PS 30-162 - Aging stands and weak growth enhancement drive biomass dynamics in US forest

Abstract: Forests account for much of the global and US land carbon (C) sinks. In the US, forests currently offset ~13% of annual US carbon emissions, however, the future strength and persistence of this C sink are unclear. Thus, partitioning the US land C sink into forest maturation and growth enhancement (ge) components can help understand the C sequestration potential of forests and inform forest management and climate change mitigation plans. Ecosystem experiments have shown about a 30% increase in forest biomass over the first decade of elevated CO₂ treatments, with some studies showing diminishing effects over time. Previous studies quantifying growth enhancement of natural forests show insignificant or weak ge effects. Current United States Department of Agriculture Forest Inventory and Analysis (FIA) data provide more power to detect ge, and the potential for ge effects to be visible in forest inventory data has grown due to continued CO₂ emissions. We used FIA data from 1997-2021, including 113,806 measurements of 57,532 plots, to quantify ge. We fit weighted non-linear least squared models to evaluate the effects of ge, climate change, and biomass loss rates on biomass vs. age, growth vs. age, and growth vs. biomass relationships by US ecoprovince.



The area-weighted mean biomass vs. age ge was 0.64±0.09 % year^-−1 for the coterminous US, which was largely driven by ge in 14 Eastern ecoprovinces (0.62±0.04 % year^-1). ge was significantly positive for half of the Eastern ecoprovinces but was negative or statistically insignificant for all Western and Pacific ones. These values indicate temporal trends in live biomass for a given stand age, but do not translate directly into regional C balances, which also depend on changes in stand age distributions. Therefore, we partitioned aboveground biomass changes into stand age-related (ΔB_age), and ge-related (ΔB_ge) components. ΔB_ge was positive in Eastern ecoprovinces but more variable and negative in some of the Interior Western and Pacific ecoprovinces. ΔB_age, however, was always positive, ranging from near 0 to 1 Mg ha^-1 year^-1 for all ecoprovinces, being driven by increases in mean stand ages for all but one ecoprovince. Thus, forest maturation is still a main driver of the US land C sink. Our analyses also show that growth enhancement is occurring in Eastern US forests but is absent in Western and Pacific forests. The pressures of prolonged drought and non-stand-age resetting, press-type disturbances (e.g., pathogens) may be outweighing ge in Western and Pacific US forests.