Detecting stress rather than damages—a new vegetation index for monitoring forest stress caused by spruce bark beetles using remote sensing

“The spruce bark beetle is the insect causing most damages to our spruce forests” according to the Swedish Forest Agency. In 2018, it destroyed 3-4 million m3 of timber in Sweden, and approximately 40 million m3 in central Europe. Accelerated by global warming, the average annual damages expected for 2021–2030 is almost six times higher than observed in 1971–2010. This is a threat to the sustainable development of forestry. One crucial strategy of damage control is removing damaged trees to prevent the spread in time, usually before the end of June in Sweden. At this time, tree crowns are however still in the early stage of infestations and show subtle visible changes. How to find the infested trees that should be removed? How early is it possible to plan the sanitation felling in advance? Remote sensing techniques have recently been proven to achieve early detection. In the past, the records of the earliest detection were June, while our latest research presents a method that achieved it before April from Sentinel-2 satellite images. Our innovative idea is that when using remote sensing data, detecting stress rather than damage would be a more efficient way to achieve early detection. A new vegetation index NDRS was developed based on this idea for large-area mapping. Implementing this idea would provide forest owners necessary monitoring of their forests and benefit damage control and sustainable management. The generated maps could also be a database for further studies about trends and environmental factors.