Nitrogen‐fixing tree abundance in higher‐latitude North America is not constrained by diversity

Citation:

Duncan N.L. Menge, Sarah A. Batterman, Wenying Liao, Benton N. Taylor, Jeremy W. Lichstein, and Gregorio Angeles-Perez. 2017. “Nitrogen‐fixing tree abundance in higher‐latitude North America is not constrained by diversity.” Ecology Letters, 20, 7, Pp. 842-851. Publisher's Version

Abstract:

The rarity of nitrogen (N)‐fixing trees in frequently N‐limited higher‐latitude (here, > 35°) forests is a central biogeochemical paradox. One hypothesis for their rarity is that evolutionary constraints limit N‐fixing tree diversity, preventing N‐fixing species from filling available niches in higher‐latitude forests. Here, we test this hypothesis using data from the USA and Mexico. N‐fixing trees comprise only a slightly smaller fraction of taxa at higher vs. lower latitudes (8% vs. 11% of genera), despite 11‐fold lower abundance (1.2% vs. 12.7% of basal area). Furthermore, N‐fixing trees are abundant but belong to few species on tropical islands, suggesting that low absolute diversity does not limit their abundance. Rhizobial taxa dominate N‐fixing tree richness at lower latitudes, whereas actinorhizal species do at higher latitudes. Our results suggest that low diversity does not explain N‐fixing trees' rarity in higher‐latitude forests. Therefore, N limitation in higher‐latitude forests likely results from ecological constraints on N fixation.