Forests are the most productive terrestrial ecosystems and account for much of the global and US land carbon (C) sinks. However, the causes of these C sinks are unclear. In the US, much of the land sink has been attributed to forest regrowth from previous land use. The role of growth enhancement (GE) due to CO2 fertilization is controversial, both globally and at the US national scale. 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 due to nutrient limitation and/or water limitation. In contrast, previous analyses of United States Department of Agriculture Forest Inventory and Analysis (FIA) data failed to show significant GE, concluding that the entire US land sink is due to forest regrowth. However, these FIA-based analyses have not been updated in over two decades. Current FIA data provide more power to detect GE, and the potential GE signal has grown due to unabated CO2 increase. We used FIA data from 1997-2021, including 208,105 inventories of 92,254 unique plot locations, to quantify GE as the temporal trend in the relationship between live biomass and stand age for different US ecoprovinces.
The area-weighted mean GE was 0.16% year-1 for the coterminous US, 0.80% for five Pacific coast ecoprovinces, -0.17% year-1 for 15 western US ecoprovinces, and 0.28% year-1 for 13 eastern US ecoprovinces. These GE values indicate trends in live biomass for a given stand age but do not translate directly into regional C balances, which also depend on shifts in stand age distributions. For comparison, the mean rate of global atmospheric CO2 increase since 1960 is 0.5% year−1, and the recent US forest C sink is equivalent to a 0.3-0.4% year−1 increase in forest C stocks. The US forest C sink − which integrates changes in land use, management, and disturbance regimes, as well physiological responses to global change drivers such as CO2 increase and nitrogen deposition – has weakened over recent decades, likely due to intensified drought and disturbance regimes in the western US and forest maturation in the eastern US. Our analysis suggests that eastern and Pacific US forests are responding positively to physiological drivers (CO2 or otherwise), but western forests are not. Without the weakly positive mean national-scale GE response, the US forest C sink would likely be weakening at a faster pace than observed.