Modelling the influence of alternative reforestation scenarios on carbon and nutrient leaching from forest sites after forest diebacks in the central-german Harz region
Headed by: Prof. Dr. Michael Rode, Prof. Dr. Karin Frank, Dr. Rico Fischer
Personel: Felix Sauke
Funding: Deutsche Bundesstifung Umwelt (DBU)
Period: 01.01.2024 - 31.12.2026
Contact: michael.rode@ufz.de , felix.sauke@ufz.de
Climate change and its associated environmental changes represent a large threat to the German forest communities, where the low mountain regions with a high proportion of coniferous monocultures, such as the Harz Mountains, recorded the highest forest damage in a nationwide comparison. Until 2023 around 75% of the spruce trees in the Harz Mountains had died, with sometimes drastic consequences on carbon and nutrient leaching from forest sites and the water quality in drinking water reservoirs. Various reforestation scenarios are therefore being discussed for the Harz region, e.g. reforestation with coniferous monocultures, reforestation with adapted mixed forest or natural succession dynamics.
It is uncertain how the development of new forest communities will influence the carbon and nutrient dynamics in forest soils as well as carbon and nutrient exports from forest sites. Depending on the flow paths that connect soil water with groundwater and surface water, the carbon and nutrient dynamics in forest soils also influence the quality of important water resources for drinking water supply. Our project addresses the interface between forest development, soil carbon and nutrient dynamics as well as their influences on water quality. It is therefore highly relevant for practical forest management measures and drinking water treatment. In order to find suitable reforestation strategies and mitigate negative consequences of forest diebacks on drinking water quality we are working in close cooperation with different research groups from the Helmholtz-Centre for Environmental Research, the Julius-Kuehn Institute, the MPI for Biogeochemistry as well as forest management and reservoir management in the Harz region.
To investigate carbon and nutrient exports from forest sites a coupled forest - soil model system was developed within the scope of this research project, consisting of the individual-based forest growth model FORMIND, the soil hydrological model LWF Brook90 and the microbial-driven Jena Soil Model. An important aim of the project was to to conduct different climate and forest management scenarios with the coupled model system for different test sites within the Harz region. These scenarios were used to investigate the effects of different reforestation strategies on the development of forest productivity, soil carbon and nutrient balances, and carbon and nutrient exports through surface and subsurface runoff (dissolved organic matter, dissolved N and P compounds) until 2100.
Results
Initial results of the project indicate that the forest calamity events between 2018 and 2023 primarily led to a short-term increase in nitrogen exports until 2025 at different test sites in the Harz region, whereas carbon and phosphorus exports were only marginally affected. The key processes driving this response were increased litter inputs, enhanced net mineralization, and reduced plant uptake, combined with the high mobility of inorganic nitrogen in the soil solution. A comparison between different simulations from experimental sites with and without forest calamities (test site Lange Bramke in the Harz spruce-dominated was severly disturbed, test site Solling beech-dominated was less severely disturbed) showed that the increase of dissolved inorganic nitrogen concentrations in the soil solutions were only present at the test site with forest calamities (test site Lange Bramke in the Harz region). In addition, we observed generally increased dissolved inorganic nitrogen concentrations at another test site in northern Saxony-Anhalt (Klötze, Altmark) which was dominated by Douglas fir. In the model those increased DIN concentrations could be explained by elevated nitrification rates in combination with decreased plant uptake.
Simulations of different reforestation scenarios revealed that the speed of forest regrowth, the nitrogen demand and the plant uptake of the respective forest stand were three very important regulatory factors controlling nitrogen export after a calamity event. Forest stands with high overall nutrient demand (from trees and other early stage successional vegetation) consequently exhibited the lowest nitrogen export rates. Reforestation influenced nitrogen
export via litterfall and nutrient demand, consequently displaying the lowest annual mean for the reforestation scenario with fast growing pioneer tree species (but also the highest variability).
When upscaling the coupled forest - soil model to 24 test sites in the Harz region with different forest types the results showed that dissolved nitrate concentrations in the topsoil were higher at coniferous forest sites than at mixed coniferous and broadleaved forest sites during the calamity event from 2021 - 2024 and also during the regrowth period from 2025 - 2053 (Figure 3). This can be explained by the overall higher mortality rates of the spruce dominated coniferous forest stands. In contrast the concentrations of dissolved organic carbon and dissolved phosphorus in the soil solution showed no difference between forest types (Figure 4 and 5), however the leaching rates of those elements under coniferous forest stands were still higher than under mixed forest stands due to higher precipitation and percolation at the coniferous forest soils in the upper Harz Mountains. In conclusion the nitrate exports were more directly influenced by forest disturbances while dissolved organic carbon and dissolved phosphorus were more influenced by soil hydrology.
