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Abstract Symbiotic nitrogen fixation (SNF) represents the largest natural input of bioavailable nitrogen into the biosphere, impacting key processes spanning from local community dynamics to global patterns of nutrient limitation and primary productivity. While research on SNF historically focused largely on herbaceous and agricultural species, the past two decades have seen major advances in our understanding of SNF by tree species in forest and savanna communities. This has included important developments in the mathematical theory of SNF in forest ecosystems, experimental work on the regulators of tree SNF, broad observational analyses of tree N-fixer abundance patterns and increasingly process-based incorporation of tree SNF into ecosystem models. This review synthesizes recent work on the local and global patterns, environmental drivers and community and ecosystem effects of nitrogen-fixing trees in natural ecosystems. By better understanding the drivers and consequences of SNF in forests, this review aims to shed light on the future of this critical process and its role in forest functioning under changing climate, nutrient cycling and land use.