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Background and Aims The vulnerability and responsiveness of forests to drought are immensely variable across biomes. Intraspecific tree responses to drought in species with wide niche breadths that grow across contrasting climatically environments might provide key information regarding forest resistance and changes in species distribution under climate change. Using a species with an exceptionally wide niche breath, we tested the hypothesis that tree populations thriving in dry environments are more resistant to drought than those growing in moist locations. Methods We determined temporal trends in tree radial growth of 12 tree populations of Nothofagus antarctica (Nothofagaceae) located across a sharp precipitation gradient (annual precipitation of 500–2000 mm) in Chile and Argentina. Using dendrochronological methods, we fitted generalized additive mixed-effect models to predict the annual basal area increment as a function of year and dryness (De Martonne aridity index). We also measured carbon and oxygen isotope signals (and estimated intrinsic water-use efficiency) to provide potential physiological causes for tree growth responses to drought. Key Results We found unexpected improvements in growth during 1980–1998 in moist sites, while growth responses in dry sites were mixed. All populations, independent of site moisture, showed an increase in their intrinsic water-use efficiency in recent decades, a tendency that seemed to be explained by an increase in the photosynthetic rate instead of drought-induced stomatal closure, given that δ18O did not change with time. Conclusions The absence of drought-induced negative effects on tree growth in a tree species with a wide niche breadth is promising because it might relate to the causal mechanisms tree species possess to face ongoing drought events. We suggest that the drought resistance of N. antarctica might be attributable to its low stature and relatively low growth rate. [ABSTRACT FROM AUTHOR]

Worldwide drought events have been reported to cause tree growth decline and mortality, thus altering the carbon (C) balance of forest ecosystems. While most of the attention has been focused on the physiological mechanisms associated with drought‐induced tree responses of a few species at specific locations, the ecological attributes of these species, like their niche breadth, may be also important in determining species' sensitivity or resilience to drought. We postulated that wide‐niche breadth tree species should be more drought‐resilient than narrow‐niche breadth species.Using the most severe 2015–2016 El Niño drought event in the last 70 years in Patagonia, we determined pre‐ and post‐drought growth (BAI, basal area increment), C reserves in the form of non‐structural carbohydrate concentrations (NSCs = starch and soluble sugars), wood isotope (δ13C, iWUE and δ18O) signalling and xylem anatomy (mean vessel diameter, mvd) in eight angiosperm tree species of contrasting niche breadth across a sharp precipitation gradient in southern Chile.All species responded in unison after the drought with a non‐water‐conservative response, maintaining BAI and NSCs, decreasing δ13C, and increasing both mvd and the soluble sugars:NSCs ratio relative to pre‐drought time. Contrary to previous results reporting species‐specific drought responses, our results show unequivocally a functional coordination of organisms' vital traits associated with a non‐water‐conservative strategy, and a drought‐induced acclimation based on starch conversion into soluble sugars in all of the tree species we examined, regardless of their niche breadth and habitat preference.We state that abiotic drivers such as drought may have selected similar interspecific responses provided that they operate at the community level rather than at the species level. These findings mark the need to revise current views about the ultimate interspecific functional coordination of organisms' vital traits when facing more frequent and intensive drought events. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Climate warming is expected to positively alter upward and poleward treelines which are controlled by low temperature and a short growing season. Despite the importance of treelines as a bioassay of climate change, a global field assessment and posterior forecasting of tree growth at annual scales is lacking. Using annually resolved tree‐ring data located across Eurasia and the Americas, we quantified and modeled the relationship between temperature and radial growth at treeline during the 20th century. We then tested whether this temperature–growth association will remain stable during the 21st century using a forward model under two climate scenarios (RCP 4.5 and 8.5). During the 20th century, growth enhancements were common in most sites, and temperature and growth showed positive trends. Interestingly, the relationship between temperature and growth trends was contingent on tree age suggesting biogeographic patterns in treeline growth are contingent on local factors besides climate warming. Simulations forecast temperature–growth decoupling during the 21st century. The growing season at treeline is projected to lengthen and growth rates would increase and become less dependent on temperature rise. These forecasts illustrate how growth may decouple from climate warming in cold regions and near the margins of tree existence. Such projected temperature–growth decoupling could impact ecosystem processes in mountain and polar biomes, with feedbacks on climate warming. [ABSTRACT FROM AUTHOR]

