According to a study led by the Spanish National Research Council (CSIC), the way trees have responded to drought in the past could be a key indicator of their risk of mortality. The study examined growth rings to compare that response in dead and surviving trees.

An international team of scientists, headed by Lucía DeSoto from the CSIC’s Arid Zone Experimental Station (EEZA), and including CREAF researcher and Autonomous University of Barcelona lecturer Jordi Martínez, has demonstrated that a tree’s rings can help predict the likelihood of it dying during drought events, which are becoming more and more frequent as a consequence of climate change. By examining their rings, it is possible to determine how greatly trees have suffered as a result of past droughts and whether they have subsequently managed to recover fully. The study shows that the trees most at risk of dying during a drought are those that have failed to make such a recovery (trees with ‘low growth resilience’). While this may seem obvious, the relationship had never previously been proven on a global scale.

A tree’s rings can help predict the likelihood of it dying during drought events

Resilience is an organism’s capacity to continue functioning during and after a stress event (e.g. a drought or a fire). The resilience of a tree can be measured retrospectively by comparing its growth ring from a year in which water was scarce with the previous and subsequent years’ growth rings. Doing so reveals how well the tree coped with and recovered from the stress event.

The study compared the rings of groups of trees from individual forests. Some of the trees were dead while others had survived drought. The study’s sample comprised over 3,500 trees of 22 species, from 118 sites around the world.

Differences between angiosperms and gymnosperms

The study found differences between the two major groups of tree species. In the case of angiosperms, resistance to drought is the most crucial factor in the risk of mortality, with trees that had previously struggled to withstand the immediate impact of drought being most at risk of dying. In the case of gymnosperms, the ability to recover from drought plays the greatest role in determining how well they will cope with future water shortages.

“Resilience to drought in the past could be a key indicator for predicting trees’ mortality risk”, says DeSoto. “So, using rings to measure resilience could be a promising means of identifying early signs of mortality and improving our capability to anticipate forest deterioration in future climates”, she adds.

“The study’s results represent a major advance given that climate change means we urgently need to preserve forests, which are particularly threatened by warming and the increased frequency and severity of dry periods”, remarks Martínez.

The trees most at risk of dying during a drought are those that have failed to fully recover from dry conditions in the past

The study was carried out as part of a project funded under a Marie Skłodowska-Curie grant agreement (H2020-MSCA-IF, no. 797188) and with the financial support of European Cooperation in Science and Technology (COST Action FP1106 STReESS). Led by the CSIC’s EEZA, it involved Spanish researchers from CREAF, the CSIC’s Pyrenean Institute of Ecology (IPE), the CIFOR-INIA Forest Research Centre, the Basque Centre for Climate Change (BC3), the University of Valladolid and Pablo de Olavide University working alongside researchers from institutions in Argentina, Finland, France, Germany, Greece, Israel, the Netherlands, Romania, Russia, Serbia, Slovenia, Switzerland and the USA.

Paper on the referenced study

DeSoto et al., 2020. “Low growth resilience to drought is related to future mortality risk in trees”. Nature Communications 11: 545. doi: 10.1038/s41467-020-14300-5