Estimating current and future fine root turnover rates at landscape scales

Background/Question/Methods Fine root turnover can account for as little as 10% and to more than 50% of net primary productivity in temperate forests. In addition, fine root turnover rates can differ by an order of magnitude among temperate tree species which makes accurate and widely applicable forest fine root turnover estimates problematic. Currently, several landscape scale models can predict future forest composition under future climate scenarios. However, we are unaware of any model applications that include species-specific information on fine root turnover and how this may shift with climate change. Here, we combine output from an existing model, DISTIB, with estimates of fine root turnover derived primarily from direct root observations and relationships with fine root traits. DISTRIB predicts future distributions of 134 common temperate tree species in the eastern United States based on landscape patterns, land use, elevation, and soil factors as well as prescribed climate. For this exercise we used a limited subset of 33 tree species and conducted our analysis at two distinct spatial scales. Our analysis was conducted first at the watershed scale in the Shale Hills Critical Zone Observatory in Central Pennsylvania (40°39'51"N 77°54'28"W) and was subsequently run across the state of Pennsylvania. Results/Conclusions Using measured importance values from Shale Hills coupled with anticipated shifts in species dominance provided by DISTRIB our analysis predicts that fine root turnover rates increase slightly from ~1.6 to 1.7 yr-1 (~6% increase) under the GCM3Hi scenario (averaged future climate scenarios of 3 Global Circulation Models assuming high CO2 emissions). There was little change predicted using GCM3Lo (low emissions scenario). Increased turnover rates with GCM3Hi were driven by a decrease in the dominance of Pinus strobus and Tsuga canadensis (relatively long-lived roots) and an increase in several Carya species and Liquidambar styraciflua (relatively short-lived roots). Across Pennsylvania, average forest fine root turnover decreased from ~1.9 to 1.7 yr-1 under GCM3Hi (12% decrease) or 1.8 yr-1 under GCM3Lo (5% decrease). This change was driven by decreases in Prunus serotina and several Acer taxa (relatively short-lived roots) and increases in Pinus echinata (relatively long lived roots). While our analysis currently discounts potential changes in turnover driven directly by climate (e.g. temperature and precipitation) it explores the utility of combining modeled tree species distributions with species-specific estimates of fine root turnover. Further analyses will include more tree species and fine tuned species-specific root turnover rates.