Divergent Pathways of Nitrogen-Fixing Trees through Succession Depend on Starting Nitrogen Supply and Priority Effects.

AbstractNitrogen-fixing trees are a major potential source of nitrogen in terrestrial ecosystems. The degree to which they persist in older forests has considerable implications for forest nitrogen budgets. We characterized nitrogen-fixing tree abundance across stand age in the contiguous United States and analyzed a theoretical model to help understand competitive outcomes and successional trajectories of nitrogen-fixing and nonfixing trees. Nitrogen-fixing tree abundance is bimodal in all regions except the northeastern United States, even in older forests, suggesting that competitive exclusion (including priority effects) is more common than coexistence at the spatial scale of our analysis. Our model analysis suggests conditions under which alternative competitive outcomes are possible and when they are transient (lasting decades or centuries) versus persistent (millennia). Critically, the timescale of the feedbacks between nitrogen fixation and soil nitrogen supply, which is thought to drive the exclusion of nitrogen-fixing trees through succession, can be long. Therefore, the long transient outcomes of competition are more relevant for real forests than the long-term equilibrium. Within these long-term transients, the background soil nitrogen supply is a major determinant of competitive outcomes. Consistent with the expectations of resource ratio theory, competitive exclusion is more likely at high and low nitrogen supply, while intermediate nitrogen supply makes coexistence or priority effects possible. However, these outcomes are modified by the nitrogen fixation strategy: obligate nitrogen fixation makes coexistence more likely than priority effects, compared with perfectly facultative fixation. These results advance our understanding of the successional trajectories of nitrogen-fixing trees and their effects on ecosystem development in secondary succession.