Tree rings reveal a forest’s vulnerability to drought
Studying tree resilience is key to forecasting whether trees species will adapt, shift their ranges or become extinct after the rapid environmental changes predicted by most climatic models. Because 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 with tree mortality to improve forest management. The EU-funded TreEsilience project has been analysing these signals to gain a better understanding of the impact of climate change. “Drought affects tree growth differently among and within populations; thus, identifying those provenances or individual trees more resilient might increase the success of future management and conservation practices,” explains Lucía DeSoto, principal investigator of the TreEsilience project. With the support of the Marie Skłodowska-Curie Actions programme, DeSoto found that trees which died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species within the same populations. “This key result is evidence of the link between mortality risk and the previous observed differences in drought resilience strategies that can be reflected in tree ring growth,” she says.
A new way of assessing potential resilience
To DeSoto’s knowledge, the project is the first to evaluate the direct links between resilience to drought and future mortality risk, “Mainly because it is difficult to empirically evaluate both resilience and mortality on the same individual tree.” The novelty is using tree ring data that allows a retrospective quantification of drought effects at annual resolution for numerous individuals, populations and species. The team, based at the Spanish National Research Council in Almería, used two databases: the International Tree Ring Data Bank (ITRDB) and the tree width growth-mortality (TRW-mortality) database. The ITRDB consists of 172 054 tree ring series found in 4 438 locations. The TRW-mortality database gathers tree ring data for 2 970 dead and 4 224 living trees from 190 sites, across 36 species, where mortality was mainly induced by stress, such as drought. “Both databases gathered tree ring data mostly from conifers and from the temperate, Mediterranean and boreal ecosystems of the northern hemisphere. We acknowledge that the spatial coverage of our data set is limited, although it still covers large variation in geographic and climatic conditions within these regions,” adds DeSoto. Since water availability and temperature affect tree performance, tree populations from suitable climates are expected to have higher growth rates that can be measured in tree rings with annual resolution. This gives researchers an insight into potentially stressful events. As DeSoto explains: “The tree ring data of each species was combined with ‘climate suitability’. Climate suitability is obtained with species distribution modelling that models the abiotic niche for every species. This makes it possible to characterise water conditions for populations and describe gradients of the environmental conditions for the species range.”
Putting the information to good use
From the work she has done, DeSoto feels it is clear species distribution models should take into account that the response to climate is not homogeneous across the whole distribution range of a certain tree species. The project’s findings will be fed into the species distribution models to identify hot spots and vulnerable tree species for conservation purposes, along with higher-resilience tree provenances suitable for more efficient afforestation. “We would like to develop a protocol for the selection of trees which have less chance of overcoming future droughts, as candidates for harvesting. This would follow an assisted forest evolution.” The project is now on the lookout for practitioners, policymakers and stakeholders who would be willing to use and assess the viability of the protocol.
TreEsilience, drought, tree rings, tree resilience, tree mortality