sierra Nevada, southeastern Spain. We compared growth responsiveness to drought in rear-edge planted vs. relic natural Scots pine (Pinus sylvestris) and coexisting Pyrenean oak (Quercus pyrenaica) stands. Our objective was to characterize and compare the different growth responses to drought between species and sites and the effect of the main physiographic factors (altitude, aspect, and slope) on these responses since the influence of these factors on post-drought resistance and resilience has received little attention to date. Our results reveal that the planted pine sites with the lowest mean growth rates displayed greater resistance during drought, and that higher altitude was associated with improved resistance and/or resilience for all species and sites. Natural pine and Pyrenean oak stands were better adapted to the dry climatic conditions of the Mediterranean region where the study was undertaken, displaying greater resistance and/or resilience and lower influence of drought on growth in comparison to stands of planted pines. These results suggest that promoting the conservation of high-elevation pine plantations and enhancing the regeneration of natural pine and oak may improve the resistance and resilience of these drought-prone forest ecosystems.     

Assessing forest vulnerability to climate warming using a process‐based model of tree growth: bad prospects for rear‐edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of ?10.7% and ?16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.     

Variation in woody plant mortality and dieback from severe drought among soils, plant groups, and species within a northern Arizona ecotone

Vegetation change from drought-induced mortality can alter ecosystem community structure, biodiversity, and services. Although drought-induced mortality of woody plants has increased globally with recent warming, influences of soil type, tree and shrub groups, and species are poorly understood. Following the severe 2002 drought in northern Arizona, we surveyed woody plant mortality and canopy dieback of live trees and shrubs at the forest–woodland ecotone on soils derived from three soil parent materials (cinder, flow basalt, sedimentary) that differed in texture and rockiness. Our first of three major findings was that soil parent material had little effect on mortality of both trees and shrubs, yet canopy dieback of trees was influenced by parent material; dieback was highest on the cinder for pinyon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma). Ponderosa pine (Pinus ponderosa) dieback was not sensitive to parent material. Second, shrubs had similar mortality, but greater canopy dieback, than trees. Third, pinyon and ponderosa pines had greater mortality than juniper, yet juniper had greater dieback, reflecting different hydraulic characteristics among these tree species. Our results show that impacts of severe drought on woody plants differed among tree species and tree and shrub groups, and such impacts were widespread over different soils in the southwestern U.S. Increasing frequency of severe drought with climate warming will likely cause similar mortality to trees and shrubs over major soil types at the forest–woodland ecotone in this region, but due to greater mortality of other tree species, tree cover will shift from a mixture of species to dominance by junipers and shrubs. Surviving junipers and shrubs will also likely have diminished leaf area due to canopy dieback.     

Different responses of Korean pine (Pinus koraiensis) and Mongolia oak (Quercus mongolica) growth to recent climate warming in northeast China

Different tree species growing in the same area may have different, or even contrasting growth responses to climate change. Korean pine (Pinus koraiensis) and Mongolia oak (Quercus mongolica) are two crucial tree species in temperate forest ecosystems. Six tree-ring chronologies for Korean pine and Mongolia oak were developed by using the zero-signal method to explore their growth response to the recent climate warming in northeast China. Results showed that Mongolia oak radial growth was mainly limited by precipitation in the growing season, while Korean pine growth depended on temperature condition, especially monthly minimum temperature. With the latitude decrease, the relationships between Korean pine growth and monthly precipitation changed from negative to positive correlation, while the positive correlation with monthly temperature gradually weakened. In the contrary, Mongolia oak growth at the three sampling sites was significantly and positively correlated with precipitation in the growing season, while it was negatively correlated with temperature and this relationship decreased with the latitude decrease. The radial growth of Korean pine at different sites showed a clearly discrepant responses to the recent warming since 1980. Korean pine growth in the north site increased with the temperature increase, decreased in the midwest site, and almost unchanged in the southeast site. Conversely, Mongolia oak growth was less affected by the recent climate warming. Our finding suggested that tree species trait and sites are both key factors that affect the response of tree growth to climate change. In addition, the suitable distribution area of Korean pine may be moved northward with the continued global warming in the future, but Mongolia oak may not shift in the same way.     

