Fine root lifespan and turnover at ecosystem and landscape scales: Sensitivity of four ecological models and new strategies for model incorporation

Background/Question/Methods

Fine root lifespan and turnover control a dominant flux of carbon from plants into soils and affect the nutrient and water uptake of a root system.  Today, terrestrial models incorporate fine root turnover which, depending on model structure, may impact rates of carbon accrual in soil as well as limit net primary productivity of the system through its effects on water and nutrient uptake.  Due to relatively sparse empirical data available to parameterize fine root turnover in models, estimates of turnover used vary by more than an order of magnitude and are often highly uncertain.  This uncertainty and lack of available data for model parameterization guides our two research efforts.  First, we tested the sensitivity of four models (CENTURY, MC1, ED2, and LandCarb) to a range of fine root turnover rates (0.1 to 5.0 yr^-1) in forested systems to determine if and how adjustments in root turnover impacts total system carbon stores.  Our second objective was to develop a species-specific approach to estimating fine root lifespan at ecosystem and landscape scales that could be used to inform estimates of fine root turnover in modeling applications.  We tested this approach against a long-term dataset of root lifespan from the DukeFACE site and applied estimates of lifespan at the landscape scale using the DISTRIB model. 

Results/Conclusions

Of the four models tested, the CENTURY and ED2 models were the most sensitive with the CENTURY model v4.5 showing a 40% decrease in total system carbon following adjustment from fast fine root turnover (5.0 yr^-1) to slow root turnover (0.1 yr^-1).  MC1, which incorporates the CENTURY model v4.0, showed less sensitivity to fine root turnover with a 10% decrease in total system carbon while LandCarb was insensitive to adjustments in fine root turnover.  These results show that some models may be highly sensitive to different root turnover rates, particularly those where primary productivity is controlled at least in part by root uptake of water and nutrients, and that caution should be used when parameterizing fine root turnover.  Preliminary results estimating fine root lifespan and turnover at ecosystem and landscape scales using species-specific estimates of fine root lifespan indicate that the approach is feasible, particularly for forests in the eastern United States.  This approach may be particularly useful as it is computationally inexpensive and may provide a tool to help modelers estimate fine root turnover for sites where empirical data are not available.