Integrated remote sensing of management variants on forest disturbance plots in Central Germany: Implications for microclimate and soil conditions

Abstract

The recent drought overstressed the response capacity of many Central European forests, resulting in large disturbed areas in forest stands of Norway spruce. Reforestation strategies are needed to limit impacts on soil structure, loss of soil organic matter and nutrients. While the ecological benefits of leaving standing deadwood in place versus complete clearing are well studied, effects of nuanced treatment variants for microclimate and soil are poorly understood, lacking both long-term in situ monitoring. In addition, there is a need for upscaling approaches with means of remote sensing for regional implications.

We carried out observations on disturbed spruce plots in low mountain regions, e.g., the Harz Mountains between 2021 and 2023. Four management variants (standing deadwood, partially standing deadwood “high stumps”, mulched and cleared open areas) were compared to a vital forest. Topsoil moisture, soil temperature, as well as surface and near-surface temperature were recorded with 30 dataloggers at 15 min interval for 36 months. Measurements were accompanied by soil surveys on basic soil chemical and physical variables. On exceptionally hot days, UAV thermal surveys were conducted, accompanied by hand-held IR measurements. Whenever available, satellite remote sensing-derived land surface temperate (Landsat-LST) was acquired and compared to in situ observations.

Surface and soil temperature as well as soil moisture differed significantly between management variants. Bare areas showed highest maximum daily temperatures during the growing season (about 6 °C more than in the standing deadwood on hot days), whereas standing deadwood mitigated temperature peaks and depletion of soil moisture effectively. As for the “high stumps”, soil temperature and moisture were more balanced during the day at the shaded sides of the stumps, thereby favoring tree recovery. Aggregate stability of the soil decreased faster with more intensive management interventions. Relative differences in surface temperature between variants were well-reflected in the thermal orthophotos with bias around 2 °C compared to surface measurements, which we attribute to atmospheric influences associated with the flight altitude (100 m). Deviations between Landsat-LST and in situ measurements can be explained, among others, by the occurrence of mixed pixels in the remote sensing product due to relatively small-scale plots. Nevertheless, relative difference in LST of open areas, standing deadwood and vital stands were distinct as well.

We conclude that the microclimate can be strongly influenced by silvicultural management of disturbed areas and that even small amounts of remaining deadwood also limited soil degradation. UAV-based remote sensing is efficient to determine the influence of management on the microclimate and associated changes in soil conditions.