Data from: Nitrogen-fixing tree abundance in higher-latitude North America is not constrained by diversity

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.