Positive feedbacks and alternative stable states in forest leaf types

The emergence of alternative stable states in forest systems has significant implications for the functioning and structure of the terrestrial biosphere, yet empirical evidence remains scarce. Here, we combine global forest biodiversity observations and simulations to test for alternative stable states in the presence of evergreen and deciduous forest types. We reveal a bimodal distribution of forest leaf types across temperate regions of the Northern Hemisphere that cannot be explained by the environment alone, suggesting signatures of alternative forest states. Moreover, we empirically demonstrate the existence of positive feedbacks in tree growth, recruitment and mortality, with trees having 4–43% higher growth rates, 14–17% higher survival rates and 4–7 times higher recruitment rates when they are surrounded by trees of their own leaf type. Simulations show that the observed positive feedbacks are necessary and sufficient to generate alternative forest states, which also lead to dependency on history (hysteresis) during ecosystem transition from evergreen to deciduous forests and vice versa. We identify hotspots of bistable forest types in evergreen-deciduous ecotones, which are likely driven by soil-related positive feedbacks. These findings are integral to predicting the distribution of forest biomes, and aid to our understanding of biodiversity, carbon turnover, and terrestrial climate feedbacks.

We warmly thank all members of the Crowther lab team, including those not listed as coauthors, for their invaluable support. The collaboration and development of the manuscript\u00A0were supported by the web-based science platform, science-i.org. We thank the Global Forest Biodiversity Initiative (GFBI) for establishing the data standards and collaborative framework. This work was supported by grants to TWC from the Bernina Foundation and DOB Ecology, and CMZ from the Ambizione Fellowship program (#PZ00P3_193646). MB was supported by a Ram\u00F3n y Cajal grant (RYC2021-031797-I) from the Spanish Ministry of Sciences. The European forest inventory data was provided by the FunDivEUROPE project, which received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no 265171. We thank the\u00A0Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, and Dr. Christian Temperli for providing Swiss National Forest Inventory (LFI)\u00A0data for the periods 1983-85, 1993-95, 2004-06, and 2009-2017. This study was also supported by the TRY initiative that is maintained by the Max Planck Institute for Biogeochemistry in Jena, Germany, and currently supported by DIVERSITAS/Future Earth and the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig. National Natural Science Foundation of China (31800374). The ReVaTene dataset is funded by the Education and Research Ministry of C\u00F4te d\u2019Ivoire, as part of the Debt Reduction-Development Contracts (C2Ds) managed by IRD. JCS considers this work a contribution to his VILLUM Investigator project \u201CBiodiversity Dynamics in a Changing World\u201D, funded by VILLUM FONDEN (grant 16549), and Center for Ecological Dynamics in a Novel Biosphere (ECONOVO), funded by the Danish National Research Foundation (grant DNRF173). AFS considers this work a contribution to his UFRN project PVA12722-2015, and is thankful to Solon J. Longhi for data sharing (Conselho Nacional de Desenvolvimento Cient\u00EDfico e Tecnol\u00F3gico 520053/1998-2). ALG considers this work a contribution to his IFFSC project (FAPESC, SDE, IMA, and CNPq grants). VS thanks for support to APVV 20-0168 from the Slovak Research and Development Agency. We are thankful to the State of S\u00E3o Paulo Research Foundation (FAPESP) for supporting the Atlantic Forest plots through the BIOTA/FAPESP Program (Project Functional Gradient 2003/12595-7, 2010/20811-7 & ECOFOR 2012/51872-5), and the Brazilian National Research Council (CNPq grant 403710/2012-0). FCT \u2013 Portuguese Foundation for Science and Technology, project UIDB/04033/2020 and ICNF-Instituto da Conserva\u00E7\u00E3o da Natureza. RAINFOR plots here were supported by a major grant from the Gordon and Betty Moore Foundation. We also thank NERC for long-term support of RAINFOR and ForestPlots.net (including NE/X014347/1, NE/N012542/1, NE/N012542/1, NE/X014347/1) and additional sources to O.L.P. including the ERC (Advanced Grant 291585, T-FORCES). P.R.B. and M.A.Z. acknowledge funding from the CLIMB-FOREST Horizon Europe Project (No 101059888) that was funded by the European Union.