Monitoring of damaged and reforested areas in the Southern Harz Mountains

Background
The last years with their hot, dry summers (2018-2020) in combination with pests have left visible traces in local forests. Particularly in spruce stands, there has been and continues to be widespread dieback - leaving behind "Dürrständer" (Fig. 1) and increasingly "bare" forest areas such as in the Harz Mountains (Fig. 2). In view of progressive climate change and the associated increasing probability of drought but also heavy rainfall events, the question arises - what's next?


Figure 1: Bare area and degraded spruce stand in the southern Harz Mountains

From an ecological perspective, disturbed areas generally represent an opportunity for biodiversity and natural reforestation, especially when deadwood is left in place. However, there are multiple services of the forest ecosystem to our society: among other things, economic livelihood, renewable resource, flood and erosion control, and recreational space are also important forest functions. This tension raises questions that cannot be answered in a blanket manner, such as: What is the "benefit" of dead spruce stands? Should degraded areas be actively reforested or left untreated? Will native tree species cope with future climatic changes?
The silvicultural options for reforestation of developed damaged areas are manifold and are intensively discussed not only within forest science and forestry. Thus, opportunities for interdisciplinary cooperation arise. Here, geographic research with its "inherently" integrative character can contribute to a scientific discussion.


Figure 2: Dead spruce stands can be seen from the observation tower of the Poppenberg in the southern Harz Mountains (June 2021).

Test site in the Southern Harz region and participation in the "Dürrständerinitiative", ThüringenForst
In 2021, a monitoring of dead spruce stands was initialized with the "Dürrständerinitiative" at the Forest Research and Competence Center Gotha, ThüringenForst AÖR. The term "Dürrständer" is used here to describe dead spruce stands that are still standing and no longer pose a risk of spreading bark beetle infestation. The objectives of monitoring such stands include:

• Collecting data on the (natural) reforestation of damaged areas that have not been logged,
• gaining knowledge about natural processes (water and material cycles, flora and fauna),
• a long-term risk assessment of the stands (safety in forestry and hunting measures), as well as
• the creation of practical examples for further education and training as well as argumentation bases for the social discussion.

For this purpose, practice-oriented variants for dealing with such stands were developed and implemented in cooperation with the Bleicherode-Südharz forestry office. The experimental design includes different variants for leaving deadwood and planting activities (Fig. 3).


Figure 3: Experimental design of the "Dürrständermonitoring": Preservation of standing dead spruces without further measures (except road protection) (0), plantings within dead spruce stand (A), leaving dead spruces up to a height of 2.5 m combined with plantings (B), area reclaimed and planted according to standard (C), leaving individual spruces up to a height of 2.5 m and planting (D), intensive treatment and reforestation (E) (Figure after aerial survey of FFK Gotha, modified).

The Department of "Cartography, GIS and Remote Sensing" of the Institute of Geography has been involved in the project since June 2021. In addition to the above-mentioned areas, neighboring areas with mixed beech and vital spruce stands are also sampled. Current activities (Fig. 4) include:

• Drone surveys in the optical and thermal spectral range.
• Taking hemispherical photos to derive biophysical parameters (leaf area index/LAI, fraction of photosynthetically active radiation/FAPAR, percent canopy cover/FCOVER)
• Continuous monitoring of the microclimate (temperature below, at and above the soil surface, soil moisture)
• Multitemporal satellite image analysis (Sentinel-2, Landsat-8, Sentinel-3, MODIS)
• Supervision of bachelor and master theses

Figure 4: Monitoring of degraded areas and reforestation measures by the department "Cartography, GIS and Remote Sensing". Top: drone flight with recordings in the optical and thermal spectral range, Middle: Derivation of biophysical parameters using digital hemispherical photography, Bottom: continuous monitoring of the microclimate on different plots.

Continuous measurements on the ground as well as remote sensing methods, both satellite-based and with the help of drones, can be used to quantitatively record changes in the land cover and land use class "forest", such as loss of vitality, area-wide dieback, reforestation and succession as well as the consequences for the microclimate. In particular, a combination of different sensors, e.g. optical and thermal remote sensing, will help to assess the impact of local forest management to foster a better adaption to climate change.
Contact: Dr. B. Putzenlechner