Climate–tree growth relationships of longleaf pine (Pinus palustris Mill.) in the Southeastern Coastal Plain, USA

Knowledge of tree growth/climate response relationships is important to dendroecological studies and dendroclimatic reconstructions, particularly in the Southeastern Coastal Plain where few such studies have been attempted. To this end, we developed tree-ring chronologies of total ring width, earlywood width, and latewood width from longleaf pine (Pinus palustris Mill.) at three sites in the Southeastern Coastal Plain to examine the climate–growth relationships for this tree species. The length of these chronologies is unprecedented for southern pine chronologies in the Southeast. We compared the tree-ring chronologies to monthly temperature, precipitation, Palmer drought severity index (PDSI), and Palmer hydrological drought index (PHDI) data from the pertinent climate divisions. We found that PDSI and PHDI have the highest correlation with longleaf pine growth, and the strongest relationships between longleaf pine growth and these variables occur between July and November. Precipitation in the spring and summer was also positively related to growth at all sites. The relationship between temperature and growth was the weakest among all climate variables, but warm summer temperatures had a consistent, negative relationship with longleaf pine growth. The climate signal in the latewood was generally more robust than for total ring width and earlywood width.     

Scots pine plantations growth adaptation to climate warming in locations at the southernmost distribution limit of the species

Under the current climate change conjuncture, understanding the forest plantations capacity of acclimation to warming and increased drought stress is crucial for forest managers. To get some understanding of their adaptability, plantations of similar provenance but located in climatically contrasting sites can be compared. Here we study the growth dynamics and their relationship with climate and drought in two Scots pine (Pinus sylvestris L.) plantations located in the center (Sierra de Guadarrama, wetter site) and south (Sierra Nevada, drier site) of Spain, the latter situated at the southernmost distribution limit of the species. Our objectives are to quantify the trends in radial growth of these plantations, to quantify the influence of climate on growth, and to project the plantations growth as a function of forecasted climate. Results reveal that the plantations from the drier site show lower, and less responsive to climate, growth and greater resilience than those from the wetter site. Furthermore, if the current climate-growth relationships continue in the future, these plantations would maintain the current limited growth rate during the 21st century. On the contrary, plantations from the wetter site show higher growth rate and more resistance to drought, and they are projected to increase growth under the warmer conditions forecasted for the 21st century. Our study shows that plantations in drier sites may have a great capacity to acclimate to local climate conditions and would not be negatively impacted by the projected climate warming.     

Drought and cold spells trigger dieback of temperate oak and beech forests in northern Spain

Dieback in temperate forests is understudied, despite this biome is predicted to be increasingly affected by more extreme climate events in a warmer world. To evaluate the potential drivers of dieback we reconstructed changes in radial growth and intrinsic water-use efficiency (iWUE) from stable isotopes in tree rings. Particularly, we compared tree size, radial-growth trends, growth responses to climate (temperature, precipitation, cloudiness, number of foggy days) and drought, and changes in iWUE of declining and non-declining trees showing contrasting canopy dieback and defoliation. This comparison was done in six temperate forests located in northern Spain and based on three broadleaved tree species (Quercus robur, Quercus humilis, Fagus sylvatica). Declining trees presented lower radial-growth rates than their non-declining counterparts and tended to show lower growth variability, but not in all sites. The growth divergence between declining and non-declining trees was significant and lasted more in Q. robur (15–30 years) than in F. sylvatica (5–10 years) sites. Dieback was linked to summer drought and associated atmospheric patterns, but in the wettest Q. robur sites cold spells contributed to the growth decline. In contrast, F. sylvatica was the species most responsive to summer drought in terms of growth reduction followed by Q. humilis which showed coupled changes in growth and iWUE as a function of tree vigour. Low growth rates and higher iWUE characterized declining Q. robur and F. sylvatica trees. However, declining F. sylvatica trees became less water-use efficient close to the dieback onset, which could indicate impending tree death. In temperate forests, dieback and growth decline can be triggered by climate extremes such as dry and cold spells, and amplified by climate warming and rising drought stress.     