Land set aside for preservation of biodiversity often has low productivity. As biodiversity generally increases with productivity, due to higher or more diverse availability of resources, this implies that some of the biodiversity may be left unprotected. Due to a lack of knowledge on the species diversity and conservation value of low‐productivity habitats, the consequences of the biased allocation of low‐productivity land for set‐asides are unknown.We examined the conservation value of boreal low‐productivity forests (potential tree growth <1 m3 ha−1 year−1) by comparing assemblages of tree‐ and deadwood‐dwelling lichens and forest stand structure between productive and low‐productivity forest stands. We surveyed 84 Scots pine‐dominated stands in three regions in Sweden, each including four stand types: two productive (managed and unmanaged) and two low‐productivity stands (on mires and on thin, rocky soils).Lichen species richness was the highest in low‐productivity stands on thin soil, which had similar amounts and diversity of resources (living trees and dead wood) to productive unmanaged stands. Stands in low‐productivity mires, which had low abundance of living trees and dead wood, hosted the lowest lichen richness. Lichen species composition differed among stand types, but none of them hosted unique species. The differences in both species richness and composition were more pronounced in northern than in southern Sweden, likely due to shorter history of intensive forestry.Synthesis and applications. Boreal low‐productivity forests can have as high conservation value as productive forests, which should be reflected in conservation strategies. However, their value is far from uniform, and conservation planning should acknowledge this variation and not treat all low‐productivity forests as a uniform group. Some types of low‐productivity forest (e.g. on rocky soil) are more valuable than others (e.g. on mires), and should thus be prioritized in conservation. It is also important to consider the landscape context: low‐productivity forests may have higher value in landscapes where high‐productivity forests are highly influenced by forestry. Finally, although low‐productivity forests can be valuable for some taxa, productive forests may still be important for other taxa. [ABSTRACT FROM AUTHOR]

Aims: Rising temperature and declining summer precipitation due to the 1970s‐climate shift in southern South America have reduced forest productivity at dry sites. Here, we worked with the most widespread Southern Hemisphere tree line species, Nothofagus pumilio, across contrasting climatic conditions and determined whether rising atmospheric CO2 concentrations as well as warmer and drier climatic conditions provoked by the 70s‐climatic shift have been causing systematic changes in tree line growth rates and intrinsic water‐use efficiency (iWUE). Location: 36–54°S, southern Andes. Time period: 1950–2010. Major taxa studied: Nothofagus pumilio. Methods: We worked at five disparate climatic tree line locations, spanning 18 degrees of latitude; at each location, we sampled trees at four different elevations, including tree line elevation. We quantified the variation in annual tree‐ring width (TRW) as a function of climate, elevation, tree age, size, annual CO2 concentrations and location, using linear mixed‐effects models and interpreted TRW trends in relation to iWUE and isotope (δ13C and δ18O) signalling. Results: Across locations, the patterns of tree line growth occurring in the 1980–2010 period exhibited a clear and significant negative trend, in contrast to the previous 1950–1980 period. We found an increase in iWUE and δ18O across time and locations. Given that an increase in δ18O indicates a decrease in stomatal conductance, we assert that drought‐induced stomatal closure appears to be causing the reduction in growth. Main conclusions: We show unequivocal evidence that warmer and drier summer conditions translated into a decrease in growth rates along the elevational tree line of the southern Andes, reverting previous growth improvements linked to climate warming. An improvement in iWUE at all locations is most likely explained by decreased stomatal conductance given the rising δ18O signal. An iWUE–growth decoupling may act as an ecological strategy to respond to drought. [ABSTRACT FROM AUTHOR]

