Abstract
The dynamics of Amazonian rainforest over long timescales connect closely to its rich biodiversity. While palaeoecological studies have suggested its stability through the Pleistocene, palaeontological evidence indicates the past existence of major expansions of savannah and grassland. Here we present integrated modelling evidence for a grassier Neotropics during the Last Glacial Maximum, congruent with palaeoecological and biological studies. Vegetation reconstructions were generated using the land processes and exchanges model, driven by model reconstructions of Last Glacial Maximum climate, and compared with palynological data. A factorial experiment was performed to quantify the impacts of fire and low CO2 on vegetation and model–data agreement. Fire and low CO2 both individually and interactively induced widespread expansion of savannah and grassland biomes while improving model–data agreement. The interactive effects of fire and low CO2 induced the greatest ‘savannafication’ of the Neotropics, providing integrated evidence for a number of biogeographically relevant open vegetation formations, including two dry corridors (paths of savannah and grassland through and around Amazonia that facilitated major dispersal and evolutionary diversification events). Our results show a bimodality in tree cover that was driven by fire and further enhanced by ‘CO2 deprivation’, which suggests biome instability in this region of climate space.
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Data availability
The model outputs from the factorial experiments that support our findings are available at the National Centers for Environmental Information repository (https://doi.org/10.25921/7zjs-0t15).
Code availability
The code for the version of LPX used in this study is available at the Zenodo repository (https://doi.org/10.5281/zenodo.4757522).
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Acknowledgements
We thank P. Bartlein for providing the LGM climate datasets. We thank L. G. Lohmann, J. L. Cracraft, J. M. Bates and H. Arakida for constructive discussions throughout the research process. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and is a contribution of the Dimensions of Biodiversity US-Biota São Paulo programme through the Fundacão de Amparo á Pesquisa do Estado de São Paulo (FAPESP 2012/50260-6) and NSF and NASA (NSF DEB 1241056) (HS and SAC). I.C.P. has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 787203 REALM) (ICP). The contribution by D.I.K. was supported by the UK Natural Environment Research Council through the UK Earth System Modelling Project (UKESM, grant no. NE/N017951/1).
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H.S. led the project and was responsible for project design, performing the experiment, analysis and writing. D.I.K. was responsible for project design, analysis, post-processing of data, figures and writing. S.J.M. was responsible for project design, analysis and writing. M.M.C. was responsible for development of the model and execution of the experiment. S.A.C. was responsible for project design, analysis and editing. I.C.P. was responsible for model development, analysis and writing.
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Extended data
Extended Data Fig. 1 Flow chart of model workflow to describe relation between inputs, model, and outputs.
Flow of model protocol from spin-up to biome assignment for each factorial experiment run (LGM climate reconstruction + factorial experiment conditions.
Extended Data Fig. 2 Diagrammatic representation of how biomes are assigned in LPX according to vegetative characteristics.
Diagram representing the biome assignment scheme. a) Division of cold and warm-hot biomes according to GDD and general organization of biomes according by fpc and height. b) Classification into more specific biomes by presence and dominance of pfts. c) Further classification of forests into seasonal and evergreen categories based on pft proportions.
Extended Data Fig. 3 Canopy density of ensemble-driven vegetation reconstruction for LGM Neotropics.
Canopy density (leaf area index) distributions for the ensemble factorial experiment in dimensionless units (m2/m2).
Extended Data Fig. 4 Canopy height of ensemble-driven vegetation reconstruction for LGM Neotropics.
Canopy height (metres) distributions for the ensemble factorial experiment.
Extended Data Table 1 List of original palynological studies used in conjunction with meta-analyses by Marchant et al.49 and Mayle et al.48 for 18 000 ± 1000 14C yr BP.
Pertinent studies are in refs. 51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85. 1:tropical humid forest, 2:tropical dry forest, 3: warm temperate forest, 4: temperate evergreen forest, 5: temperate deciduous forest, 6: boreal evergreen forest, 7: boreal deciduous forest, 8: tropical savanna, 9: sclerophyll woodland, 10: temperate parkland, 11: boreal parkland, 12: dry grass/shrubland 13: hot desert, 14: shrub tundra, 15: tundra.
Extended Data Table 2 Affinity matrix for LPX biomes to compute ‘distance’ between biomes in trait space.
Thf = Tropical humid forest, Tdf = Tropical dry forest, wtf = warm temperate forest, tef = temperate evergreen forest, tdf = temperate deciduous forest, bef = boreal evergreen forest, bdf = boreal deciduous forest, Ts = Tropical savanna, sw = sclerophyll woodland, tp = temperate parkland, bp = boreal parkland, g = dry grass/shrubland, d = desert, st = shrub tundra, t = tundra.
Extended Data Table 3 Correspondence legend between pollen reconstructed and model assigned biomes.
Scheme to compare model and regionally specific pollen-derived biomes using the Discrete Manhattan metric.
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Sato, H., Kelley, D.I., Mayor, S.J. et al. Dry corridors opened by fire and low CO2 in Amazonian rainforest during the Last Glacial Maximum. Nat. Geosci. 14, 578–585 (2021). https://doi.org/10.1038/s41561-021-00777-2
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DOI: https://doi.org/10.1038/s41561-021-00777-2
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