Periodic Reporting for period 2 - THRESHOLD (Thresholds and tipping points in ecosystem responses to global warming)
Période du rapport: 2022-07-01 au 2023-12-31
There is little consensus on how the functioning of terrestrial ecosystems will change under projected scenarios of global warming, or when we will reach or surpass thresholds and tipping points. This is largely because most studies have failed to unravel ecosystem responses to increasing temperatures in terms of the underlying non-linear responses of plants, soil organisms, and their communities. Since plants and their associated soil organisms (i.e. pathogens, mutualists, and decomposers) can vary in their responses to changes in temperature, global warming may disrupt or decouple interactions among coexisting and co-evolved species. This may have unforeseen consequences for key ecosystem functions, such as carbon and nutrient cycling.
Terrestrial ecosystems are globally important in providing key services to humankind, such as elemental cycling and climate regulation. However, under global warming, ecosystem services may be at risk and the detrimental effects of increasing air and soil temperatures are already evident. Among these, terrestrial carbon cycling has received particular attention, since destabilized ecosystem carbon fluxes, caused by global warming, can strongly contribute to amplifying climate change. This is exemplified when terrestrial ecosystems switch from their current role as a carbon sink to a carbon source as temperatures increase.
The overall objective is to develop a mechanistic understanding of non-linear and threshold responses of plants and soil organisms, as well as and their interactions, to better explain and predict the rate, timing, and, hence, vulnerability of ecosystem functioning to climate change. By taking a multi-level experimental approach, we will step by step unravel ecosystem responses to elevated temperatures in terms of the underlying non-linear responses of plants, soil organisms, and the communities they comprise.
Terrestrial ecosystems are globally important in providing key services to humankind, such as elemental cycling and climate regulation. However, under global warming, ecosystem services may be at risk and the detrimental effects of increasing air and soil temperatures are already evident. Among these, terrestrial carbon cycling has received particular attention, since destabilized ecosystem carbon fluxes, caused by global warming, can strongly contribute to amplifying climate change. This is exemplified when terrestrial ecosystems switch from their current role as a carbon sink to a carbon source as temperatures increase.
The overall objective is to develop a mechanistic understanding of non-linear and threshold responses of plants and soil organisms, as well as and their interactions, to better explain and predict the rate, timing, and, hence, vulnerability of ecosystem functioning to climate change. By taking a multi-level experimental approach, we will step by step unravel ecosystem responses to elevated temperatures in terms of the underlying non-linear responses of plants, soil organisms, and the communities they comprise.
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.
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.
We know surprisingly little about temperature response functions, and how the shape of response functions of one group of organisms (plants) depend on the response functions of an associated group of organisms (soil biota). Further, while the major research fields of organismal, community, and ecosystem ecology have seen much recent activity and advancements in the context of global warming, they have not yet been brought together in a unified way, which limits our capacity to predict ecological responses as climate warming proceeds. Our THRESHOLD project tackles this issue directly by using a cross-disciplinary approach of dissecting ecosystem responses to warming down to the underlying non-linear species and community responses. This allows us to break new ground in predicting how ecosystem functioning will change under projected scenarios of global warming, and when we would reach or surpass critical thresholds and tipping points. This knowledge is of current interest and global importance because these threshold and tipping points are at the very basis of understanding global change effects on the provisioning of ecosystem services.
Towards the end of the project, data on process, community, and individual responses to temperature will be synthesized using conceptual and quantitative approaches. This will help in positioning our overall findings within a broader context of understanding ecosystem dynamics under global change. While this project is focused on global forest-alpine ecotones, the concepts that it develops will be widely applicable to other ecosystems under temperature stress, particularly those where large shifts in plant community composition (and associated soil communities) are to be expected. The expected breakthrough findings will not only advance the field of experimental ecology, but also pave the way for improving the accuracy of model predictions. In particular, determining whether plant and soil biota traits align in a predictable way in response to temperature stress has been identified as one of the key ‘ways forward’ in improving the predictive capacity of process-based models of biogeochemical cycling, and for understanding the large-scale and long-term responses of carbon dynamics to global change.
Towards the end of the project, data on process, community, and individual responses to temperature will be synthesized using conceptual and quantitative approaches. This will help in positioning our overall findings within a broader context of understanding ecosystem dynamics under global change. While this project is focused on global forest-alpine ecotones, the concepts that it develops will be widely applicable to other ecosystems under temperature stress, particularly those where large shifts in plant community composition (and associated soil communities) are to be expected. The expected breakthrough findings will not only advance the field of experimental ecology, but also pave the way for improving the accuracy of model predictions. In particular, determining whether plant and soil biota traits align in a predictable way in response to temperature stress has been identified as one of the key ‘ways forward’ in improving the predictive capacity of process-based models of biogeochemical cycling, and for understanding the large-scale and long-term responses of carbon dynamics to global change.