With the DenDrought2018 initiative, an international team of researchers is now able to tell a story about drought stress from the perception of 21 tree species across Europe. Results of their joint effort have been published in Nature Communications under the title “The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests” (doi: 10.1038/s41467-021-27579-9)

Enduring the heatwave in the shade of trees

Anyone who has ever biked on a hot summer day knows the relief of reaching the shade that the forest canopy provides. But what about the trees? They are fully exposed to the elements and have to cope with the consequences. This makes one wonder how trees can deal with extremely hot and dry conditions, like the ones prevailing during the summer of 2018, when European average temperature was 1.3°C higher than usual.

The rhythm of tree stems

Although trees appear as static elements in a landscape, you might be surprised to find out that tree stems actually move in rhythm with their environment. The tree stem shrinks during daytime due to transpiration-induced water transport from roots to leaves and emptying of water reserves in the stem. At night, the stem expands again when water is taken up from the soil to allow for stem tissues to refill. This is also the time when trees  grow. However, during long hot and dry periods, soil water reserves are used up, and the stem cannot fully refill and expand. Sensors can pick up these tiny changes in stem radius, which are invisible from outside, and detect when exactly trees experience drought stress - a powerful biological measure for an atmospheric phenomenon!

Dendrometers as a key tool to monitor drought stress and tree growth

Dendrometers are sensors installed on the tree stem to monitor diameter fluctuations with a high spatial (trees) and temporal (hours) resolution. The data is stored on a logging device or directly sent to the server. Dendrometers come in different shapes and sizes. Commonly, a point dendrometer provides the most sensitive reading at a specific stem location. It looks like a metal rod resting on the tree bark and records tiny (1/1000 mm) changes in the stem radius. Although dendrometer measurements have been collected by scientists over years and across hundreds of research sites, they have rarely been combined into larger networks to show responses across forest sites and species.

Start of the DenDROUGHT2018 project

The 2018 European heatwave was a unique opportunity to illustrate the power of an extensive Dendrometer network initiated during the EU COST Action STReESS (Studying tree responses to extreme events). Scientists from all over Europe joined forces and used dendrometer measurements to quantify the effects of the 2018 European heatwave and summer drought on tree growth across different forest landscapes and tree species.

Heatwaves, like the one occurring in large parts of Europe in the summer of 2018, are expected to occur more often in the future. The question arises on how European tree species growing under various site conditions experienced such extreme conditions.

In January 2019, during a workshop supported by the European Forest Institute (EFI), this question was discussed by an international and interdisciplinary team. The group decided to compile a European-wide dendrometer dataset of different tree species to gather the scientific evidence that would eventually allow answering the question. There has been a call for automatically collected dendrometer records and corresponding weather data with a minimum temporal resolution of 1 hour, covering at least the period from 2016 to 2018. Hundreds of individual dendrometer data series from 21 species were provided by enthusiastic colleagues from all over Europe. Despite being challenged by the amount and complexity of received data, the analyses delivered the desired information and provided quite unexpected results.

The story evolved: drought stress recorded by the tree stems at micrometer scale

As expected, tree species responded very differently to the 2018 heatwave, and – surprisingly - the drought stress did not necessarily translate into growth limitations, at least not on short notice. This was likely explained by the timing of the heatwave, which was quite late during the growing season at many locations when most trees had already achieved most of their stem growth. But this does not mean that trees did not suffer from temporary stress! Many trees ran into trouble, as 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 cause problems for some species under certain site conditions. Trees might also be negatively affected by carryover effects in the years after the extreme event, which must be further explored.

Today, in 2022, we know that on many sites, especially where trees grow on soils with a low water-holding capacity or where planted species do not match the climatic or site conditions, some of the stressed trees would die in the years following the 2018 heatwave.

Tree monitoring “in the picture”

The DenDrought2018 initiative has not only provided important insights into the response of tree species to heat waves and droughts, but has also assisted putting high-resolution monitoring of trees "in the picture", and not only in our research community. Dendrometer observations can be used as an important early-warning system to detect tree stress and measure species resilience before more severe responses such as shedding of leaves occur. This opens up new applications, for example, to guide forest management practices and species selection for reforestation.

Many initiatives started or are underway to integrate tree monitoring with remotely sensed information and classical forest inventories. Nowadays tree growth and, hence, also the impact of extreme climate events on trees growing at different locations can be followed online, e.g. on TreeWatch.net, Tree Talker and Treenet.info. New sensing techniques are being developed and more trees equipped, allowing us to keep the network connected and growing. Our initiative also showed the added value of integrating data to jointly find answers to pressing questions such as how our tree species cope with climatic extreme events and how forests management can be adapted to the changing environment.

So, let’s work for the best, as it is much nicer to bike in the shade of trees.