19 Oct 2022
19 Oct 2022
Status: this preprint is currently under review for the journal BG.

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

Benjamin Seth Felzer Benjamin Seth Felzer
  • Earth and Environmental Sciences, Lehigh University, 1 W. Packer Ave., Bethlehem, PA 1807

Abstract. Modeling the 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 upon 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. Modeled LULC include plant functional types (PFTs) of potential vegetation, as well as managed cropland, pastureland, and urban areas. 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. From 2000–2014 the modeled Net Ecosystem Productivity (NEP) is 989 TgCyr-1, but only -15 TgCyr-1 if accounting for carbon lost from land use transitions and management.

The hypothesis is that the initial state of the vegetation and soils significantly affects the future state of the terrestrial carbon sink. In this study, LULC remains constant in the future, with the NCAR CCSM4 RCP8.5 climate used to force the TEM-Hydro model. 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 (AVERAGE). 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. Water fluxes are dominated by environmental factors, but can be slightly dependent upon the underlying carbon dynamics. It is therefore necessary to account for past disturbances when modeling future changes in carbon dynamics.

Benjamin Seth Felzer

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-208', Anonymous Referee #1, 07 Nov 2022
  • RC2: 'Comment on bg-2022-208', Anonymous Referee #2, 11 Nov 2022

Benjamin Seth Felzer

Benjamin Seth Felzer


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Short summary
The future of the terrestrial carbon sink depends upon the legacy of past land use, which determines the stand age of the forest and nutrient levels in the soil, both of which affect vegetation growth. This study uses a modeling approach to determine the effects of land use legacy in the conterminous U.S. from 1750 to 2099. Not accounting for land legacy results in a low carbon sink and high biomass, while water variables are not as highly affected.