Climate-induced shifts in plant investment strategies regulate ecosystem carbon cycling across alpine grasslands

Climate change can impact ecosystem carbon cycling directly by altering kinetic rates of biological processes and indirectly through climate-induced changes in the functional composition of the plant community. Species dominance and complementarity are posed as important mechanisms underlying community regulation of ecosystem processes. Assessing processes regulating ecosystem carbon exchange along climatic gradients can provide crucial knowledge for understanding how carbon cycling can be affected by climate change. Our study was conducted in twelve semi-natural grasslands spanning broad temperature and precipitation gradients. At each site, we conducted extensive measurements of net ecosystem exchange (NEE), gross primary production (GPP) and ecosystem respiration (R_eco). We measured plant functional traits related to plant size and the leaf economics spectrum at each site and computed community-weighted mean trait values (CWM) to characterize the effect of dominant traits and various diversity measures (richness, diversity, evenness and functional dispersion; FDis) to assess effects of complementarity on ecosystem carbon exchange. Plant communities in warmer, drier sites characterized by tall species and high leaf N% had higher GPP and R_eco compared to communities with shorter growing species with smaller leaves, high LDMC and lower N content in colder, wetter sites. This shows that a shift in plant investment strategy along climatic gradients drives ecosystem carbon exchange. Furthermore, more functionally diverse communities had lower further GPP and R_eco further highlighting the importance of dominant traits for ecosystem carbon exchange. Species richness had moderate positive effects on ecosystem carbon exchange giving limited evidence for complementarity, but it may point to the importance of subordinate species for ecosystem stability. Direct effects of climate played a larger role in regulating R_eco than GPP, where temperature enhanced R_eco more than GPP, and high amounts of precipitation promoted GPP while it tended to lower R_eco. Synthesis. Our study shows that shifts in plant functional composition along climatic gradients strongly mediate ecosystem carbon exchange. The greater importance of climate on R_eco compared to GPP has important consequences for net ecosystem exchange and potential climate feedbacks, as plant community assemblage processes will lag behind changes in temperature and precipitation with climate change.