Data from: Patterns of nitrogen-fixing tree abundance in forests across Asia and America

Symbiotic nitrogen (N)‐fixing trees can provide large quantities of new N
to ecosystems, but only if they are sufficiently abundant. The overall
abundance and latitudinal abundance distributions of N‐fixing trees are
well characterised in the Americas, but less well outside the Americas.
Here, we characterised the abundance of N‐fixing trees in a network of
forest plots spanning five continents, ~5,000 tree species and ~4 million
trees. The majority of the plots (86%) were in America or Asia. In
addition, we examined whether the observed pattern of abundance of
N‐fixing trees was correlated with mean annual temperature and
precipitation. Outside the tropics, N‐fixing trees were consistently rare
in the forest plots we examined. Within the tropics, N‐fixing trees were
abundant in American but not Asian forest plots (~7% versus ~1% of basal
area and stems). This disparity was not explained by mean annual
temperature or precipitation. Our finding of low N‐fixing tree abundance
in the Asian tropics casts some doubt on recent high estimates of N
fixation rates in this region, which do not account for disparities in
N‐fixing tree abundance between the Asian and American tropics. Synthesis.
Inputs of nitrogen to forests depend on symbiotic nitrogen fixation, which
is constrained by the abundance of N‐fixing trees. By analysing a large
dataset of ~4 million trees, we found that N‐fixing trees were
consistently rare in the Asian tropics as well as across higher latitudes
in Asia, America and Europe. The rarity of N‐fixing trees in the Asian
tropics compared with the American tropics might stem from lower intrinsic
N limitation in Asian tropical forests, although direct support for any
mechanism is lacking. The paucity of N‐fixing trees throughout Asian
forests suggests that N inputs to the Asian tropics might be lower than
previously thought.