On how a distributed experimental approach informs our understanding of the processes underlying context-dependencies in the ecosystem respiration response to a warming tundra

Empirical assessments are valuable sources of knowledge to evaluate impacts of global change on organisms and ecosystems. Experimental data are especially valuable as they offer controlled conditions for testing hypotheses and establishing process understanding. However, these approaches are also notoriously difficult to upscale to broad geographic extents as they require detailed and often labor-intensive studies in multiple field sites. Meta-analyses based on shared protocols and ‘distributed experiments’, that is, experiments replicated across broad geographic or environmental extents, offer opportunities to overcome these challenges. The International Tundra Experiment (ITEX) is one of the largest and longest-running distributed experiments in plant and ecosystem science. In this talk, we will present a recent ITEX data synthesis project that used experimental data to assess the processes underlying increased ecosystem respiration in the warming tundra.   

Arctic and alpine tundra ecosystems are large reservoirs of organic carbon, and climate warming may stimulate ecosystem respiration and release carbon into the atmosphere. The magnitude and persistence of this stimulation and the environmental mechanisms that drive its variation remain uncertain. To address this knowledge gap, we synthesized 136 datasets from 56 open-top chamber in situ warming experiments located at 28 arctic and alpine tundra ITEX sites that have been running for up to 25 years. We show that a mean rise of 1.4 °C in air and 0.4 °C in soil temperature results in an increase in growing season ecosystem respiration by 30%, due to increases in both plant-related and microbial respiration. There was substantial variation in the warming effects on respiration, however. Such context-dependencies have often frustrated attempts at generalizations in ecology, but we show how the distributed experimental approach allowed us to disentangle the ecological processes underlying these variations. We found that tundra sites with stronger nitrogen limitation, and sites in which warming stimulated plant and microbial nutrient turnover, seemed particularly sensitive in their respiration response to warming. This knowledge may improve the accuracy of global land carbon–climate feedback projections. Our study highlights how empirical approaches that enable process understanding of context-dependent ecological variation may allow generalization and prediction of complex ecological phenomena.