Quantifying post-fire effects and recovery in a disturbed landscape: Quesenbank fire, Harz National Park

Anthropogenic climate change increases the risk of forest fire following drought periods in temperate forests of Central Europe. Areas with an increased proportion of standing deadwood are often considered to be at risk. Especially in national parks, deadwood is not removed, forming an essential part of the local ecosystem.

In the Harz National Park, we aim at a comprehensive impact assessment following a fire in a spruce forest that was already disturbed after a massive bark beetle infection to understand deadwood breakdown, vegetation succession, surface erosion and changes in soil properties. The Quesenbank fire of August 2022 burned an area of approx. 13 ha within four days. We scanned 10 ha of burned compared to unburned areas using unoccupied aerial vehicles (UAVs) equipped with multispectral, thermal, high-resolution RGB and light-detection and ranging (LiDAR) sensors. Derived orthoimages, 3D point clouds and canopy height models (CHM) are employed to estimate standing deadwood, fractional cover and succession indicators, thermal ground regime alterations and small-scale morphological changes. To capture the gradual breakdown of deadwood, we collected ground truth on vegetation biophysical parameters, such as fractional cover, plant area index (PAI) and fraction of absorbed photosynthetically active radiation (FAPAR) from upward-directed digital hemispherical photos. The surveys were conducted 2, 9, 11 and 12 months post-fire together with the UAV campaigns in diffuse or near-dusk light conditions.

The analysis of the digital CHM and ground models reveal a decline in the detection rate of tree crowns (tree height ≥ 2 m) by 15 %, crown area by 74 %, and a corresponding loss of surface material affecting at least 0.9 ha between October 2022 and October 2023, respectively.

The ground reference data confirmed considerably lower fractional cover on burned areas. PAI and FAPAR in burned standing deadwood was lower in unburned stands, altering light, soil moisture and temperature regimes. This is reflected in the occurrence of typical post-fire and light-demanding species such as Epilobium spec. on burned areas, though in lower coverage compared to an unburned, logged site. As the variation in reference data was relatively low over the observation period, we suggest that the main dynamics of the breakdown of standing deadwood had already happened several weeks after the fire. Interestingly, we found a very heterogeneous microtopography due to granite boulders, with subsurface tunneling and unstable ground, influencing post-fire recovery. Upcoming analysis will include analyses of fire-influenced soil properties, morphodynamics and biogeochemical cycling in a region that still shows traces of past land use associated with the mining history of the Harz.

We acknowledge the collaboration with the Harz National Park Authority. A preliminary data set from two months after the fire can be accessed via Zenodo: Jackisch, R., Putzenlechner, B., & Dietze, E. (2023). UAV data of post fire dynamics, Quesenbank, Harz, 2022 (orthomosaics, topography, point clouds) (1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.7554598