Soil respiration response to decade-long warming modulated by soil moisture in a boreal forest

The effects of long-term climate warming on soil respiration and its drivers remain unclear in forests, which store approximately 40% of global soil carbon. Here we conducted a climate change experiment for 13 years in forest plots planted with tree juveniles at two southern boreal forest sites. Treatments included simultaneous above- and below-ground warming (ambient, +1.7 °C and +3.3 °C) under different rainfall scenarios (100% and 60% of summer rainfall) and contrasting overstory canopy openness (open and closed). Soil respiration increased by 7% and 17% under +1.7 °C and +3.3 °C warming, respectively, averaged across all sites, treatments and years. These increases in respiration were higher than impacts per degree warming of the only two prior long-term, but soil-only, forest warming experiments. Moreover, warming effects on soil respiration varied significantly over time. Under almost all conditions, moist soil exhibited a greater increase in respiration in response to warming than dry soil. Our results suggest that a realistic range of anticipated conditions, including both above- and below-ground temperature and moisture, should be accounted for when predicting warming effects on soil respiration. Soil moisture greatly affects the response of soil respiration to warming, according to 13 years of warming experiments in a boreal forest.