Effect of land-use legacy on the future carbon sink for the conterminous US

[摘要] Modeling the effects of the terrestrial carbon sink in the future dependsupon not just current-day land use and land cover (LULC) but also thelegacy of past LULC change (LULCC), which is often not considered. The agedistribution of trees in the forest depends upon the history of pastdisturbances, while the nutrients in the soil depend upon past LULC. Thus,establishing the correct initial state of the vegetation and soil is crucialto model accurately the effect of biogeochemical cycling with environmentalchange in the future. This study models the effects of LULCC from 1750 to2014 using the land-use harmonization dataset (LUH2) of land-use transitionswith the terrestrial ecosystems model (TEM) for the conterminous US.Modeled LULC include plant functional types (PFTs) of potential vegetation,as well as managed cropland, pastureland, and urban areas. LULCC is treatedusing a cohort approach, in which a separate cohort occurs every year thereis a land-use transition, thereby ensuring proper age structure of forestsand regrowth with the correct soil nutrients. From 2000–2014 the modeled netecosystem productivity (NEP) is 989 TgC yr −1 for the conterminousUS but only −15  TgC yr −1 if accounting for carbon lost from land-usetransitions and management. The hypothesis is that the initial state of the vegetation and soilssignificantly affects the future state of the terrestrial carbon sink. Inthis study, LULC remains constant in the future, with the NCAR CCSM4 RCP8.5climate used to force the TEM-Hydro model. The following experiments are runfrom 2015 to 2100, including (a) restarting from existing cohorts in 2014(RESTART), (b) reinitializing in 2015 based on condensing the cohorts foreach PFT into a single cohort (CONDENSED), and (c) restarting from averagecohort conditions for each PFT (AVERAGE). The NEP is too low when usingcondensed cohorts without reinitializing due to a larger increase inheterotrophic respiration ( R h ) resulting from the assumption of matureforests. The carbon stocks are larger than using all the cohorts ifcondensed cohorts are reinitialized due to the assumption of mature,equilibrated forests. Where nitrogen-limited, forest regrowth is enhanced ifregrowth starts from more nutrient-rich conditions. Water fluxes aredominated by environmental factors but can be slightly dependent upon theunderlying carbon dynamics. It is therefore necessary to account for pastdisturbances when modeling future changes in carbon dynamics.

[学科分类] 大气科学