Succession and soil carbon cycling after forest disturbance

Forest soils store large amounts of carbon (C) and act as globally important sink for atmospheric CO2. Forest disturbances, e.g. by windthrow, are increasing due to a changing climate. This represents a key risk for soil organic C (SOC) storage, potentially turning soils into a distinct C source to the atmosphere until succession returns back to a forested state. Across Europe, tree regeneration is often inhibited by ungulate herbivory and a dense herbaceous ground 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 fungal communities, decomposition processes and SOC storage. It is hypothesized that distinct soil fungal communities 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 mycorrhizal fungi, it is hypothesized that ectomycorrhizal fungi increase the 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 soil fungal communities measured by DNA-based techniques and soil C fluxes. In microcosms, it will be determined under controlled conditions whether plant types and associated mycorrhizal fungi differ in stimulating SOC decomposition and associated C fluxes. Overall, the project will provide novel insights into the complex field of disturbance ecology which has so far focused on above-ground 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 helps to better assess C sequestration in forests and their potential contribution to mitigate climate change. This project will be hosted by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), ETH domain