Sensitivity of four ecological models to adjustments in fine root turnover rate

Large uncertainties surrounding root-specific parameters limit model descriptions of belowground processes and ultimately hinder understanding of belowground carbon (C) dynamics and terrestrial biogeochemistry. Despite this recognized shortcoming, it is unclear which processes warrant attention in model development, given the computational cost of additional model complexity. Here, we tested the sensitivity of four models to adjustments in fine root turnover in forested systems: CENTURY, ED2, MC1, and LANDCARB. In general, faster root turnover rates resulted in lower total system carbon (C) and within model changes ranged from 1% to 38% following 100-year simulations. However, the underlying mechanisms driving these changes differed among models as some expressed lower net primary productivity (NPP) with faster turnover rates and others had similar NPP but large shifts in C allocation away from wood to fine roots. Based on these findings we expect that different model responses to changes in fine root turnover will be determined by (1) whether C is allocated to fine roots as fixed portion of NPP or to maintain a fixed biomass ratio between fine roots and leaves or stems and (2) whether soil nutrient and water uptake is a function of both resource availability and fine root biomass or based on resource availability alone. These results suggest that better constrained estimates of fine root turnover will represent a valuable improvement in many terrestrial biosphere models.