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.
Bibliographical noteFunding Information:
We thank everyone involved in the collection of the vast quantity of data for the 44 CTFS-ForestGEO plots considered, including the hundreds of volunteers and field and laboratory staff who tirelessly contributed to the repeated censuses of each plot. The CTFS-ForestGEO network has received major support from the Smithsonian Institution ? particularly the Smithsonian Tropical Research Institute, the Arnold Arboretum of Harvard University, the National Science Foundation (multiple grants), the Rockefeller Foundation, the John Merck Fund, the John D. and Catherine T. MacArthur Foundation, the Andrew W. Mellon Foundation, the Frank Levinson Family Foundation, the HSBC Climate Partnership, the Bromley Charitable Trust, John Swire & Sons Pty Ltd, Celerity, F. H. Levinson Fund, Small World Institute Fund and Jennifer and Greg Johnson. Plot-specific acknowledgements are listed in Table S1. Part of the analyses was conducted during the 2016 CTFS-ForestGEO Workshop in Hainan, China, supported by a National Science Foundation Dimensions of Biodiversity award (DEB-1545761 to S.J.D.). R.A.C. and T.F. are supported by a grant to R.A.C. from James S. McDonnell Foundation (#220020470). D.N.L.M. acknowledges support from NSF DEB-1457650. This is study # 760 of the technical series of the Biological Dynamics of Forest Fragment Project BDFFP?INPA/STRI. In addition, we thank David A. Wardle, Amelia Wolf, Benton Taylor, Andrew Quebbeman and Sian Kou-Giesbrecht for comments on the manuscript.
- Smithsonian ForestGEO
- nitrogen fixation
- nutrient limitation