https://hal.archives-ouvertes.fr/hal-00457602/document

A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests

▶ Addresses a wide range of timely environment, economic and energy topics ▶ A forum to review, analyze and stimulate the development, testing and implementation of mitigation and adaptation strategies at regional, national and global scales ▶ Contributes to real-time policy analysis and development as national and international policies and agreements are discussed and promulgated ▶ 94% of authors who answered a survey reported that they would definitely publish or probably publish in the journal again

Mitigation and Adaptation Strategies for Global Change

http://sylvain-delzon.com/wp-content/uploads/2013/02/Lindner-et-al-2010-FEM.pdf

Climate change impacts, adaptive capacity, and vulnerability of European forest ecosystems

Patterns, mechanisms, projections, and consequences of tree mortality and associated broad-scale forest die-off due to drought accompanied by warmer temperatures—“hotter drought”, an emerging characteristic of the Anthropocene—are the focus of rapidly expanding literature. Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortality-relevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO2] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.

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On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009

Increasing temperatures are driving rapid upward range shifts of species in mountains. An altitudinal range retreat of 10 m is predicted to translate into a ∼10-km latitudinal retreat based on the rate at which temperatures decline with increasing altitude and latitude, yet reports of latitudinal range retractions are sparse. Here, we examine potential climatic, biological, anthropogenic and methodological explanations for this disparity. We argue that the lack of reported latitudinal range retractions stems more from a lack of research effort, compounded by methodological difficulties, rather than from their absence. Given the predicted negative impacts of increasing temperatures on wide areas of the latitudinal distributions of species, the investigation of range retractions should become a priority in biogeographical research.

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The altitude-for-latitude disparity in the range retractions of woody species

Provenance tests of forest trees, which were originally intended to identify suitable seed sources for planting at different locations, provide valuable data for assessing the response of populations to environmental change. Environmental differences between the location of origin and the planting (test) site have been calculated by principal component analysis and termed ecological distance. Based on ecological distance values, the growth response of tree populations can be modeled as a function of the test site macroclimate. These models can then be used to predict the effects of climatic change on growth and survival. The growth response model predicts that increasing annual mean temperatures will result in accelerated growth if precipitation is sufficient, but only within the limits characteristic of the species. At the southern limits of distribution, growth and competitive ability of the species will decline, leading to successional changes.

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Modeling climate change effects with provenance test data

Common garden testing of populations of different origin started with forest trees more than two hundred years ago. Since then, so-called provenance tests have been established with most commercially important species. Beyond the strictly silvicultural goals, the tests offer excellent opportunities to study intraspecific genetic variation patterns and represent probably the most powerful available tool for testing hypotheses of climatic adaptation in trees.

Climatic adaptation of trees: rediscovering provenance tests

As tree habitats shift towards the poles in response to climate change, we must study the neglected, trailing edges of forests, warns Csaba Matyas — they are economically and ecologically important.

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Forecasts needed for retreating forests

Xeric (trailing) forest range limits are particularly vulnerable to impacts of predicted climate change. Regional modelling studies contribute to the identification of potential local climatic threats and may support appropriate management strategies. We carried out bioclimatic distribution modelling of two climate-dependent, dominant tree species, beech and sessile oak, to determine the most influential climatic variables limiting their distributions and to predict their climate-induced range shifts over the twenty-first century in the forest-steppe biome transition zone of Hungary. To exclude confounding effects of edaphic conditions, only data of zonal sites were evaluated. For both species, temperature and precipitation conditions in late spring and summer appear as principal variables determining the distribution, with beech particularly affected by summer drought. Projections from the applied fine-scale analysis and modelling results indicate that climate change may lead to drastic reduction in macroclimatically suitable sites for both forest types. Regarding the stands in zonal position, 56–99% of present-day beech forests and 82–100% of sessile oak forests might be outside their present bioclimatic niche by 2050. Phenotypic plasticity, longevity, endurance of non-zonal stands and prudent human support may brighten these dire predictions. Nevertheless, an urgent adjustment of forest management and conservation strategies seems inevitable.

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Csaba Mátyás

Papers

The altitude-for-latitude disparity in the range retractions of woody species

Modeling climate change effects with provenance test data

Climatic adaptation of trees: rediscovering provenance tests

Forecasts needed for retreating forests

Present and forecasted xeric climatic limits of beech and sessile oak distribution at low altitudes in Central Europe