Warming in the subarctic: Impacts on soil food webs and carbon cycling

Terrestrial ecosystems provide key ecosystem services, yet their stability is increasingly threatened by global warming. There is, however, little consensus on how ecosystem functioning will respond to projected warming scenarios or when thresholds and tipping points may be crossed. This uncertainty arises largely from our limited understanding of the underlying non-linear responses of plants and soil organisms to temperature changes. Since plants and soil organisms often respond differently to warming, it can disrupt or decouple interactions among coexisting and co-evolved species, potentially leading to unforeseen consequences for key ecosystem functions, such as carbon and nutrient cycling.

Our ERC-THRESHOLD project aims to unravel these dynamics by investigating how non-linear temperature responses manifest across levels of ecological organization, including soil micro-organisms and soil fauna. We use forest-tundra and forest-alpine ecotones in seven countries across five continents to assess how plants, soil organisms, and ecosystem carbon cycling respond to increasing temperatures and how these responses may cross critical thresholds.

Preliminary findings show two key patterns. First, the slope of temperature profiles differs between aboveground and belowground measurements, with a steeper decline aboveground. This means that the difference between aboveground and belowground temperature declines with elevation. This has important implications for studying the effects of warming on soil food webs. Second, the shape of carbon flux responses along temperature gradients varies widely across transects and countries, indicating strong regional context dependence. Ongoing analyses of soil microorganisms and soil fauna aim to further elucidate these patterns

We also conduct growth chamber experiments to estimate how warming influences ecosystem carbon fluxes through the reorganization of plant and soil communities. In one experiment, subarctic heath vegetation monoliths were incubated at five warming levels, ranging from ambient to +9°C. While nematode density and community composition at the feeding group level remained relatively stable across warming treatments, individual nematode families exhibited diverse linear and non-linear responses. Soil micro-arthropods, including mites and springtails, showed generally weak responses to (short-term) warming, with patterns influenced by the dominant plant species. In another experiment, using the same temperature treatments, we are examining the warming responses of constructed tundra meadow communities and associated biogeochemical processes, both in absence and presence of soil microfauna. This experiment also tests the responses and effects of ‘encroaching’ ectomycorrhizal tree seedlings, specifically Betula pubescens subsp. czerepanovii.

Our ongoing work focuses on identifying the shapes of temperature "response functions" for plants, soil organisms, their communities, and the ecosystem processes they drive. By distinguishing linear from non-linear responses, we aim to better understand the mechanisms underlying ecosystem resilience and susceptibility to warming. Defining these response functions represents a critical frontier in global change research, offering insights into how terrestrial ecosystems may transition under future climate scenarios.