Climate change and rising atmospheric CO 2 concentrations ( c a ) are expected to affect forests worldwide. The effects of climate change, however, have not been deeply assessed in humid forest biomes from the southern Hemisphere where climate is not warming but drying. This is the case of the temperate rainforest in southern Chile, where the endemic and threatened long-living gymnosperm Fitzroya cupressoides occurs. We assessed how radial growth, intrinsic water-use efficiency (iWUE) and tree-ring δ 18 O responded to increasing c a and decreasing precipitation in F. cupressoides and companion species. We hypothesized that F. cupressoides , a long-lived and probably less plastic species, will show less acclimation to global-change effects than co-occurring Nothofagus species which show broader climatic niche. Thus, F. cupressoides should display iWUE increases different from the c i / c a constant scenario, which represents an active mechanism to increase intercellular CO 2 concentrations ( c i ) as c a rises. Although cool and wet conditions during the growing season enhanced growth of all species, particularly in F. cupressoides , growth of F. cupressoides declined noticeably since the 1980s in response to a decrease in precipitation. Current drier conditions led to increased iWUE in Nothofagus species. According to δ 18 O values, this increased in iWUE should be due to a decrease in stomatal conductance. Fitzroya cupressoides , however, displayed a decrease in iWUE in response to drier conditions, shifting from an active c i / c a scenario to a more passive c i / c a scenario, and maintaining a relatively constant stomatal conductance. Using multiple bodies of evidence, our findings indicate a poor adaptability of the long-lived F. cupessoides to drier conditions despite rising c a . Thus, not all species are having similar and expected responses to increasing c a , which should be a call of attention in the case of long-lived, endangered and narrow-distributed species, like F. cupressoides . [ABSTRACT FROM AUTHOR]

If isotherms shift upslope or poleward due to climate warming, treeline trees will be exposed to more or less thermal constraints of growth depending on treeline shift rates. Climate warming is expected to enhance tree growth at alpine treelines. Isotherms may move upslope, but treeline trees may not; our study focused on trees growing at the altitudinal or latitudinal limits of tree existence today regardless of where the treeline will be in the future. [Extracted from the article]

Background and Aims There is a growing concern about how forests will respond to increased herbivory associated with climate change. Carbon (C) and nitrogen (N) limitation are hypothesized to cause decreasing growth after defoliation, and eventually mortality. This study examines the effects of a natural and massive defoliation by an insect on mature trees' C and N storage, which have rarely been studied together, particularly in winter-deciduous species. Methods Survival, growth rate, carbon [C, as non-structural carbohydrate (NSC) concentration] and nitrogen (N) storage, defences (tannins and total polyphenols), and re-foliation traits were examined in naturally defoliated and non-defoliated adult trees of the winter-deciduous temperate species Nothofagus pumilio 1 and 2 years after a massive and complete defoliation caused by the caterpillar of Ormiscodes amphimone (Saturniidae) during summer 2009 in Patagonia. Key Results Defoliated trees did not die but grew significantly less than non-defoliated trees for at least 2 years after defoliation. One year after defoliation, defoliated trees had similar NSC and N concentrations in woody tissues, higher polyphenol concentrations and lower re-foliation than non-defoliated trees. In the second year, however, NSC concentrations in branches were significantly higher in defoliated trees while differences in polyphenols and re-foliation disappeared and decreased, respectively. Conclusions The significant reduction in growth following defoliation was not caused by insufficient C or N availability, as frequently assumed; instead, it was probably due to growth limitations due to factors other than C or N, or to preventative C allocation to storage. This study shows an integrative approach to evaluating plant growth limitations in response to disturbance, by examining major resources other than C (e.g. N), and other C sinks besides storage and growth (e.g. defences and re-foliation). [ABSTRACT FROM AUTHOR]

Background and Aims Multi-stemmed trees (tree clusters) in Nothofagus pumilio, a dominant tree species in Patagonia, are very uncommon and are restricted to the edge of second-growth forests following human-provoked fires. No vegetative reproduction has been reported so far. The genetic structure of multi-stemmed trees of this species was investigated and it was hypothesized that genets within a cluster were more closely related than average in the population. Methods Fifteen clusters (composed of at least three purported stems) and 15 single trees were sampled at the edge of a second-growth forest and genotyped using two amplified fragment length polymorphism (AFLP) primer pairs. We obtained 119 polymorphic markers that allowed clonality to be determined, together with sibship structure and relatedness among samples. Key Results Clonality was detected in seven clusters but all clusters had at least two different genotypes. Full sibs were found exclusively within clusters and in all clusters. Within a cluster, stems that were not identified as full sibs were often half sibs. Relatedness values for the full sibs and half sibs were higher than the theoretical values of 0·5 and 0·25 but the relatedness between clusters was very low. Conclusions Tree clusters that are merged at the edge of the second-growth forest of N. pumilio are composed of stems of the same genotype and of other genotypes that are highly related (but not always). It is suggested that this peculiar genetic structure results from a combination of several causes, including selection for merging of related individuals. [ABSTRACT FROM AUTHOR]