Evenness mediates the global relationship between forest productivity and richness
Authors: Hordijk Iris , Maynard Daniel S. , Hart Simon P. , Lidong Mo , ter Steege Hans , Liang Jingjing , de‐Miguel Sergio , Nabuurs Gert‐Jan , Reich Peter B. , Abegg Meinrad , Adou Yao C. Yves , Alberti Giorgio , Almeyda Zambrano Angelica M. , Alvarado Braulio V. , Esteban Alvarez‐Davila , Alvarez‐Loayza Patricia , Alves Luciana F. , Ammer Christian , Antón‐Fernández Clara , Araujo‐Murakami Alejandro , Arroyo Luzmila , Avitabile Valerio , Aymard C Gerardo A. , Baker Timothy , Bałazy Radomir , Banki Olaf , Barroso Jorcely , Bastian Meredith L. , Bastin Jean‐Francois , Birigazzi Luca , Birnbaum Philippe , Bitariho Robert , Boeckx Pascal , Bongers Frans , Bouriaud Olivier , Brancalion Pedro H. S. , Brandl Susanne , Brienen Roel , Broadbent Eben N. , Bruelheide Helge , Bussotti Filippo , Cazzolla Gatti Roberto , César Ricardo G. , Cesljar Goran , Chazdon Robin , Chen Han Y. H. , Chisholm Chelsea , Cienciala Emil , Clark Connie J. , Clark David B. , Colletta Gabriel , Coomes David , Cornejo Valverde Fernando , Corral‐Rivas Jose J. , Crim Philip , Cumming Jonathan , Dayanandan Selvadurai , de Gasper André L. , Decuyper Mathieu , Derroire Géraldine , DeVries Ben , Djordjevic Ilija , Iêda Amaral , Dourdain Aurélie , Nestor Laurier Engone Obiang , Enquist Brian , Eyre Teresa , Fandohan Adandé Belarmain , Fayle Tom M. , Ferreira Leandro V. , Feldpausch Ted R. , Finér Leena , Fischer Markus , Fletcher Christine , Frizzera Lorenzo , Gamarra Javier G. P. , Gianelle Damiano , Glick Henry B. , Harris David , Hector Andrew , Hemp Andreas , Hengeveld Geerten , Hérault Bruno , Herbohn John , Hillers Annika , Honorio Coronado Eurídice N. , Hui Cang , Cho Hyunkook , Ibanez Thomas , Bin Jung Il , Imai Nobuo , Jagodzinski Andrzej M. , Jaroszewicz Bogdan , Johanssen Vivian , Joly Carlos A. , Jucker Tommaso , Karminov Viktor , Kartawinata Kuswata , Kearsley Elizabeth , Kenfack David , Kennard Deborah , Kepfer‐Rojas Sebastian , Keppel Gunnar , Khan Mohammed Latif , Killeen Timothy , Kim Hyun Seok , Kitayama Kanehiro , Köhl Michael , Korjus Henn , Kraxner Florian , Laarmann Diana , Lang Mait , Lewis Simon , Lu Huicui , Lukina Natalia , Maitner Brian , Malhi Yadvinder , Marcon Eric , Marimon Beatriz Schwantes , Marimon‐Junior Ben Hur , Marshall Andrew Robert , Martin Emanuel , Martynenko Olga , Meave Jorge A. , Melo‐Cruz Omar , Mendoza Casimiro , Merow Cory , Miscicki Stanislaw , Mendoza Abel Monteagudo , Moreno Vanessa , Mukul Sharif A. , Mundhenk Philip , Nava‐Miranda Maria G. , Neill David , Neldner Victor , Nevenic Radovan , Ngugi Michael , Niklaus Pascal A. , Oleksyn Jacek , Ontikov Petr , Ortiz‐Malavasi Edgar , Pan Yude , Paquette Alain , Parada‐Gutierrez Alexander , Parfenova Elena , Park Minjee , Parren Marc , Parthasarathy Narayanaswamy , Peri Pablo L. , Pfautsch Sebastian , Phillips Oliver L. , Picard Nicolas , Piedade Maria Teresa , Piotto Daniel , Pitman Nigel C. A. , Polo Irina , Poorter Lourens , Poulsen Axel Dalberg , Poulsen John R. , Pretzsch Hans , Ramirez Arevalo Freddy , Restrepo‐Correa Zorayda , Rodeghiero Mirco , Rolim Samir , Roopsind Anand , Rovero Francesco , Rutishauser Ervan , Saikia Purabi , Salas‐Eljatib Christian , Schall Peter , Schepaschenko Dmitry , Scherer‐Lorenzen Michael , Schmid Bernhard , Schöngart Jochen , Searle Eric B. , Šebeň Vladimír , Serra‐Diaz Josep M. , Sheil Douglas , Shvidenko Anatoly , Silva‐Espejo Javier , Silveira Marcos , Singh James , Sist Plinio , Slik Ferry , Sonké Bonaventure , Souza Alexandre F. , Stereńczak Krzysztof , Svenning Jens‐Christian , Svoboda Miroslav , Swanepoel Ben , Targhetta Natalia , Tchebakova Nadja , Thomas Raquel , Tikhonova Elena , Umunay Peter , Usoltsev Vladimir , Valencia Renato , Valladares Fernando , van der Plas Fons , Tran Do Van , Van Nuland Michael E. , Vasquez Martinez Rodolfo , Verbeeck Hans , Viana Helder , Vibrans Alexander C. , Vieira Simone , von Gadow Klaus , Wang Hua‐Feng , Watson James , Werner Gijsbert D. A. , Wiser Susan K. , Wittmann Florian , Wortel Verginia , Zagt Roderick , Zawila‐Niedzwiecki Tomasz , Zhang Chunyu , Zhao Xiuhai , Zhou Mo , Zhu Zhi‐Xin , Zo‐Bi Irie Casimir , Crowther Thomas W. 
Published: May 2023

1. Biodiversity is an important component of natural ecosystems, with higher species richness often correlating with an increase in ecosystem productivity. Yet, this relationship varies substantially across environments, typically becoming less pronounced at high levels of species richness. However, species richness alone cannot reflect all important properties of a community, including community evenness, which may mediate the relationship between biodiversity and productivity. If the evenness of a community correlates negatively with richness across forests globally, then a greater number of species may not always increase overall diversity and productivity of the system. Theoretical work and local empirical studies have shown that the effect of evenness on ecosystem functioning may be especially strong at high richness levels, yet the consistency of this remains untested at a global scale.

2. Here, we used a dataset of forests from across the globe, which includes composition, biomass accumulation and net primary productivity, to explore whether productivity correlates with community evenness and richness in a way that evenness appears to buffer the effect of richness. Specifically, we evaluated whether low levels of evenness in speciose communities correlate with the attenuation of the richness–productivity relationship.

3. We found that tree species richness and evenness are negatively correlated across forests globally, with highly speciose forests typically comprising a few dominant and many rare species. Furthermore, we found that the correlation between diversity and productivity changes with evenness: at low richness, uneven communities are more productive, while at high richness, even communities are more productive.

4.Synthesis. Collectively, these results demonstrate that evenness is an integral component of the relationship between biodiversity and productivity, and that the attenuating effect of richness on forest productivity might be partly explained by low evenness in speciose communities. Productivity generally increases with species richness, until reduced evenness limits the overall increases in community diversity. Our research suggests that evenness is a fundamental component of biodiversity–ecosystem function relationships, and is of critical importance for guiding conservation and sustainable ecosystem management decisions.