Thresholds and tipping points in ecosystem responses to global warming

Background/Question/Methods

Terrestrial ecosystems are important in providing key ecosystem services, but under global warming the provisioning of these services is at risk. There is, however, little consensus on how ecosystem functioning will change under projected scenarios of global warming, or when we will reach or surpass thresholds and tipping points. This is largely because we have failed to unravel ecosystem responses to warming in terms of the underlying non-linear responses of plants and soil organisms. Since plants and their associated soil organisms can vary in their responses to temperature change, warming may disrupt or decouple interactions among coexisting and co-evolved species. This may have unforeseen consequences for key ecosystem functions, such as carbon cycling. Here, I will introduce and present the first results of our new ERC-THRESHOLD project where we aim to advance our knowledge of how non-linear temperature responses transcend levels of ecological organization. We use forest-tundra and forest-alpine ecotones to assess how responses of ecosystem carbon cycling to increasing temperature will be pushed across thresholds and tipping points. We further perform mesocosm experiments under a range of different temperatures, to estimate how ecosystem process responses to warming can be predicted from the reordering of plant and soil communities.

Results/Conclusions

For one of our ecotones, an elevational gradient in subarctic Sweden, we show that slopes of temperature profiles depend on the position of the measurement, with a steeper decline aboveground than belowground. This means that the difference between aboveground and belowground temperature declines with elevation. Here, we also show that the shapes of temperature responses for morphological and chemical plant root traits are often non-linear. For community-level traits, intraspecific variation was often more important than species turnover, pointing at the importance of trait plasticity in temperature responses. In our mesocosm experiments, we show that plant growth responses to temperature depend on interactions with their associated soil biota. We also show that the shape of plant responses to increasing temperature depend on water availability. This indicates that indirect effects of global warming on plants and soil biota through changes in soil moisture can modify (or counteract) the direct effects of global warming, and hence, could shift when we should expect threshold and tipping point responses in ecosystem functioning under global warming. While the THRESHOLD project is focused on forest-alpine and forest-tundra ecotones, the results and concepts that it develops will be widely applicable to other ecosystems under temperature stress.