Patterns of nitrogen‐fixing tree abundance in forests across Asia and America
Authors: Menge Duncan N. L. , Chisholm Ryan A. , Davies Stuart J. , Abu Salim Kamariah , Allen David , Alvarez Mauricio , Bourg Norm , Brockelman Warren Y. , Bunyavejchewin Sarayudh , Butt Nathalie , Cao Min , Chanthorn Wirong , Chao Wei‐Chun , Clay Keith , Condit Richard , Cordell Susan , da Silva João Batista , Dattaraja H. S. , de Andrade Ana Cristina Segalin , de Oliveira Alexandre A. , den Ouden Jan , Drescher Michael , Fletcher Christine , Giardina Christian P. , Savitri Gunatilleke C. V. , Gunatilleke I. A. U. Nimal , Hau Billy C. H. , He Fangliang , Howe Robert , Hsieh Chang‐Fu , Hubbell Stephen P. , Inman‐Narahari Faith M. , Jansen Patrick A. , Johnson Daniel J. , Kong Lee Sing , Král Kamil , Ku Chen‐Chia , Lai Jiangshan , Larson Andrew J. , Li Xiankun , Li Yide , Lin Luxiang , Lin YiChing , Liu Shirong , Lum Shawn K. Y. , Lutz James A. , Ma Keping , Malhi Yadvinder , McMahon Sean , McShea William , Mi Xiangcheng , Morecroft Michael , Myers Jonathan A. , Nathalang Anuttara , Novotny Vojtech , Ong Perry , Orwig David A. , Ostertag Rebecca , Parker Geoffrey , Phillips Richard P. , Abd. Rahman Kassim , Sack Lawren , Sang Weiguo , Shen Guochun , Shringi Ankur , Shue Jessica , Su Sheng‐Hsin , Sukumar Raman , Sun I‐Fang , Suresh H. S. , Tan Sylvester , Thomas Sean C. , Toko Pagi S. , Valencia Renato , Vallejo Martha I. , Vicentini Alberto , Vrška Tomáš , Wang Bin , Wang Xihua , Weiblen George D. , Wolf Amy , Xu Han , Yap Sandra , Zhu Li , Fung Tak 
Published: May 2019

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.