Tree cover shows strong sensitivity to precipitation variability across the global tropics

Abstract

Aim Vegetation is sensitive to mean annual precipitation (MAP), but the sensitivity of vegetation to precipitation variability (PV) is less clear. Tropical ecosystems are likely to experience increased PV in the future. Here we assessed the importance, magnitude and mechanism of PV effects on tree cover in the context of covarying environmental drivers such as fire, temperature and soil properties. Location Tropical land. Time period 2000–2010. Major taxa studied Trees. Methods We compiled climate, soil and remotely‐sensed tree cover data over tropical land. We then comprehensively assessed the contribution of PV at different time‐scales to tropical tree cover variations and estimated the sensitivity of tree cover to PV changes by conducting rolling‐window regression and variance decomposition analyses. We further adopted a mechanistic modelling approach to test whether water competition between trees and grasses can explain the observed effect of PV. Results We find that PV contributes 33–56% to the total explained spatial variation (65–79%) in tree cover. The contribution of PV depends on MAP and is highest under intermediate MAP (500–1,500 mm). Tree cover generally increases with rainy day frequency and wet season length but shows mixed responses to inter‐annual PV. Based on the estimated sensitivity, tropical tree cover can decrease by 3–5% overall and by up to 20% in Amazonia under a 20% decrease in rainy days. Mechanistic modelling analysis reproduced the continental differences in tree cover along an MAP gradient. Main conclusions Under intermediate rainfall regimes (500–1,500 mm), PV can be a more important determinant of tropical tree cover than conventionally proposed drivers such as MAP and fire. The effect of PV likely results from the sensitivity of tree–grass competition to the temporal distribution of water resources. These results show that climate variability can strongly shape the biosphere.

Soil texture mediates tree responses to rainfall intensity in African savannas

“…On sandier soils, particularly at low rainfall, tree cover instead increased with rainfall intensity (a result that was robust across both Landsat and MODIS tree cover data). This contrasts with other recent analyses that downplay the role of soil in mediating rainfall variability (Xu et al ., ), but generally this result (along with the positive effect of sand in general) is consistent with widely observed patterns of higher savanna tree cover on sandy soils than on clay soils (Williams et al ., ; Sankaran et al ., ; Fensham et al ., ; Staver et al ., ) as well as higher rates of woody encroachment (O'Connor et al ., ; Case & Staver, ). Soil hydrology offers a number of potential hypotheses, worthy of further study, which could explain this pattern.…”

“…Intra‐annual rainfall variability modified overall trends, and those effects depended on interactions, especially with soils – resolving previously conflicting lines of evidence that trees can respond either positively or negatively to less frequent, more intense rainfall. Overall, tree cover was lower where rainfall arrived as fewer, more intense storms, consistent with previous analyses at continental and global scales (Good & Caylor, ; Xu et al ., ). Longer wet seasons exacerbated the negative effect of rainfall intensity on tree cover, and MODIS tree cover also generally declined with wet season length, results which seem surprising given previous findings of tree cover increasing with wet season length (Staver et al ., ; Xu et al ., ) or peaking at an optimum (Feng et al ., ; Guan et al ., ), but which make sense when considering the interrelatedness of rainfall variables analyzed here.…”

“…Overall, tree cover was lower where rainfall arrived as fewer, more intense storms, consistent with previous analyses at continental and global scales (Good & Caylor, ; Xu et al ., ). Longer wet seasons exacerbated the negative effect of rainfall intensity on tree cover, and MODIS tree cover also generally declined with wet season length, results which seem surprising given previous findings of tree cover increasing with wet season length (Staver et al ., ; Xu et al ., ) or peaking at an optimum (Feng et al ., ; Guan et al ., ), but which make sense when considering the interrelatedness of rainfall variables analyzed here. When one controls for rainfall amount and intensity, as we do in this analysis, a longer wet season also corresponds to more intermittent rainfall, which could outweigh the benefits of an extended growing season.…”

“…MAR w (hereafter ‘rainfall’) was the strongest predictor of tree cover by an order of magnitude, but α w (hereafter ‘rainfall intensity’) was also an important predictor, and T w modified the effects of both rainfall amount and intensity. Overall, savanna tree cover declined with increasing rainfall intensity, consistent with previous continental‐scale analyses and theoretical predictions (Good & Caylor, ; Xu et al ., , ). However, the response of tree cover to rainfall intensity varied substantially depending on soil context.…”

Droughts and the ecological future of tropical savanna vegetation

“…Here, I focus on the effects of altered rainfall regimes, specifically droughts, on savanna dynamics. My focus on drought is prompted by the recognition that: (a) water availability is a key determinant of savanna structure and function (Bond, Midgley, & Woodward., ; Bucini & Hanan, ; D'Onofrio, Hardenberg, & Baudena, ; Good & Caylor, ; Lehmann et al, ; Sankaran et al, ; Sankaran, Ratnam, & Hanan, ; Scholes & Archer, ), (b) changes in precipitation regimes, coupled with warmer temperatures that increase evaporative demand, are projected to lead to more intense and frequent regional‐scale droughts across large parts of the globe, including savanna regions (Dai, ; IPCC, ; Mitchell, O'Grady, Hayes, & Pinkard, ; Trenberth et al, ), (c) savannas are expected to be particularly responsive to precipitation changes given the tight coupling between rainfall and production in these systems (Knapp & Smith, ; Sala, Gherardi, Reichmann, Jobbagy, & Peters, ; Sala et al, ; Xu et al, ), and (d) in contrast to forests, the role of droughts in regulating tropical savanna dynamics has received relatively less attention. Importantly, inferring savanna responses based on studies carried out in other ecosystems is not straightforward.…”