The energy–water limitation threshold explains divergent drought responses in tree growth,needle length,and stable isotope ratios

Predicted increases in extreme droughts will likely cause major shifts in carbon sequestration and forest composition. Although growth declines during drought are widely documented, an increasing number of studies have reported both positive and negative responses to the same drought. These divergent growth patterns may reflect thresholds (i.e., nonlinear responses) promoted by changes in the dominant climatic constraints on tree growth. Here we tested whether stemwood growth exhibited linear or nonlinear responses to temperature and precipitation and whether stemwood growth thresholds co-occurred with multiple thresholds in source and sink processes that limit tree growth. We extracted 772 tree cores, 1398 needle length records, and 1075 stable isotope samples from 27 sites across whitebark pine's (Pinus albicaulis Engelm.) climatic niche in the Sierra Nevada. Our results indicated that a temperature threshold in stemwood growth occurred at 8.4°C (7.12–9.51°C; estimated using fall-spring maximum temperature). This threshold was significantly correlated with thresholds in foliar growth, as well as carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios, that emerged during drought. These co-occurring thresholds reflected the transition between energy- and water-limited tree growth (i.e., the E–W limitation threshold). This transition likely mediated carbon and nutrient cycling, as well as important differences in growth-defense trade-offs and drought adaptations. Furthermore, whitebark pine growing in energy-limited regions may continue to experience elevated growth in response to climate change. The positive effect of warming, however, may be offset by growth declines in water-limited regions, threatening the long-term sustainability of the recently listed whitebark pine species in the Sierra Nevada.     

Coupled plant traits adapted to wetting/drying cycles of substrates co-define niche multidimensionality

Theories attempting to explain species coexistence in plant communities have argued in favour of species' capacities to occupy a multidimensional niche with spatial, temporal and biotic axes. We used the concept of hydrological niche segregation to learn how ecological niches are structured both spatially and temporally and whether small scale humidity gradients between adjacent niches are the main factor explaining water partitioning among tree species in a highly water-limited semiarid forest ecosystem. By combining geophysical methods, isotopic ecology, plant ecophysiology and anatomical measurements, we show how coexisting pine and oak species share, use and temporally switch between diverse spatially distinct niches by employing a set of functionally coupled plant traits in response to changing environmental signals. We identified four geospatial niches that turned into nine, when considering the temporal dynamics of the wetting/drying cycles in the substrate and the particular plant species adaptations to garner, transfer, store and use water. Under water scarcity, pine and oak exhibited water use segregation from different niches, yet under maximum drought when oak trees crossed physiological thresholds, niche overlap occurred. The identification of niches and mechanistic understanding of when and how species use them will help unify theories of plant coexistence and competition.     

Tree-ring growth of Pinus nigra Arn. subsp. pallasiana under different climate conditions throughout western Anatolia

Pinus nigra Arn. subsp. pallasiana (black pine) is one of the most widely grown tree in Turkey. It is the third most widely distributed tree species after Quercus L. and Pinus brutia Ten. Black pine grows in 20% of all forested areas in Turkey. In this dendroecological study, we identified the most important climate factors affecting radial growth of black pine in western Anatolia and classified its responses to climate. Twenty-eight site chronologies developed by different researchers were used in the analysis. Response functions were calculated for each chronology to identify the effect of climate on radial growth. Hierarchical cluster analysis was used to sort response functions and to classify the chronologies into groups based on climate responses. The individual responses of these chronologies to temperature and precipitation were classified in four main groups. Climatic and phytogeographic differences were the major factors influencing the formation of clusters. The results suggest that the major limiting factor is drought caused by low precipitation, especially in May, in almost all sites. The drought effect is much stronger in the transition region to the steppe, Central Anatolia and Mediterranean Regions than the Black Sea Region. Black pine trees respond positively to higher temperature at the beginning of growing season in almost all areas except in transition region to the steppe.     

