Functional Diversity in Land Surface Modeling: Where and When Does It Matter for the Terrestrial Carbon Cycle?

Plant functional traits moderate ecosystem responses to climate and exchanges of water and carbon between the land surface and the atmosphere. However, the extent to which diversity in functional traits influences global carbon and hydrological cycles is a major unknown. The scaling gap between site-level analyses and global biogeochemical cycling makes it difficult to develop informed protocols for representing physiologically diverse organismal responses in a parsimonious manner suitable for land surface models used in Earth system model projections. Here, we used a perturbed parameter ensemble with the Community Land Model (CLM5) that varied hydraulic, carbon economy, and stomatal parameters across 500 global simulations of the land surface. Parameters were perturbed independently for each plant functional type (PFT), resulting in variation across ensemble members in trait means and ranges for PFTs co-occurring in the same land surface grid cell, while preserving the same number of PFTs. We calculated metrics of ecosystem drought sensitivity and used gaussian process emulators to quantify the relative importance of stomatal, carbon economy, and hydraulic trait diversity in moderating carbon and water fluxes. We found that the type of trait regulating vegetation productivity, drought sensitivity, and stress varies with resource limitation globally. Hydraulic trait diversity showed widespread importance in regulating water and carbon exchange during drought in regions where model structure permits multiple interacting PFTs. Interestingly, increasing functional diversity tended to increase the sensitivity of ecosystem carbon fluxes to drought, contrary to expectations from ecological theory. However, we show this finding is a numerical consequence of sampling across nonlinear functions and is not behavior emergent in the interaction between different PFTs.