Forest ecosystem recovery following disturbance is often constrained by nitrogen. Nitrogen can be replenished through biological nitrogen fixation (BNF), but in temperate forests BNF declines rapidly after a few decades of recovery. The long term effect of such transitory BNF on forest composition 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. In particular, legacy BNF may also favor the establishment and growth of arbuscular mycorrhizal tree species, which appear to benefit from faster rates of nitrogen mineralization relative to ectomycorrhizal tree species. However, few datasets offer a direct test of these ideas. Using a record of permanent vegetation plots at the Coweeta Hydrologic Laboratory, we estimated BNF by Robinia pseudoacacia, an early successional N-fixing tree, across forest plots over the 90 years following selective logging, and in watershed 7 (WS7) 40 years after clear-cut logging. We hypothesized that BNF would increase biomass accretion rates and the relative dominance of arbuscular mycorrhizal trees. 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.
Results/Conclusions
BNF by Robinia pseudoacacia varied widely across plots in the Coweeta basin and in WS7 over the first 40 years after disturbance. In the basin, median summed BNF over this period was 80 kg N/ha following the selective logging and loss of Chestnut in the 1920 and 1930s. In contrast, median summed BNF was 614 kg N/ha in the 40 years following clearcut logging in WS7. Although rates of aboveground biomass accretion were highly variable across plots, total BNF was a strong predictor of biomass accretion. The fraction of aboveground biomass comprised by arbuscular mycorrhizal tree species increased to 25% on average across the basin, 90 years after selective harvesting, and approached 50% in WS7, 40 years after clear-cut logging. In both cases, the relative dominance of arbuscular mycorrhizal trees increased as a function of BNF, and this relationship was stronger in WS7. Our findings suggest that BNF locally enriches soil nitrogen pools, thereby favoring the establishment and growth of arbuscular mycorrhizal tree species over several decades following disturbance. Such legacy effects of BNF may contribute to heterogeneity in species composition and carbon and nitrogen cycling across temperate forests.