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Widespread increase of boreal summer dry season length over the Congo rainforest

Abstract

Dry season length strongly influences tropical rainforest vegetation and is largely determined by precipitation patterns1,2. Over the Amazon, the dry season length has increased since 1979 and severe short-term droughts have occurred3,4. However, similar changes have not been investigated for the world’s second largest rainforest, the Congo Basin, where long-term drying and large-scale declines in forest greenness and canopy water content were reported5. Here we present observational evidence for widespread increases in the boreal summer (June–August) dry season length over the Congo Basin since the 1980s, from both hydrological and ecological perspectives. We analysed both dry season onset and dry season end via multiple independent precipitation and satellite-derived vegetation datasets for the period 1979–2015. The dry season length increased by 6.4–10.4 days per decade in the period 1988–2013, primarily attributed to an earlier dry season onset and a delayed dry season end. The earlier dry season onset was caused by long-term droughts due to decreased rainfall in the pre-dry season (April–June). The delayed dry season end resulted from insufficiently replenished soil moisture, which postpones the start of the next wet season and hinders vegetation regrowth. If such changes continue, the enhanced water stress in a warming climate may affect the carbon cycle and alter the composition and structure of evergreen rainforest1,6.

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Data availability

The daily and monthly GPCC precipitation datasets are available at https://doi.org/10.5676/DWD_GPCC/FD_D_V1_100. The 5-d GPCP precipitation data are available at https://precip.gsfc.nasa.gov/. The 5-d CMAP precipitation data are available at https://www.esrl.noaa.gov/psd/data/gridded/data.cmap.html. The satellite-observed NDVI datasets are available from the NASA Earth Exchange (NEX) website (https://nex.nasa.gov/nex). VOD and SIF datasets are available upon request from L.Z.

Code availability

The Python language was used to generate all results. Scripts are available upon request from L.Z.

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Acknowledgements

This study is supported by National Science Foundation (NSF No. AGS-1535426). MERRA-2 reanalysis data were obtained from the Goddard Earth Sciences Data and Information Services Center. The ECMWF interim reanalysis data were obtained from the ECMWF data server.

Author information

Y.J. and L.Z. designed the research. All authors collectively analysed the data. Y.J. prepared the figures and wrote the first draft of this paper. L.Z., C.J.T., A.R., W.H., Y.Y.L. and J.J. contributed to refining the ideas and revising this paper.

Correspondence to Liming Zhou.

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The authors declare no competing interests.

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Peer review information: Nature Climate Change thanks Hans Verbeeck, Jessica Baker and other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Table 1, Supplementary Figs. 1–11 and Supplementary References.

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Fig. 1: Spatial patterns of linear trends of the JJA DSL from four precipitation datasets for the period 1988–2013.
Fig. 2: Regionally aggregated interannual variations and linear trends of the JJA DSL and DSO.
Fig. 3: JJA dry season changes estimated by NDVI and VOD.
Fig. 4: Interannual variability of standardized regional mean precipitation, soil moisture and vegetation parameters for the period 1980–2015.
Fig. 5: JJA dry season changes estimated by NDVI and SIF for the period 2007–2015.