Effect of Land Use and Land Cover Change in Context of Growth Enhancements in the United States Since 1700: Net Source or Sink?

Abstract

Many flux-based data and related models indicate that the United States is currently a strong carbon sink, but this flux imbalance does not account for carbon lost from previous disturbance. Here we take a modeling approach that involves a full carbon accounting to include the effect of previous human-induced land use changes and management during a period of abiotic changes to the environment since the 1700s. The goal is to show how land use and land cover change has affected carbon loss in the context of other environmental changes like climate, elevated CO 2 , and nitrogen deposition that tend to enhance growth. Here we show that land use and land cover change has led to the loss of carbon since 1700 (42 PgC), while growth enhancements since the start of the 20th century have only partially countered this loss (1.8 PgC). Fertilized croplands are the largest carbon sink in the first decade of the 21st century (477 TgC/year), followed by forests (266 TgC/year) and grazing pastures (223 TgC/year), if not accounting for land use conversion fluxes, product decomposition, and livestock respiration (i.e., net ecosystem productivity). When accounting for these disturbance fluxes, only returning forests (40 TgC/year) and grasslands (16 TgC/year) remain a carbon sink. A carbon accounting of human disturbances should be considered when determining the climate and policy implications of forest regrowth on the terrestrial carbon sink.

Understanding forest dynamics by integrating age and environmental change

“…In addition, land‐use change can create disturbances not followed by a typical recovery, often replacing the original forest ecosystem with one that has faster C turnover and smaller C storage capacity (Luo & Weng, 2011). For example, since 1700, land‐use change has led to a net loss of 42 Pg C in terrestrial C in the USA (Felzer & Jiang, 2018). Given the significant C loss caused by reduced forest recovery, it is necessary to closely examine forest regrowth to predict the future terrestrial C sink.…”

Author's reply to reviewer comments

“…Many land surface models did incorporate empirical relationships on management effect on soil and vegetation carbon. A recent literature is Felzer and Jiang (2018), who assessed…”

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

“…Globally, during the 21 st century, land use and land cover change (LULCC) has accounted for 14% of the total anthropogenic carbon emissions (Friedlingstein et al, 2020). LULCC have been responsible for the largest losses of carbon from the land in the conterminous U.S. since the 1700s, with growth enhancements from CO2 fertilization and nitrogen deposition only partially countering this loss since the 1950s (Felzer and Jiang, 2018). Reforestation and afforestation have been the primary drivers for this enhanced sink (Kondo et al, 2018), especially growing back with rising CO2 levels (Strassmann et al, 2008).…”

“…Only a few models (e.g. (Felzer and Jiang, 2018;Shevliakova et al, 2009)) have included forest demography, to accurately track the effects of disturbance in regrowing forests. Krause et al (2020) showed that including land legacy effects increases future carbon storage as ecosystems regrow and adapt to higher levels of CO2 and N deposition.…”