Monitoring drought damage in forests using artificial neural networks.

The drought summer of 2018 led to the death of many millions of trees in Central Europe; conifers were disproportionately affected.

In recent decades, ongoing climate change has led to increased occurrence of droughts worldwide. Germany also is increasingly affected. Prolonged droughts, coupled with intense heat waves, occurred especially in the summers of 2018 and 2019. The drought summer of 2018 caused large scale forest damages. This resulted in the death of many millions of trees in Germany alone. The forestry industry recorded losses in the millions. As climate change progresses, the impact of droughts and other disturbances on forests is expected to increase.

To better understand the effects of increased disturbances and provide foresters, as well as other stakeholders, with information for future-proof forest management, it is important to monitor forests on a large spatial and fine temporal scale. Traditional monitoring methods such as forest inventories are usually limited to smaller areas and are not conducted annually. In addition, these methods are relatively costly in terms of time, money, and personnel.

Preliminary results from a pilot project in Luxembourg show that there were ~950% more dead forest trees in 2019 than in 2017, representing about 1% of the country's forest area. The classification into deciduous and coniferous trees showed that conifers were proportionally much more affected by the event. Observations suggest that spruce suffered heavily from the summer drought which was exacerbated by bark beetle calamities. Thus, restructuring forests toward more deciduous trees could increase resilience to droughts in the future.  
Using the large-scale tree mortality data, further questions can be explored. Due to the data’s spatially explicit nature, site factors that contribute to tree mortality under drought conditions can be explored. In addition, the already trained algorithm can be applied to new aerial imagery. Thus, forest condition information can be updated regularly, with little effort.

Associated Institute at KIT: Institute for Meteorology and Climate Research Atmospheric Environmental Research (IMK-IFU), KIT-Campus Alpin, Garmisch-Partenkirchen
Author: Selina Schwarz (Oct. 2021)