In the hot summer of 2018, the average temperature in Europe was 1.3 °C higher than usual. The drought visibly affected the vegetation in many places. How can forest trees cope with such extremely hot and dry conditions? An international team of researchers with the participation of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL investigated this question with the help of measurements of the stem radius variations of trees.
The rhythm of the tree stems
The thickness of tree stems varies with the environmental conditions. During the day, when the trees release water vapour through the leaves, the roots cannot supply water quickly enough. A negative pressure develops, water reservoirs along the trunk empty and all elastic tissues contract. The trunk shrinks. At night it expands again when more water is absorbed from the soil than evaporates at the top of the crown and the trunk tissue fills up again. Night is also the time when trees grow. During long hot and dry periods, however, the water reserves in the soil are used up. The trunk cannot completely fill and expand again and the tree cannot grow.
These fluctuations in radial stem size amount to only fractions of a millimetre. They can be measured with special devices called dendrometers (see box). These are widely used in forest monitoring programmes around the world. "This is an effective biological measurement method for an atmospheric phenomenon," says Richard Peters of WSL and the University of Basel, one of the two lead authors of the study.
Tree trunks shrank by record numbers in 2018
For the study, the researchers collected dendrometer data from 21 tree species at 53 forest sites across Europe. Over the course of summer 2018, many trees showed record-high stem shrinkage. The trees were unable to replenish their stem water stores during the night. They therefore suffered from drought stress.
Not all tree species were affected equally: Coniferous tree species reacted more sensitively to the 2018 heatwave than deciduous tree species. They are less able to replenish depleted water reserves quickly. " The wood of conifers generally conducts water more poorly than that of deciduous trees. Conifers are also less able to absorb water from dry soils than oaks, for example," says Roman Zweifel, co-author of the study and ecophysiologist at WSL.
Given the extreme weather conditions, tree growth was surprisingly little affected in 2018 and, accordingly, the annual ring was hardly narrower. When the weather became hot and dry in July, growth was already largely complete at many forest sites. Moreover, forest trees only grow for a short period of time throughout the year. A few moist nights may be enough, even during a dry period, to generate the necessary cell growth.
While most trees cope well with a single, short heat phase, repeated and long heat waves are critical for some tree species. Tree growth is strongly influenced by the previous history of the trees, e.g. how much a tree was able to invest in buds and carbon storage last year and how large its crown is. The tree can then benefit from this the following year. This is called the legacy effect.
Correspondingly, however, the poor conditions in summer 2018 also had after-effects. Other recent studies show that the legacy effect can influence tree physiology and growth for four or more years. This has been confirmed especially for spruce, which is not very drought-resistant. After 2018, spruce growth continued to decline. The weakened trees were susceptible to bark beetles and many spruce died.
Today, drought and heat cause problems primarily where soils have low water-holding capacity or where planted species are not acclimated to weather or site conditions.
Tree signals as an early warning system
This study was conducted as part of the European initiative DenDrought2018. It concludes that trees can serve effectively as early warning systems for the ecological impacts of extreme events. Therefore, monitoring networks with stem radius measurements are currently being expanded throughout Europe. In Switzerland it is Treenet.info, which measures stem radius changes on over 350 trees and automatically evaluates current drought stress and growth. To be applicable for early detection of possible tree damage, the network needs a central data processing system that can collect and analyse data in near real-time. Since last year, TreeNet has been publishing daily drought and growth nowcasts, a kind of daily weather forecast for the condition of our forest trees in Switzerland.
Other dendrometer networks in Europe are TreeWatch.net or Tree Talker. They will be further expanded, with new sensor techniques and more trees equipped with dendrometers. This will help us to understand better and better how forests respond to climate change.