Symbiotic nitrogen fixation in trees: patterns, controls and ecosystem consequences.

This article reviews recent advances in understanding symbiotic nitrogen fixation (SNF) by trees and shrubs in forest and savanna ecosystems, emphasizing its ecological roles, regulation, and biogeographic patterns. SNF, the biological conversion of atmospheric nitrogen (N₂) into bioavailable nitrogen by plant–bacteria symbioses, is primarily carried out by rhizobial bacteria in the Fabaceae family and Frankia bacteria in actinorhizal plants within the Rosid I clade. The review highlights that SNF rates and the abundance of N-fixing trees are regulated by complex interactions of abiotic factors (such as soil nitrogen and phosphorus availability, temperature, light, and water) and biotic factors (including plant and bacterial taxonomy, competition, and herbivory). It also discusses the evolutionary history and latitudinal distribution of N-fixing trees, noting a shift from rhizobial dominance in tropical regions to actinorhizal dominance at higher latitudes, largely driven by ecological rather than phylogenetic constraints. Finally, the article addresses the implications of global change on SNF, the need for improved mechanistic modeling in Earth system models, and identifies key research priorities to better understand and predict the role of woody SNF in future forest functioning.