Relationships between above‐ground plant traits and carbon cycling in tundra plant communities

The trait composition and trait diversity of plant communities are globally applicable predictors of ecosystem functioning. Yet, it is unclear how plant traits influence carbon cycling. This is an important question in the tundra where vegetation shifts are occurring across the entire biome, and where soil organic carbon stocks are large and vulnerable to environmental change. To study how plant traits affect carbon cycling in the tundra, we built a model that explained carbon cycling (above-ground and soil organic carbon stocks, and photosynthetic and respiratory fluxes) with abiotic conditions (air temperature and soil moisture), and the averages and within-community variabilities of three above-ground traits: plant height, leaf dry matter content (LDMC) and SLA. These functional parameters were represented by abundance-weighted means and standard deviations of species traits. The data were collected from an observational study setting from northern Finland. The explanatory power of the models was relatively high, but a large part of variation in soil organic carbon stocks remained unexplained. Average plant height was the strongest predictor of all carbon cycling variables except soil carbon stocks. Communities of larger plants were associated with larger CO2 fluxes and above-ground carbon stocks. Communities with fast leaf economics (i.e. high SLA and low LDMC) had higher photosynthesis, ecosystem respiration and soil organic carbon stocks. Within-community variability in plant height, SLA and LDMC affected ecosystem functions differently. Variability in SLA and LDMC increased CO2 fluxes and soil organic carbon stocks, while variability in height increased the above-ground carbon stock. The contributions of within-community trait variability metrics to ecosystem functioning within the study area were about as important as those of average SLA and LDMC. Synthesis. Plant height, SLA and LDMC have clear effects on tundra carbon cycling. The importance of within-community trait variability highlights a potentially important mechanism controlling the vast tundra carbon pools that should be better recognized. More research on root traits and decomposer communities is needed to understand the below-ground mechanisms regulating carbon cycling in the tundra.