Résumé
Until 2017, up to 90% of the Harz Mountains were forested, mostly with Norwegian spruce. As a result of storms and the drought 2018-2020, the large-scale spruce monoculture stands (proportion of spruce in the tree population: approx. 80%) became infested by spruce bark beetle and subsequently collapsed. Formerly forested areas, including slopes, ridges, and hilltops, are now fallow land (Fig. 1). Their future existence is influenced by various factors: natural succession (in natural dynamic zones), near-natural afforestation (in areas designated as FFH), or commercially-driven afforestation (in the remaining areas). The latter approaches can be categorized into traditional monoculture or mixed forest methods. Regions in Germany and Europe that have experienced forest dieback—such as the Bavarian Forest and its continuation in the Czech Republic, which suffered from acid rain during the 1980s—are seeing significant impacts in several areas: local and regional meteorology, runoff and groundwater recharge patterns, the mobilization or release of nutrients and pollutants, and aquatic ecology, primarily due to changes in water quality and physical conditions. local and regional meteorology, runoff and groundwater recharge patterns, the mobilization or release of nutrients and pollutants, and aquatic ecology, primarily due to changes in water quality and physical conditions. However, the Harz Mountains with the Rappbode dam as Germany’s largest drinking water reservoir, is a fundamental source for millions of people including the economy. Moreover, Harz Mountains are a hotspot of regional biodiversity given their isolated location in the North-German lowlands. The proposed changes could significantly impact hydrology and ecology, which may ultimately affect the socio-economic landscape. Additionally, the strategies aimed at transforming the land—from natural succession to commercial monocultural afforestation—may influence these areas in various ways over the long term. Therefore, we set up an observation to monito the different aspects of the environment based on the future land use: natural succession in the Harz National Park, moderated succession, and commercial afforestation with either mixed forests or monoculture coniferous forests. For decades, the Harz region has suffered from atmospheric nitrogen inputs in the form of nitrogen oxides (NOx) that exceed critical load limits for eutrophying nitrogen. This excess leads to an increased risk of nitrate discharges into both groundwater and surface water. The situation has worsened due to the decline in nitrogen absorption by former forests, which has notably impacted the aquatic communities in the typically nutrient-poor creeks and streams. The extensive deforestation not only changes water quality, but also runoff dynamics. Usually, forest catchment areas have an attenuating function on runoff behavior and are therefore considered as kind of decentralized flood protection. Extensive deforestation has the opposite effect, presumably intensifying the dynamics of runoff, particularly during intense rainfall events, becoming more and more usual due to climate change. Reforestation is to be seen as positive, but will only achieve its buffering effect in a few years or decades. It must also be assessed how the change from spruce monocultures to deciduous stands or mixed forest will affect runoff peaks as a result of snowmelt in the long term. And considering groundwater, the change in land surface from forest to (afforested) fallow land can lead to changes in groundwater recharge. On the one hand, evapotranspiration due to the vegetation cover is considerably lower. On the other hand, missing forest and tree shading will significantly increase the solar and global irradiation and thus also the temperatures of the land surface and of channels as well as the surface exposition to wind is considerably higher, increasing evaporation, especially in summer. The Upper Harz plateau is characterized by annual precipitation of 800-1,000 mm, while the prevailing magmatic (granites) and crystalline rocks (clay shales, quartzites, greywackes) allow only moderate infiltration of water along fissures and faults into the subsurface. The rocks are overlain by the very variable but rather thin weathering zone, in which water can percolate and be stored. This loose rock cover represents the rapidly reacting aquifers. Waterlogged zones and bogs often occur in levelling areas. The moderate infiltration capacity of the rocks leads to preferential surface runoff in the form of streams and ditches, which also drain the waterlogged zones. The year-round waterlogged zones and peat bogs will be particularly affected by this and will presumably dry out in summer. The aim of the installed Harz Mountain Observatory is to determine the short- to long-term effects of: the large-scale loss of forest cover and the subsequent replanting with different forest types. the large-scale loss of forest cover and the subsequent replanting with different forest types. In principle, we assume that the abrupt change in land cover can and will have corresponding consequences on the seasonal soil water balance, groundwater recharge, the quantity and intensity of surface runoff, the aquatic ecology of watercourses, nutrient discharge (C, N, P) and the mobilization of heavy metals. Four observation sites have been selected accordingly and equipped with sensors to monitor: groundwater levels, surface water and soil moisture in different depths. The physical properties as well as the chemical and isotopic composition of: soil water, groundwater and surface water is determined by regular manual and automatized sampling applying a high-capacity sampler with evaporation protection. groundwater levels, surface water and soil moisture in different depths. The physical properties as well as the chemical and isotopic composition of: soil water, groundwater and surface water is determined by regular manual and automatized sampling applying a high-capacity sampler with evaporation protection. In addition, continuous in-situ measurements of pH, T, EC, turibidity, NO 3 applying multiprobe devices are undertaken. The biodiversity and ecosystem functioning of streams are continuously monitored.