While people tend to escape the sun during hot summers under the tree canopy in parks and forests, trees are fully exposed. They have to fight the heat and drought. How do trees actually perceive this stress? And does drought stress translate into less growth? A group of researchers across Europe used dendrometers to record tiny changes in stem size on hundreds of trees to find out.
The DenDrought2018 initiative is a spin-off of the COST Action “STReESS” (Studying Tree Responses to extreme events). Ute Sass-Klaassen, associate professor at Wageningen University & Research and chair of COST STReESS explains how the idea for the initiative was born: “When I was biking in the heat during my summer holidays in 2018, I appreciated the shadow of the trees more than ever. This was when I thought: we have to use our COST STReESS dendrometer network to understand how trees are able to bear this terrible heat”.
Invisible changes in stem diameter
Although trees appear as static elements in a landscape, their stems actually move in rhythm with their environment. Tree stems shrink during daytime, because of transpiration-induced water transport from roots to leaves, and emptying of water reserves in the stem. At night the stems expand again, when water is taken up from the soil and stem tissues are refilled. This is also the time when trees grow. However, during long hot and dry periods, soil water reserves are used up. Tree stems can no longer fully refill and expand. Dendrometers pick up this heat and drought-induced stem shrinkage.
Dendrometers are sensors which are attached to the stem and can measure the growth and shrinkage of trees every hour up to micrometers. In the DenDrought2018 initiative records from 377 trees across Europe were integrated and analysed to tell a story about how the 2018 heatwave was experienced by 21 European tree species. The results of this joint effort were recently published in Nature Communications, led by Kathy Steppe of Ghent University.
Ute Sass-Klaassen: “We showed that tree monitoring with dendrometers can be used as an early-warning system to detect drought stress, and measure species resilience before more severe responses such as shedding of leaves occur. This opens up new applications to e.g. guide forest management practices and species selection for reforestation.”
The effect of the 2018 heatwave
Tree species responded very differently to the 2018 heatwave. The drought stress did not necessarily translate into growth limitations, at least not on short notice. This likely relates to the timing of the heatwave, which at many locations was quite late during the growing season. Most trees already had achieved most of their stem growth. But this does not mean that trees did not suffer from temporary stress. Many stems showed considerable shrinkage. The inability of trees to refill stem water reservoirs during the night suggests limited resilience to heatwaves and droughts. In particular, conifer species were more sensitive to the 2018 heatwave than broadleaf species. While a single stress event might not be problematic, repeated heatwaves will likely cause problems for some species under certain site conditions.
In the Netherlands, data records from more than 100 dendrometers, established and managed by co-authors Gert-Jan Nabuurs and Bas Lerink of Wageningen University & Research, have been partly operating sinds 2007. These data are waiting for analysing the recovery of Dutch trees species from the hot and dry years 2018-2020.