Nonlinear interactive effects of temperature and soil moisture on plant biomass and carbon fluxes

Climate warming is affecting plant growth and CO ₂ fluxes in high‐latitude ecosystems. However, soil moisture – and how it responds to warming – can alter the shape of the relationship between temperature and both plant performance and ecosystem processes. We conducted a microcosm experiment to test the interactive effects of temperature, soil moisture, and plant species identity on plant biomass and CO ₂ fluxes. The experiment included five temperature levels (ambient to 9°C above ambient) and three soil moisture treatments, including ambient and drought conditions simulating severe drought stress predicted for future dry summers. We grew four ecologically contrasting plant species commonly found in sub‐Arctic alpine meadow under these conditions to explicitly test for non‐linear temperature responses in key ecosystem processes: above‐ and belowground plant biomass, gross primary productivity (GPP), ecosystem respiration (R _eco ) and net ecosystem exchange (NEE). Overall, warming enhanced all measured processes non‐linearly, with stronger responses at higher temperatures. However, the magnitude and shape of these relationships varied depending on the interaction between soil moisture and plant species identity. For example, reducing soil moisture from 55% (ambient) to 33% significantly suppressed warming‐induced increases in above‐ and belowground biomass, GPP, NEE and R _eco at the warmer end of the temperature gradient – especially for Phleum alpinum , which showed a greater reduction than did the other species. Taken together, our findings indicate that the positive effects of warming on plant biomass and CO ₂ flux rates are constrained by lower soil moisture levels. This suggests that the impacts of climate warming on high‐latitude meadow ecosystems will be strongly shaped by warming‐induced changes in soil moisture and shifts in precipitation regimes.