Pine invasion impacts on plant diversity in Patagonia: invader size and invaded habitat matter

Conifers, which are widely planted as fast growing tree crops, are invading forested and treeless environments across the globe, causing important changes in biodiversity. However, how small-scale impacts on plant diversity differ according to pine size and habitat context remains unclear. We assessed the effects of different stages of pine invasion on plant communities in forest and steppe sites located in southern Chile. In each site, we sampled plant diversity under and outside the canopy of Pinus contorta individuals, using paired plots. We assessed the relative impact of pine invasion on plant species richness and cover. In both sites, richness and cover beneath pine canopy decreased with increasing pine size (i.e. height and canopy area). A significant negative impact of pines on species richness and plant cover was detected for pines over 4 m in height. The impact of pines on plant richness and cover depended on pine size (i.e. canopy area) and habitat type. Larger pines had more negative impacts than smaller pines in both sites, with a greater impact for a given pine size in the Patagonian steppe compared to the A. araucaria forest. Species composition changed between under and outside canopy plots when pines were 4 m or taller. Pine presence reduced cover of most species. The impacts of pine invasions are becoming evident in forested and treeless ecosystems of southern Chile. Our results suggest that the magnitude of pine invasion impacts could be related to how adapted the invaded community is to tree cover, with the treeless environment more impacted by the invasion.     

Growth patterns in relation to drought-induced mortality at two Scots pine (Pinus sylvestris L.) sites in NE Iberian Peninsula

Drought-related tree mortality has become a widespread phenomenon. Scots pine (Pinus sylvestris L.) is a boreal species with high ecological amplitude that reaches its southwestern limit in the Iberian Peninsula. Thus, Iberian Scots pine populations are particularly good models to study the effects of the increase in aridity predicted by climate change models. A total of 78 living and 39 dead Scots pines trees were sampled at two sites located in the NE of the Iberian Peninsula, where recent mortality events have been recorded. Annual tree rings were used to (1) date dead trees; (2) investigate if there was an association between the occurrence of tree death and severe drought periods characterized by exceptionally low ratios of summer precipitation to potential evapotranspiration (P/PET); and (3) to compare the growth patterns of trees that died with those of surviving ones. Mixed models were used to describe the relationships between tree growth (in terms of basal area increment, BAI, and the percentage of latewood, LW%) and climate variables. Our results showed a direct association between Scots pine mortality and severe drought periods characterized by low summer water availability. At the two sites, the growth patterns of dead trees were clearly distinguishable from those of the trees that survived. In particular, the BAI of dead trees was more sensitive to climate dryness (low P/PET_summer, high temperatures) and started to decline below the values of surviving neighbors 15–40 years before the time of death, implying a slow process of growth decline preceding mortality.     

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.     

Drought-induced tree growth decline in the desert margins of Northwestern China

Water deficiency is the primary limiting factor for tree growth in arid and semi-arid areas. Droughts associated with rising temperatures have increased in severity and frequency globally over the past few decades, making the trees in the drought-prone sites first be affected by water shortages. However, our understanding of tree growth status in these areas, and of their response to drought, is currently insufficient; especially in the context of global warming. Here, we studied 94 Chinese pine (Pinus tabulaeformis) and 86 spruce (Picea crassifolia) trees from different altitudes [2,100–2400 m above sea level (a.s.l.)] distributed at the desert margins of Northwestern China to explore tree growth and drought response from multiple perspectives using dendroecological approaches. Significant growth decline, across all tree species and altitudes, was detected in response to an interdecadal trend towards a drier climate. Moreover, the extent of tree growth decline, the proportion of affected trees, and the degree of moisture dependence have all tended to increase in each sample site, most likely due to enhanced drought severity and duration in recent decades. The more sensitive and susceptible trees were found at lower elevations (drier sites) and may signify a higher vulnerability to heating-induced drought stress. Tree resistance to drought showed strong negative correlation with drought severity across all sample sites. However, the connection between post-drought tree resilience and drought intensity is weak, perhaps because the samples were all collected from living trees, while those that had died were not sampled. The priority for future work should be to combine surviving and dead trees simultaneously, thus achieving a more representative view of tree resilience to drought; this will improve our knowledge of forest dynamics and even ecosystem succession in these vulnerable and sensitive environments.     

