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Effects of interannual climate variability on tropical tree cover


Climatic warming is substantially intensifying the global water cycle1 and is projected to increase rainfall variability2. Using satellite data, we show that higher climatic variability is associated with reduced tree cover in the wet tropics globally. In contrast, interannual variability in rainfall can have neutral or even positive effects on tree cover in the dry tropics. In South America, tree cover in dry lands is higher in areas with high year-to-year variability in rainfall. This is consistent with evidence from case studies suggesting that in these areas rare wet episodes are essential for opening windows of opportunity where massive tree recruitment can overwhelm disturbance effects, allowing the establishment of extensive woodlands. In Australia, wet extremes have similar effects, but the net effect of rainfall variability is overwhelmed by negative effects of extreme dry years. In Africa, effects of rainfall variability are neutral for dry lands. It is most likely that differences in herbivore communities and fire regimes contribute to regulating tree expansion during wet extremes. Our results illustrate that increasing climatic variability may affect ecosystem services in contrasting, and sometimes surprising, ways. Expansion of dry tropical tree cover during extreme wet events may decrease grassland productivity but enhance carbon sequestration, soil nutrient retention and biodiversity3.

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Figure 1: Interactive effects of MAP and the coefficient of interannual variation in rainfall (CV) on tropical tree cover.
Figure 2: Dry-land tree cover response to extreme wet events.
Figure 3: Schematic diagram of the windows of opportunity theory depicting how a climatic pulse may trigger tree cover expansion.


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This research was partly financially supported by the ERC-Early Warning grant and Spinoza award received by M.S. We thank C. J. F. Ter Braak for insightful discussions on the statistical approaches taken and G. Mazzochini for advice on spatial linear modelling. The data reported in this paper are extracted as described in the supporting online material from the publicly available sites of MODIS ( and CRU (

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M. Holmgren and M.S. conceived and wrote the paper. M. Hirota collected the data. All authors analysed and interpreted the data, and revised the manuscript.

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Correspondence to Milena Holmgren.

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Holmgren, M., Hirota, M., van Nes, E. et al. Effects of interannual climate variability on tropical tree cover. Nature Clim Change 3, 755–758 (2013).

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