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Relation between rainfall intensity and savanna tree abundance explained by water use strategies
Friday, 2015/10/23 | 08:31:38

 Xiangtao Xu, David Medvigy, and Ignacio Rodriguez-Iturbe

ECOLOGY

Significance

Savannas account for 20% of global land area and support 30% of terrestrial net primary production. The biome is characterized by the coexistence of trees and grasses. Tree abundance strongly influences savanna ecosystem dynamics. Maximum tree abundance in tropical savannas is found to be negatively correlated with rainfall intensity, which remains unexplained. Through combining in situ observations, a biophysical tree–grass competition model, and a stochastic rainfall generator, we present that differentiated tree and grass water use strategies are essential to explain the phenomenon. Our findings show the importance of vegetation physiology in determining tree abundance in the biome and enhance our ability to predict future ecosystem composition and dynamics under global change.

Abstract

Tree abundance in tropical savannas exhibits large and unexplained spatial variability. Here, we propose that differentiated tree and grass water use strategies can explain the observed negative relation between maximum tree abundance and rainfall intensity (defined as the characteristic rainfall depth on rainy days), and we present a biophysical tree–grass competition model to test this idea. The model is founded on a premise that has been well established in empirical studies, namely, that the relative growth rate of grasses is much higher compared with trees in wet conditions but that grasses are more susceptible to water stress and lose biomass more quickly in dry conditions. The model is coupled with a stochastic rainfall generator and then calibrated and tested using field observations from several African savanna sites. We show that the observed negative relation between maximum tree abundance and rainfall intensity can be explained only when differentiated water use strategies are accounted for. Numerical experiments reveal that this effect is more significant than the effect of root niche separation. Our results emphasize the importance of vegetation physiology in determining the responses of tree abundance to climate variations in tropical savannas and suggest that projected increases in rainfall intensity may lead to an increase in grass in this biome.

 

See: http://www.pnas.org/content/112/42/12992.abstract.html?etoc

PNAS October 20 2015; vol. 112 no. 42: 12992–12996

 

Fig. 1.

Differentiated water use strategies of C4 grass (black) and tree (red). (A) Transpiration. (B) Net growth. Net growth is defined as net primary production (NPP) minus biomass turnover. Relative soil moisture is volumetric soil moisture normalized by soil porosity. The intersections of growth curves (sg0 for C4 grasses and st0 for trees) with the zero-growth curve (blue) are shown in the Inset of B. The curves are generated by a saturating function of soil moisture (SI Appendix, Eq. S5) with parameters based on field and laboratory observations (Methods and SI Appendix, Table S1). Incorporation of radiative transfer and shading can induce small changes in the location of the curves

 

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