Divergent pathways of nitrogen-fixing trees through succession depend on starting nitrogen supply and priority effects

Nitrogen-fixing trees are a major potential source of nitrogen into terrestrial ecosystems. The degree to which they persist into 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-centuries) vs. 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 to 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.

Methods
Data are from the Forest Inventory Analysis (version 5.1) and are publicly available in their raw form at https://www.fia.fs.fed.us/

In processing the data, we calculated the percent basal area of nitrogen (N)-fixing trees for each plot. These data (plot number and percent basal area of N-fixing trees) are displayed in our dataset together with the plot stand age, the absolute basal area of all trees in a plot, the absolute basal area of all N-fixing trees in a plot, the latitude and longitude of the plot, and the 50 year age bin for the plot. Plantations and plots with harvest activities were excluded. N-fixing taxa were identified following Menge et al. (2017), which used actinorhizal and rhizobial N-fixing classifications from Huss-Danell (1997) and Sprent (2009), respectively. We used data for the contiguous United States.

Literature cited:

Huss-Danell, K. 1997. Actinorhizal symbioses and their N2 fixation. New Phytologist 136:375–405.

Menge, D. N. L., S. A. Batterman, W. Liao, B. N. Taylor, J. W. Lichstein, and G. Ángeles-Pérez. 2017. Nitrogen-fixing tree abundance in higher-latitude North America is not constrained by diversity. Ecology Letters 20:842–851.

Sprent, J. I. 2009. Global Distribution of Legumes. Legume Nodulation: A global perspective.