Heatwaves deaden the beat of central Europe’s trees

Close your eyes and think of the most solid living organism you know of. Did you think of a tree? Trees are solid, still, and impervious to the passing of time, right? Well, wrong actually. In reality, the trunk of a tree beats: it shrinks in the daytime due to water loss and swells at night as it rehydrates via the tree’s roots. That beat, according to a study published recently in Nature Communications, is weakened by heatwaves. Specifically, the study found that in the summer of 2018, central European trees shrank twice as much as they would have in normal conditions because they lost too much water during the day, owing to the heatwave affecting the continent, and were unable to rehydrate sufficiently at night.

“Shrinkage on that scale is a sign of dehydration in the tree, which will suffer the affects for years afterwards, as it will be less resilient and more vulnerable to possible environmental changes stemming from the climate crisis, such as infestations or droughts.”

Rafael Poyatos, CREAF researcher and co-author of the study

Nonetheless, the research, which was led by Roberto Salomón, of Ghent University (Belgium), and Richard Peters, of Ghent University and the Swiss Federal Institute for Forest, Snow and Landscape Research (Switzerland), did not detect differences, relative to previous years, in the trees’ growth during the heatwave and drought of 2018, which occurred between mid and late summer. That finding can be attributed to the trees having largely completed their annual growth phase by that time of year, so the lack of water in the period in question was not critical in that respect. “Things would be much more complicated if the same situation arose in spring or early summer, which is the most crucial time for trunk diameter growth,” says Poyatos.

To carry out the study, a team of scientists from 59 institutions in 17 countries recorded tree diameter variations every 30 minutes at 50 sites across Europe, compiling data on more than 400 trees of 21 different species.

Beating to the rhythm of water

A tree pumps water up from its roots and out into the atmosphere to replace the water it loses through leaf transpiration.

A tree beats to the rhythm of the water inside its trunk. Just as a heart pumps blood, a tree pumps water up from its roots and out into the atmosphere to replace the water it loses through leaf transpiration; the water moves through the trunk from bottom to top. At dawn, the trunk is at its most swollen and water begins to move upwards inside the tree, until it is released through its leaves as a result of evaporation caused by heat or photosynthesis. At midday, the trunk has lost a large amount of water and contracted, this being the point at which its diameter is smallest. In the afternoon and, in particular, at nightfall, the tree rehydrates, absorbing water via its roots, and the cycle — a beat of constant expansion and contraction — beings again.

The results presented in the article show that, in extremely hot conditions, the trees of central Europe lose much more water through their leaves due to evaporation in the daytime than they are able to absorb via their roots at night. Consequently, their water reserves become more and more depleted, resulting in dehydration that gets worse the longer a drought lasts.

Rhythms at risk: the Norway spruce and the Scots pine

The study found that the effect in question varies substantially from species to species. Oak trees, for example, are better able to rehydrate their trunks at night, as their roots are well suited to finding deep soil water. In contrast, the Scots pine (Pinus sylvestris) and, in particular, the Norway spruce (Picea abies) are less efficient and suffer much more during heatwaves. The study does not include data from the Iberian Peninsula and the threat it describes mainly applies to species unaccustomed to water shortages.

Norway spruce (Picea abies). Image from Unsplash.

An electrocardiogram for trees 

For their research, the team of scientists used a huge database containing records of the beating of more than 400 trees across Europe, taken at half-hourly intervals. They were able to do so thanks to a European network of automatic dendrometers (metal bands placed around tree trunks) capable of measuring such changes, which are undetectable to the naked eye. Trunk diameter variations are recorded in a data receiver every 30 minutes and the result shows how each trunk beats, like an electrocardiogram for trees. Of course, in addition to trunk beats, the dendrometers also record when and how much the trees grow.

Trunk diameter variations are recorded every 30 minutes and the result shows how each trunk beats.

As it is very important to study how tree girth changes to find out how forests work and how they react to climate change, many countries around the world use dendrometers and monitor trees of different species and in different habitats. The study discussed in the article is a first step for DendroGlobal, a worldwide network that ultimately aims to combine the information recorded by research centres all over the world in a single database.

“To achieve that, we need to coordinate, organize and find a way to unify the data to ensure they are useful to and can be reused by as many people as possible; the study was our first step in that more ambitious project,” concludes Poyatos.

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Article: Salomón, R. L., Peters, R. L., Zweifel, R., Sass-Klaassen, U. G., Stegehuis, A. I., Smiljanic, M., Poyatos, R.... & Steppe, K. (2022). The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests. Nature communications, 13(1), 1-11.