Effect of land use legacy and future land use on the future carbon sink for the conterminous U.S

Modeling effects of the terrestrial carbon sink in the future depends upon not just current-day land use and land cover (LULC), but also the legacy of past LULC change (LULCC), which is often not considered. The age distribution of trees in the forest depends upon the history of past disturbances, while the nutrients in the soil depend up past LULC. Thus establishing the correct initial state of the vegetation and soil is crucial to model accurately the effect of biogeochemical cycling with environmental change in the future. This study models the effects of LULCC from 1750 to 2014 using the Land Use Harmonization dataset (LUH2) of land use transitions with the Terrestrial Ecosystems Model (TEM) for the conterminous U.S. LULCC is treated using a cohort approach, in which a separate cohort occurs every year there is a land use transition, thereby ensuring proper age structure of forests and regrowth with the correct soil nutrients. Experiments involve both constant LULC using NCAR CCSM4 RCP8.5 and LULCC using SSP3/RCP7. The following experiments are run from 2015 to 2100, including a) restarting from existing cohorts in 2014 (RESTART), b) reinitializing in 2015 based on condensing the cohorts for each PFT into a single cohort (CONDENSED), and c) restarting from average cohort conditions for each PFT (AVERAGED).

From 2000-2014 the modeled Net Ecosystem Productivity (NEP) is 860 TgCyr-1, but only -21 TgCyr-1 if accounting for carbon lost from land use transitions and management. With constant future LULC, the NEP is too low when using condensed cohorts without reinitializing due to a larger increase in heterotrophic respiration (Rh) resulting from the assumption of mature forests. The carbon stocks are overestimated if condensed cohorts are reinitialized due to the assumption of mature, equilibrated forests. Where nitrogen-limited, forest regrowth is enhanced if regrowth starts from more nutrient-rich conditions. While accounting for past disturbance is necessary for the correct soil moisture, water fluxes are dominated by extant environmental factors. Starting with correct initial conditions rather than reinitializing corrects most of these problems. Further results will show the effect of land use legacy under assumptions of changing LULC in the 21st century.