Our new study, led by Duncan Menge, from Columbia University, and Tak, on global patterns of nitrogen-fixing tree abundance has just been published in Journal of Ecology. Symbiotic nitrogen-fixing (N-fixing) trees can supply large amounts of N to forests, and are important for ecosystem functioning. Patterns of abundance of N-fixing trees have been well studied in the Americas, but less well in other biogeographic regions.
In the new study, which included 82 co-authors from the CTFS-ForestGEO network, we examined patterns and drivers of N-fixing tree abundance in forests across different biogeographic regions. We used tree census data from 44 large CTFS-ForestGEO forest plots, comprising in total approximately 5,000 tree species and 4 million trees. Most of these plots were situated in America and Asia. In America, we confirmed the previously known pattern of a decrease in N-fixing tree abundance by an order of magnitude moving from the tropics towards the poles. In contrast, in Asia, we found no latitudinal gradient in N-fixing tree abundance; instead, abundance was uniformly low, and in particular N-fixing tree abundance in the Asian tropics was approximately six times lower than in the American tropics.
Our results suggest that recent estimates of N fixation rates used in global models may be too high, because they do not account for the Asian tropics having lower N-fixing tree abundance than the American tropics.