Forest mycorrhizal dominance depends on historical land use and nitrogen-fixing trees.
Abstract
Most forests are recovering from human land use, making it critical to understand the effect of disturbance on forest recovery. Forests of the eastern United States have a long history of land use, but it is unknown whether historical disturbances have contributed to their transition from ectomycorrhizal (ECM) to arbuscular mycorrhizal (AM) tree dominance. Disturbance may promote nitrogen (N)-fixing trees in early succession, which can elevate soil N availability even after they die. Higher soil N availability may facilitate the competitive success of AM trees over ECM trees, but such 'N fixer founder effects' have not been empirically tested. Here, we analysed data from three land-use disturbances in a temperate forest historically dominated by ECM trees: selective-cutting (ranging from 0 to 52 m2 ha-1), clear-cutting and agricultural abandonment. These disturbances occurred at different times, but long-term data capture 3-7 decades of forest recovery. We found that the AM tree fraction in contemporary forests was 2, 4, and 6-fold higher following selective-cutting, clear-cutting and agricultural abandonment, respectively, compared to forest composition in 1934. Across these disturbances we also observed an increasing abundance of the N fixer black locust immediately following disturbance. Using a simulation model parameterized by data from black locust, we estimated historical rates of symbiotic N fixation to understand the relationship between N fixation and AM dominance in individual plots. We found that N fixation was positively associated with the growth of ECM trees generally, and oak and hickory specifically, only following light selective-cutting (<12 or <18 m2 ha-1 basal area extraction, respectively). Following higher levels of selective-cutting and clear-cutting, N fixation was positively associated with the growth of AM trees, particularly red maple and tulip poplar. Agricultural abandonment led to AM dominance regardless of N fixation rates. Synthesis and applications. Our findings suggest that common land use practices and black locust, a native N fixer, can reduce the dominance of ECM trees. If N fixers are likely to proliferate following disturbance, we might maintain ECM dominance by cutting trees at low densities and by applying prescribed fire to remove N.