Forest ecosystem recovery following disturbance is often constrained by nitrogen. Nitrogen can be replenished through biological nitrogen fixation (BNF), but in temperate forests BNF is supported by certain early-successional species that decline rapidly after a few decades of recovery. The long term effect of such transitory BNF is unclear. If fixed N remains in the plant-soil system it may continue to facilitate forest biomass accretion, long after N-fixers have declined. Legacy BNF may also help speed recovery from future disturbance. However, few datasets offer a direct test of these ideas. Using a record of permanent vegetation plots at the Coweeta Hydrologic Laboratory, we determined the effect of Robinia pseudoacacia, an early successional N-fixing tree, across 274 forest plots (20 x 40 m) over 90 years of forest recovery. In the decades prior to the long-term record, Coweeta experienced two major disturbances: the loss of American chestnut and timber harvesting. We hypothesized that R. pseudoacacia BNF in the first 40 years after disturbance, would increase biomass accretion rates over the long-term record. We estimated plot-level BNF over time using relationships among stem density, age and fixation rates, and determined the role of BNF in aboveground biomass accretion rates using linear models.
Robinia pseudoacacia BNF significantly accelerated forest recovery from disturbance in these southern Appalachian forests. While R. pseudoacacia stems were frequent (occurring in 87% of plots), their contribution to aboveground standing biomass was at most 3% over census years. However, R. pseudoacacia fixed an average total of 125 kg N/ha (range of 0 – 953 kg N/ha) over the first 40 years of recovery. Although rates of aboveground biomass accretion were highly variable across plots (-5.1 – 11.4 Mg/ha/yr), total BNF was a strong predictor of this rate. Each kg of fixed N resulted in 292 kg of additional aboveground biomass growth per ha over the long-term record. Following the extreme drought events of the 1980s, legacy BNF (from 1930 – 1970s) increased the rate of aboveground biomass recovery in plots that experienced widespread tree mortality. Thus, in spite of its low direct contribution to biomass and its early-successional status, R. pseudoacacia indirectly increased the rate of forest recovery for 90 years. Our study suggests that legacy BNF determines the rate of biomass accretion and provides resiliency to repeated disturbance events in forests, even several decades after N-fixers have declined in the ecosystem.