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Global patterns of intraspecific variation in tree resilience to drought

Periodic Reporting for period 1 - TreEsilience (Global patterns of intraspecific variation in tree resilience to drought)

Reporting period: 2018-05-01 to 2020-04-30

What is the problem/issue being addressed?
Forests provide essential long-term ecosystem services, such as the conservation of biodiversity or the reduction of greenhouse gas emissions. Consequently, preserving forests has been the focus of many conservation policies (i.e. the Europe 2020 Strategy). Drought events associated with climate change reduce tree growth and prompt tree mortality episodes, impacting globally and severely on forest ecosystems. TreEsilience project has studied the ability of trees to resist and recover from drought (tree resilience) that will be decisive to maintain the functioning of these ecosystems.

Why is it important for society?
The results have provided insights into novel strategies to improve forestry efficiency by identifying early signals of tree mortality, useful for forest practitioners, policy-makers and stakeholders in order to mitigate the detrimental effects of land degradation and desertification on human wellbeing due to climate change.

What are the overall objectives?
TreEsilience has explored tree climate-growth relationship and resilience to drought across tree species and populations as key for predicting the effect of climate change, in particular drought-induced mortality. First, climate sensitivity may depend on species-specific traits leading physiological and anatomical responses to temperature and water availability. Second, different tree populations may have faced contrasting climatic conditions, and then within-species response to climate change may differ geographically due to local adaptation. In order to reach this ambitious goal, TreEsilience has used an interdisciplinary approach that combines dendrochronology, phylogenetic comparative methods and species distribution modelling, thus bringing tools from the spatial and evolutionary ecology to the study of forest management and conservation.

Conclusion. Our findings confirm that trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. This key result evidences the link between mortality risk and the previous observed differences in drought resilience strategies that can be reflected in tree-ring growth.
Note that, due to the CoVID-19 pandemic and the familiar commitments, the action has experienced some difficulties for its implementation and some tasks are still in progress.

The work performed by TreEsilience project was divided in three phases. First, two databases were gathered selecting and compiling data of tree growth (i.e. tree-ring width, TRW) of existing open databases: The International Tree Ring Data Bank (ITRDB) and TRW-mortality database. ITRDB compiles 172,054 tree-ring series that were manually grouped within 93,320 tree identities and 4,438 sites. TRW-mortality database gathers tree-ring data of 2,970 dead and 4,224 living trees from 190 sites (36 species) where mortality was mainly induced by stress, such as drought.

Second, drought events were identified for each site in the TRW-mortality database and, then, resilience of tree-ring growth to drought was computed. We found that, across the regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk was associated with lower resistance (low capacity to reduce impact of the initial drought), while it was related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The outstanding significance of these results allowed us to publish them as a paper in the top journal Nature Communications. This finding was covered for different media and thus, the results were highly disseminated between the general public.

Tree growth responds to climatic conditions displaying intraindividual variability in annual growth. I have explored the differences in intraindividual variation in tree growth among sites and species of the ITRDB. Moreover, maintaining variation might have costs in other functions and eventually might have consequences in tree survival. Here, I have analysed the intraindividual variability in radial growth and mortality in the populations and species of the TRW-mortality database. Preliminary results indicate that trees that died during water shortages show higher plasticity in growth than surviving trees.

Third, I will develop a protocol to select tree individuals less likely to overcome future droughts as candidates for harvesting within woodland stands. This protocol will follow Adaptive Forest Management, working in an iterative process of interaction between scientists, practitioners, policy-makers and stakeholders.

Besides the publication in the multidisciplinary journal Nature Communications (2020, 11:545), I have disseminated the research results of TreEsilience to the scientific community by publishing two more papers. I have been involved in dissemination activities for general public such as the “Science Week” and the “European Researchers’ Night”. Main results have been also disseminated in different general media, newspapers, radio and internet. Finally, I made a short-film about how tree rings can be used to detect tree sensitivity to drought and future mortality risk.

This work was done in collaboration with Prof J.M. Gómez (EEZA), Dr C. Armas (EEZA), Dr M. Cailleret (INRAE), Dr J. Martínez-Vilalta (CREAF), Dr R. Mateo (UAM) and Dr R. Torices (URJC).
TreEsilience has yielded significant outcomes that will impact on the European Research Area and attitudes of European Society. Previous studies have related tree growth patterns to drought-induced mortality both at local and global scales. However, TreEsilience provides, to our knowledge, the first empirical evidence linking low growth resilience to past droughts with increased risk of tree mortality across species and regions. Moreover, the ability to resist the immediate impacts of drought is linked to long-term mortality risk in angiosperms, whereas recovery capacity appears to control the likelihood of drought-induced mortality in gymnosperms. Future research should therefore consider the intraspecific patterns of the tree response to drought.

TreEsilience results will be relevant to forest management. Since the effect of drought on tree growth can affect tree survival and hence timber productivity and forest conservation, it is crucial to assess how climate change can be mitigated by identifying early signals associated to tree mortality to improve forest management. Drought affects tree growth differently among and within populations; thus, selecting those provenances or individual trees more resilient might increase the success of future management and conservation practices. In this case, the TreEsilience’s final goal is to promote more sustainable afforestation and forest practices improving the quality of life in Europe and the world extensively. Therefore, the specific design of the TreEsilience will continue in the academia-industry in the near future.
Picea abies surviving and dead trees from a drought event in Swiss Alps