We have made good progress with the establishment of a global network of forest-alpine ecotone sites (that is, temperature gradients on par with projected global warming scenarios). These sites are used to quantify the shape of response functions and capture threshold responses of ecosystem carbon cycling, and identify the rates and magnitudes of changes in these key ecosystem processes with increasing temperature. In addition, we also monitor how the functional trait composition of plant and soil communities changes along these temperature gradients and test whether non-linearity in plant and soil community responses can be used to predict where thresholds and tipping points in ecosystem responses occur. We started the global sampling campaign in Australia (early '23) and are currently working in northern Sweden and Colorado, USA (Jul-Aug '23). In the coming year, we will expand our network with new sites in Argentina, New Zealand, France, and Japan, with help of our local collaborators. This network of sites is needed to include a broader biogeographic variety of sites, allowing us to test the context-dependency of warming effects on plants, soils, and ecosystems.
Meanwhile, we have set up our new ‘THRESHOLD lab’ which contains a Temperature Gradient Plate and five large climate chambers. These chambers can be programmed to a range of temperature scenarios, from ambient to well beyond what these communities naturally experience (that is, +0, +2.25 +4.5 +6.75 and +9°C). To mimic realistic diurnal and seasonal cycles, we make use of our long-term temperature measurements in northern Sweden. We have been using this infrastructure for our first experiments testing how plants and soil organisms, their interactions, and their carbon functions respond to shifts in temperature. Some of these experiments focus on herbaceous plants while others focus on Mountain Birch, which is the treeline-forming species in the Scandinavian mountains. These experiments are explicitly designed to test for non-linear temperature responses. Currently, the PhD students working on the THRESHOLD project are analyzing their samples and data. Furhter, we are currently collecting turfs of subarctic tundra vegetation in northern Sweden to set up a new large climate chamber experiment to test shifts in plant and microbial physiology and accompanying changes in aboveground-belowground carbon and nutrient assimilation abilities.
Finally, we are currently harvesting the first field experiment that ran from 2022 to 2023. This experiment tested how shifts in temperature influence the performance of tree seedling establishment in the forest, at the treeline, and (far) above the treeline. Here, we will link seedling performance not only to aboveground and belowground temperature, but also to their associated rhizosphere microbial communities as well as other biotic and abiotic environmental variables. Another field experiment is being set up in northern Sweden this summer (2023) - this experiment will test how plant and microbial communities adjust their nitrogen and phosphorus assimilation abilities and resulting carbon-nitrogen-phosphorus stock under warming. For this experiment, we use a design based on space-for-time substitution along an elevational temperature gradient.