Species-specific growth-climate responses of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) in the Greater Khingan Range,northeast China

Responses of tree growth to climate are usually spatially heterogeneous. Besides regionally varying external environments, species specificity is a crucial factor in determining said spatial heterogeneity. A better understanding of this species specificity would improve our estimations of the warming effects on forests. In this study, we selected two widely-distributed boreal conifers, Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica), to compare their growth-climate responses, including long-term growth-climate correlations and short-term growth resilience to drought. We sampled 160 trees and 481 tree-ring cores from the two species in two pure and two mixed forests, located in the Greater Khingan Range, northeast China. We found that Dahurian larch was generally positively correlated with spring temperature and negatively correlated with summer temperature. In contrast, Mongolian pine was more sensitive to summer moisture. Our results suggest that the main climatic limitations were low spring temperatures for Dahurian larch and summer moisture deficits for Mongolian pine. Dahurian larch represented higher growth resistance to drought, while Mongolia pine represented higher recovery. Based on this, we inferred that Dahurian larch was more vulnerable to extreme droughts, while Mongolian pine was more vulnerable to frequent droughts. We also demonstrated the effects of forest type on growth-climate responses. The negative effects of summer temperatures on Mongolian pine seemed to be more significant in mixed forests. As warming continued, Mongolian pine in this area would suffer severer moisture deficits, especially when coexisting with Dahurian larch. Our results suggest that Dahurian larch gained an advantage in the competition with Mongolian pine during high moisture stress. Driven by the warming trends, the species specificity in growth response would ultimately promote the separation of the two species in distribution. This study will help improve our estimations of the warming effects on forests and develop more species-targeted forest management practices.     

Water availability,fundamental niches and realized niches: A case study from the Cape flora

The ability of plants to survive drought or waterlogging constitutes an important niche parameter, which might be particularly significant in explaining species coexistence in the species‐rich and seasonally dry Cape Floristic Region of South Africa. However, the degree of physiological adaptation and specialization to these eco‐hydrological parameters (the fundamental niche) cannot be readily inferred from correlative studies based on species distributions and spatial variation in environmental parameters (the realized niche). We used an ex situ greenhouse experiment to compare the fundamental hydrological niches (different mean annual precipitation, rainfall seasonality and soil drainage) of six eco‐hydrologically divergent African Restionaceae species. Juvenile plants were subjected to six different watering treatments, ranging from no watering to waterlogging, to determine drought and waterlogging susceptibility and optimal growth conditions. We used the rate of biomass accumulation and survival rate as response measures. We found that species from dry and mesic (but well‐drained) habitats had optimal or near‐optimal growth at benign conditions (under which most restio species grow well). All species performed worse when droughted and died when not watered. Species from dry habitats tended to perform better (assessed in growth) than species from wet habitats under droughting. Species from wet habitats performed best when waterlogged, whereas species from dry habitats performed very poorly when waterlogged – thus showing that realized and fundamental niches covaried at the wet end of the hydrological gradient. We conclude that eco‐hydrological parameters are part of the fundamental niche, and fundamental and realized species niches are approximately correlated along them. The distribution of wet habitat species appears not to reflect their drought tolerance, suggesting that it may not be predicted by bioclimatic variables, but rather by soil drainage characteristics.     

