Abstract
Vegetation greening has been suggested to be a dominant trend over recent decades, but severe pulses of tree mortality in forests after droughts and heatwaves have also been extensively reported. These observations raise the question of to what extent the observed severe pulses of tree mortality induced by climate could affect overall vegetation greenness across spatial grains and temporal extents. To address this issue, here we analyse three satellite-based datasets of detrended growing-season normalized difference vegetation index (NDVIGS) with spatial resolutions ranging from 30 m to 8 km for 1,303 field-documented sites experiencing severe drought- or heat-induced tree-mortality events around the globe. We find that severe tree-mortality events have distinctive but localized imprints on vegetation greenness over annual timescales, which are obscured by broad-scale and long-term greening. Specifically, although anomalies in NDVIGS (ΔNDVI) are negative during tree-mortality years, this reduction diminishes at coarser spatial resolutions (that is, 250 m and 8 km). Notably, tree-mortality-induced reductions in NDVIGS (|ΔNDVI|) at 30-m resolution are negatively related to native plant species richness and forest height, whereas topographic heterogeneity is the major factor affecting ΔNDVI differences across various spatial grain sizes. Over time periods of a decade or longer, greening consistently dominates all spatial resolutions. The findings underscore the fundamental importance of spatio-temporal scales for cohesively understanding the effects of climate change on forest productivity and tree mortality under both gradual and abrupt changes.
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Data availability
The tree-mortality sites can be found at https://www.iufro.org/science/task-forces/tree-mortality-patterns (https://doi.org/10.6084/m9.figshare.24847698) (ref. 79). The Google Earth sub-metre high-resolution satellite images can be found at https://doi.org/10.6084/m9.figshare.23243915 (ref. 80). The climate, vegetation and soil data can be found at https://doi.org/10.6084/m9.figshare.24847788 (ref. 81). The combined grids of 270 m and about 8 km can be found at https://doi.org/10.6084/m9.figshare.24850734 (ref. 82). The Landsat NDVI (EVI), MODIS NDVI (EVI), DEM (including elevation, slope and aspect) and TerraClimate database (including precipitation and PDSI product) were calculated on GEE, which is available at https://code.earthengine.google.com/. The GIMMS NDVI data can be obtained from https://poles.tpdc.ac.cn/en/data/9775f2b4-7370-4e5e-a537-3482c9a83d88/. The land cover data were downloaded from http://maps.elie.ucl.ac.be/CCI/viewer/. The world continental boundaries were obtained from https://hub.arcgis.com/datasets/esri::world-continents/about. The sub-metre high-resolution satellite images were downloaded from Google Earth, which is available at https://earth.google.com/. The SPEI data are available from https://digital.csic.es/handle/10261/202305. The precipitation data can be retrieved from https://data.ceda.ac.uk/badc/cru/data/cru_ts. The available water-storage capacity, soil clay and soil sand data were downloaded from https://daac.ornl.gov/SOILS/guides/HWSD.html. The canopy height can be obtained from https://webmap.ornl.gov/ogc/dataset.jsp?dg_id=10023_1. The global maximum rooting depth was derived from https://wci.earth2observe.eu/thredds/catalog/usc/root-depth/catalog.html. The tree density was derived from https://elischolar.library.yale.edu/yale_fes_data/1/. The native plant species richness was downloaded from https://anthroecology.org/.
Code availability
Java, MATLAB, Python and R codes for the analysis of these data can be obtained from https://github.com/YCY-github-YCY/Tree.
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Acknowledgements
This study was supported by the National Natural Science Foundation of China (grant no. 41988101 to S.P.). A.C. was supported by a US Department of Energy grant (grant no. DE-SC0022074). We thank D. Zhu, W. Lang, Y. Yan and Y. Deng for their useful suggestions for this paper, and H. Zhuang and M. Li for their help with the experiments. Any use of trade, product or firm names in this paper is for descriptive purposes only and does not imply endorsement by the US government.
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S.P. designed the research. Y.Y. performed analysis and drafted the figures. Y.Y., S.P., S.H. and A.C. wrote the first draft of the manuscript. W.M.H. collected the tree-mortality sites. C.D.A., W.M.H., S.M.M., R.B.M. and H.X. revised the manuscript. All authors contributed to the interpretation of the results and to the text.
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Yan, Y., Piao, S., Hammond, W.M. et al. Climate-induced tree-mortality pulses are obscured by broad-scale and long-term greening. Nat Ecol Evol 8, 912–923 (2024). https://doi.org/10.1038/s41559-024-02372-1
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DOI: https://doi.org/10.1038/s41559-024-02372-1