Succession and soil carbon cycling after forest disturbance

Forest soils store large amounts of carbon (C) and are a globally important sink for atmospheric CO2. Forest disturbances, e.g. by windthrow, are increasing due to a changing climate and thus pose a significant threat to soil organic C (SOC) storage. After disturbance, SOC stocks decline with soils typically becoming a distinct C source until succession and/or forest management return the ecosystem back to a forested state. Across Europe, tree regeneration is often inhibited by ungulate herbivory and dense herbaceous vegetation, and ecosystems remain in grass-dominated states for decades. The impact of these diverging successional pathways either as tree regeneration or as prolonged grass cover on SOC dynamics is highly unknown because a multitude of plant-soil feedbacks are involved and underlying processes have hardly been studied. The proposed project aims to link successional plant groups, associated soil fungi, decomposition and SOC storage. It is hypothesized that soil fungi associated with each succession type will exert a dominant influence on SOC dynamics. Most temperate trees form symbiosis with ectomycorrhizal fungi, while grasses are primarily associated with arbuscular mycorrhizal fungi. In comparison to arbuscular mycorrhizae associated with grasses, symbiosis of ectomycorrhizae with trees is hypothesized to increase decomposition of older SOC. The project is based on a series of approaches ranging from naturally disturbed forest stands to controlled microcosm experiments. At windthrow sites with tree regeneration and prolonged grass covers, soils will be density fractionated and analyzed for their isotopic composition and radiocarbon (14C) ages, which allows the quantification of C inputs from new plant-derived sources and C losses from older SOC pools. These results will then be linked to fungal communities measured by DNA-based techniques and soil C fluxes. In microcosms, it will be determined whether plant types and associated mycorrhizal fungi differ in their C inputs into soils and in stimulating SOC decomposition. Overall, the project will provide novel insights into the complex field of disturbance ecology with a focus on understudied belowground processes by combining advanced techniques with innovative experimental set-ups from the plant to the ecosystem scale. The findings of this study will be of interest for microbiologists, functional ecologists, ecosystem modelers and will be published in high profile journals. Moreover, the project is highly relevant for policy makers and forest practitioners, as it will help to better evaluate how disturbance regimes and succession influence the climate change mitigation potential of forest management. This project will be hosted by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), ETH domain.

Forest soils store large amounts of carbon (C) and are a globally important sink for atmospheric CO2, helping to mitigate climate change. However, natural disturbances such as windthrow and bark beetle outbreaks are becoming more frequent due to a changing climate and pose a significant threat to soil carbon storage. Following such disturbances, soil carbon stocks often decline and soils can become a source of CO until forest regeneration returns the ecosystem to a forested state. Across Europe, tree regeneration is often hindered by grazing animals and dense grass establishment, leaving many areas dominated by grasses rather than trees for decades. In this project, we investigated the effects of these two contrasting successional pathways after forest disturbance - tree regeneration versus prolonged grass cover - on soil carbon dynamics by analysing the underlying interactions between vegetation, soil microorganisms and nutrient cycling. We used a combination of approaches, ranging from studies in naturally disturbed forest stands to controlled microcosm experiments. Our methods included measurements of carbon and nutrient pools and fluxes, as well as innovative isotopic and DNA-based techniques. Soil carbon and nitrogen stocks were found to increase under longer grass cover. Three decades after forest disturbance, these stocks were about a third higher than in areas where forests had regrown. This increase was partly due to carbon inputs from the fine roots of the grasses. The microbial community also differed significantly between the two systems. In the grass-dominated areas, ectomycorrhizal fungi, which mainly live in symbiosis with trees, were almost absent. Instead, saprotrophic decomposer- and pathogenic fungi were more abundant. Grass-dominated systems also showed increased nitrogen cycling rates, with enhanced nitrification and mineralisation driven by more abundant bacteria and archaea. This faster cycling is likely to have created a feedback loop, supporting grass growth and contributing additional root-derived carbon to the soil. While grass-dominated systems improved soil carbon storage and stability, their overall carbon sequestration potential was lower than that of regenerating forests with trees. Grass-dominated systems lacked the woody biomass carbon that forests naturally accumulate. Regenerating forest stands achieved higher total carbon sequestration over 30 years. These results provide insights into the complex plant-microbe-soil interactions in post-disturbance landscapes. They highlight the critical role of plant type in shaping the carbon sequestration potential of disturbed forest ecosystems. This project was carried out by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and BOKU University, with support from the Austrian Federal Forests (ÖBf AG), the National Park Kalkalpen GmbH and the Austrian Environment Agency (UBA).