Spatiotemporal variability of stone pine (Pinus pinea L.) growth response to climate across the Iberian Peninsula

Climate warming and increasing aridity have impacted diverse ecosystems in the Mediterranean region since at least the 1970s. Pinus pinea L. has significant environmental and socio-economic importance for the Iberian Peninsula, so a detailed understanding of its response to climate change is necessary to predict its status under future climatic conditions. However, variability of climate and uncertainties in dendroclimatological approach complicate the understanding of forest growth dynamics. We use an ensemble approach to analyze growth-climate responses of P. pinea trees from five sites along a latitudinal gradient in Spain over time. The growth responses to April-June precipitation totals were stronger in the north than in the south. Since the 1950s, the sensitivity of growth to April-June precipitation increased in the north and decreased in the south. Meteorological drought usually started in May in the southern sites, but in June-July in the northern sites. The water deficit in the southern sites is thus greater and more limiting for tree growth, and this likely accounts for the lower growth sensitivity during these months. Our results indicate that P. pinea has a high degree of plasticity, suggesting the species will withstand changing climatic conditions. However, growth response to drought regimes varies among P. pinea populations, suggesting that different populations have different capacities for acclimation to warmer and drier climate, and this may influence future vegetation composition.     

Long- and short-term impacts of a defoliating moth plus mistletoe on tree growth,wood anatomy and water-use efficiency

Climate warming and biotic stressors are expected to reduce tree radial growth and performance at short and long time scales. However, the impacts of different biotic stressors on performance throughout a tree’s life are largely understudied. Here we assessed the effects of a past nun moth (Lymantria dispar) outbreak and related defoliation on Scots pine (Pinus sylvestris) trees, which were later severely infested by the hemiparasite mistletoe (Viscum album subsp. austriacum). We compared the responses of trees severely infested or not infested by mistletoe in a wet vs. a dry site to quantify the relative importance of biotic stressors under different climate conditions. We used dendrochronology to quantify: long- and short-term changes in radial growth (resilience), differences in wood anatomy during the outbreak, and recent changes in intrinsic water-use efficiency (WUEi). The outbreak caused a sharp growth reduction in 1953 (50% decrease in basal area increment –BAI) and the formation of tracheids of small transversal lumen diameter (33% decrease in diameter). Recent mistletoe infestation caused a difference in growth between infested and non-infested trees lasting 34 and 21 years in the wet and dry sites, respectively. Growth (BAI) decreased more steeply in severely infested than in non-infested trees, the post-drought resilience decreased in severely infested trees, and the WUEi increased, particularly in the dry site. The BAI of severely infested trees was more negatively impacted by warm and dry conditions during the growing season than in non-infested trees, particularly in the dry site. Tree rings recorded historical effects of biotic stressors (L. monacha outbreak), which may constrain responses to recent stressors (mistletoe).     

Spatio-temporal growth variability of three Pinus species of Northeast Himalaya with relation to climate

This research aimed to evaluate spatio-temporal growth variability of three Pinus species viz. Pinus kesiya (Khasi pine), Pinus merkusii (Merkus pine) and Pinus wallichiana (Blue pine) along with the existence of species differentiation among the taxa in northeast India. Several statistical analyses were used, namely Pearson correlation and multivariate approaches involving UPGMA Cluster Analysis; ordination methods by Principal Component Analysis (PCA) and Non-metric multidimensional scaling (NMDS) on tree-ring width chronologies from 13 sites. The tree growth-climate relationships were assessed with both correlation and bootstrap response function using regional climate datasets of each sampling site prepared by averaging the nearest grid points of 0.5 × 0.5° of CRU TS-2.1 climate dataset. Pronounced species differentiation in the growth pattern among the three Pinus taxa was inferred. The observed spatio-temporal variability revealed inter-species tree growth variations were not uniform suggesting no common factor influenced the radial tree growth in this region, which may be related to anthropogenic impact or non-climatic factors. The tree growth-climate relationship showed that climatic factors limiting the radial growth of Pine are mostly similar for intra-species but diverse in inter-species. This study is extremely relevant in terms of species and site selection for the long-term climate reconstruction and forest management in the Northeast Himalaya.