Patterns of nitrogen‐fixing tree abundance in forests across Asia and America

Abstract

1. 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.2. 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.3. 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. K E Y W O R D Sforest, legume, nitrogen fixation, nutrient limitation, Smithsonian ForestGEO, symbiosis Correspondence Tak Fung

Global macroecology of nitrogen‐fixing plants

“…Menge et al. (2019) also found that N‐fixing trees in the Americas are more prevalent in areas with high mean annual temperature and precipitation. However, large‐scale studies of tropical forests and savannas in Africa and South America (Pellegrini et al., 2016) as well as of Neotropical forests (Gei et al., 2018) have shown an increase of the share of N‐fixing trees in vegetation with decreasing precipitation.…”

“…Menge et al. (2019) found a latitudinal gradient in N‐fixing tree distribution in the Americas, but no such trend was present in Asia despite similar climatic gradients. Because symbiotic nitrogen fixation is an expensive process requiring both large carbon investment for construction of bacteria‐harbouring nodules and energy for nitrogen reduction and bacterial life, N‐fixing plants typically grow in open habitats (Vitousek & Field, 1999).…”

“…The current empirical information about the large‐scale distribution of N‐fixing plant species and the factors influencing the distribution patterns is fragmentary, as most published data concern N‐fixing trees. Two recent studies (Menge et al., 2019; Steidinger et al., 2019) mapped the distribution of the relative cover of N‐fixing trees and found that N‐fixing trees dominate in the tropics. Climate and soil conditions have been the main explanatory factors for N‐fixing tree distribution and latitudinal gradient.…”

Substrate stoichiometry determines nitrogen fixation throughout succession in southern Chinese forests

“…S11). We measured the C, N, and P concentrations and stoichiometry and N fixation rates in the soil, forest floor, and moss (N-fixing trees are rare in the tropics of Asia, including in southern China; Menge et al 2019). Given that plant and soil N concentrations may increase (via N deposition and fixation) throughout succession, which may change substrate C and P availability and stoichiometry, we applied long-term (3-13 years) N-addition treatments (0-150 kg N ha À1 year À1 ) to explore whether resource availability or stoichiometry control N fixation.…”

Soil nitrogen concentration mediates the relationship between leguminous trees and neighbor diversity in tropical forests

“…However, a high number of N-fixing legume species are also found in undisturbed tropical forests 24 and there is little evidence of decline in N-fixer abundance in forest chronosequence studies 8,25,26 . Legume richness and abundance also increases from subtropical to tropical areas 14,27,28 , but across the tropics their distribution varies widely, with higher abundance in the Neotropics and Africa than in South East Asia 29 .…”