Global grassland productivity and carbon storage benefit from functional diversity under climate change.

Functional diversity plays an important role for ecosystem services of managed grasslands such as their gross primary productivity (GPP) and soil organic carbon (SOC) stocks. Yet vegetation models assessing functional diversity have been limited to small spatial scales. We used the Lund-Potsdam-Jena managed land (LPJmL) competitor stress-tolerant ruderal (CSR) model to assess the role of functional diversity for managed grassland GPP and SOC under climate change at the global scale. LPJmL-CSR simulates functional diversity using plant functional types representing different ecological strategies based on functional traits connected to CSR strategies. Our simulation experiments assumed low and high functional diversity levels and moderate and strong climate change scenarios. Results show that functional diversity plays an important role for GPP and SOC stocks already under moderate climate change. Strong loss of functional diversity prohibits communities' adaptation to climate change, resulting in loss of GPP and SOC. Spatial patterns of GPP and SOC stocks show substantial differences between the low and high functional diversity scenarios dependent on regional conditions. These results are consistent with empirical findings and support the importance of functional diversity as insurance against climate change. The results of our global-scale model assessment show that enhancing functional diversity could be crucial for maintaining grassland productivity and thus human well-being under global change. In combination with empirical findings, they underpin the need for management practices that support functional diversity of managed grasslands.