Subtle stress signals made visible: Even before damage caused by drought becomes apparent, stresses on forest trees can be recorded by specially equipped drones, a study shows: multi-spectral cameras can provide images of tree tops that literally reflect signs of the plant’s condition. The method can thus be used to research plant stress reactions and to search for tree species and varieties that cope particularly well with the challenges of climate change.
Water shortages, heat and other pollution are increasingly causing problems for Europe’s forests. Discoloration and the loss of leaves or needles will then eventually become visible to the naked eye. But when traces of the stress are evident, the damage is often comparatively advanced and possibly irreversible. As a result, processes of damage formation can no longer be traced in detail. Practical methods for the early detection of plant reactions are therefore particularly important for research into the drought tolerance of forest trees. Specifically, some research groups are investigating the question of how forests can be made more resistant to the effects of climate change. They are looking for tree species or varieties that are particularly suitable for afforestation.
Multispectral cameras look at tree tops
There are already analytical methods that can provide indications of acute pollution, for example during a hot summer in certain parts of the forest. But they are comparatively complex or only a few trees can be assessed. As part of their study, researchers from the Federal Research Institute for Forests, Snow and Landscape (WSL) in Birmensdorf have now examined to what extent drone images can contribute to the precise recording of the condition of forest trees. The aircraft were equipped with multispectral cameras so that they can deliver particularly informative images. They enable a particularly detailed breakdown of the wavelengths emitted by the treetops when exposed to daylight.
During the experimental drone measurements, the researchers set their sights on a piece of forest in the Valais, on which the WSL is carrying out a long-term experiment: Scots pines (Pinus sylvestris) have been exposed to different irrigation regimes at this relatively dry location since 2003. Through their investigations, the researchers have now been able to show that subtle changes in the plant pigments chlorophyll and carotenoid reflect how much a tree invests in photosynthesis, and thus in growth, or in other processes that are caused by scarcity of resources. “In simple terms, one can say that the sunlight that is reflected from the treetops contains information about the state of the tree,” says first author Petra D’Odorico.
Subtle pigment changes recorded
As she explains in more detail, leaves or needles are unbalanced when there is a lack of water, while at the same time the light radiation is often strongest. They absorb more energy than they need for photosynthesis because they have to close their tiny breathing openings (stomata) to prevent dehydration. In order to dissipate the excess energy, the needles then crank the conversion of pigments. It is precisely this activity that can be observed using multispectral imaging. “It’s like seeing the invisible stress build up in the tree. In this way, scientists can now see in trees that still seem healthy whether there is any sign of damage from drought, ”says D’Odorico.
Compared to classic physiological measurements on the ground, remote observation using drones means that many more trees can now be examined in a short time, the scientists emphasize. This could result in an important addition to the previous techniques in forest condition monitoring from the process. “Detecting tree stress at an early stage is important in order to understand the increasing effects of drought on our forests and to identify tree species that are better adapted to drought than others,” summarizes the WSL.
Source: Federal Research Institute for Forests, Snow and Landscape