The drone captures thousands of frames during the survey campaigns, which take place regularly between July and November. »The image sections are chosen in such a way that 80% of the area captured by each frame overlaps with the frames taken before and after,« explains Sören Hese. The individual frames are subsequently converted into an overall image. This method also allows the researchers to create detailed, precise elevation models of the treetops, as the drones capture each point from different perspectives and can measure their position in real time.
The photographs captured 2019 paint a dramatic picture. Hese is currently evaluating them to map changes in the forest following the hot summer of 2018. Even outside the hotspots on exposed hillsides, beech trees are dying in great numbers. »Up to 30% of the trees are already dead in several areas,« states Hese. This is confirmed by the latest ground-based reference studies that national park authorities have conducted. It remains to be seen whether partially defoliated trees and those with only a few tiny leaves will recover next year.
Beeches whose treetops are in the highest area of the forst are under particular threat. These huge trees are around 180 years old, and some are even older. »These older trees are the first to die,« says Hese as he opens an animated elevation model of Hainich National Park generated from 1.2 billion elevation points. The defoliated treetops are easy to spot during the drone flights, as they individual peaks protrude from the canopy below. This problem may primarily be affecting beeches, because they have more superficial roots than other trees, such as oaks, and therefore cannot reach water reserves deep underground.