A new study conducted by researcher Petra D ‘ Odorico from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL shows how tree stress can be detected before it becomes visible to the naked eye by extracting information from multispectral drone pictures. Recognizing stressed trees early is important to understand the impact of dryness on forests. Fascinating.
A new study by the Swiss Federal Research Institute WSL now shows that the stress level of trees can be detected from the air with a multispectral camera. During a series of tests, the project manager Petra D’Odorico and teammates carried out drone flights over pines in the Pfynwald (Valais). She explains: “In simple terms, one can say that the sunlight that is reflected from the tree canopy contains information about the state of the tree”. Their measurements have shown that the reflected light causes changes in the photosynthetic pigments (chlorophyll and carotenoids). By analyzing these pigment changes it can be deduced how much a tree invests in photosynthesis, and thus in growth, or in other processes that are caused by scarcity of resources.
Extreme dry years are expected to increase over the next few decades and have a decisive impact on the health of the forest. When the least amount of water is available, the light radiation is often strongest. The leaves or needles are unbalanced as a result: they absorb more energy than they need for photosynthesis because they close their stomata to prevent dehydration. In order to dissipate the excess energy, the needles stimulate the conversion of pigments. This activity can be observed via multispectral imaging, a technique that reveals information that is invisible to the human eye. “It’s like seeing how invisible stress builds up in the tree,” explains D’Odorico.
The drone measurements took place during the 2019 and 2020 vegetation periods in the Pfynwald, where the WSL is carrying out a unique long-term experiment . At this location in Valais, one of the driest inner-alpine alpine valleys in Europe, pines ( Pinus sylvestris ) have been exposed to different irrigation regimes since 2003 . The new study also came to the conclusion that the life history of trees and thus past environmental conditions have an impact on how they react to stress today.
Remote observation with drones enables many more trees to be measured in a short time than with classic physiological measurements on the ground. Even if the method cannot yet be used operationally in forest management, it will in future be indispensable as a supplement to other techniques in order to be able to record the forest’s reaction to warmer and drier conditions at an early stage.
The authors would like to take this opportunity to thank the community of Leuk, the local forest service, and the company for electricity and water power FMV for the longstanding cooperation. This work was supported by the Swiss National Science Foundation (SNSF) Spark funding program for innovative research at Petra D’Odorico.