Forest research eagle looking at stressed trees

Show hidden stress signals: Even before drought damage becomes apparent, stresses on forest trees can be recorded by specially equipped drones, a study shows: Multispectral cameras can provide images of treetops that literally reflect signs of plant condition. The method can thus be used to investigate plant stress interactions and to search for tree species and varieties that deal well with climate change challenges.

Water shortages, heat and other pollution are increasingly causing problems for Europe’s forests. The color and loss of leaves or needles will eventually become visible to the naked eye. But when the effects of stress become apparent, the damage is often relatively advanced and may not be repaired. As a result, the processes of damage formation can no longer be traced in detail. Therefore, practical methods for early detection of plant interactions are of particular interest in research into drought tolerance of forest trees. Specifically, some research groups are investigating the question of how to make forests more resilient to the effects of climate change. They are looking for tree species or cultivars particularly suitable for afforestation.

Multispectral cameras look at the tops of trees

Analytical methods already exist that can provide indications of acute pollution, for example during hot summers in certain parts of the forest. But it is relatively complex or only a few trees can be evaluated. As part of their study, researchers from the Federal Research Institute for Forests, Snow and Landscape (WSL) in Birmensdorf examined the extent to which drone images can contribute to the accurate recording of the state of forest trees. The aircraft is equipped with multispectral cameras so that it can provide specially informative images. It enables a particularly detailed analysis of the wavelengths emitted by the treetops when exposed to daylight.

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During the experimental drone measurements, the researchers set their sights on a piece of forest in the Valais, where WSL is conducting a long-term experiment: Scots pines (Pinus sylvestris) were exposed to different irrigation regimes in this relatively drought. site since 2003. Through their investigations, the researchers are now able to show that subtle changes in plant pigments chlorophyll and carotenoids reflect how much the tree is invested in photosynthesis, and thus in growth, or in other processes caused by resource scarcity. “In simple terms, one could say that sunlight reflected from the treetops contains information about the state of the tree,” says first author Petra Dodurico.

Minute chromosomal changes were recorded

As I explained in more detail, the leaves or needles are unbalanced when there is a lack of water, while at the same time the light radiation is often stronger. They absorb more energy than they need for photosynthesis because they have to close their small breathing openings (stomata) to prevent dehydration. In order to dissipate the excess energy, the needles then convert the dyes. This activity can be specifically observed using multispectral imaging. “It’s like seeing invisible stress build up in a tree. That way, scientists can now see in trees that still look healthy if there’s any sign of drought damage,” Dodurico says.

Compared to classic physiological measurements on the ground, remote monitoring with drones means that many trees can now be examined in a short time, scientists assert. This could result in an important addition to previous methods of monitoring the state of forests from the process. “Detecting tree stress at an early stage is important for understanding the increasing effects of drought on our forests and for identifying tree species that are better adapted to drought than others,” summarizes WSL.

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those: Federal Research Institute of Forest, Snow and Landscape

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