Soil moisture and plant stress dynamics along the Kalahari precipitation gradient

[1] We present an analysis of water balance and plant water stress along the Kalahari precipitation gradient using a probabilistic model of soil moisture. The rainfall statistical characteristics, obtained from daily data of four stations along the transect, show that the rainfall gradient (from 950 to 300 mm/year) is mostly due to a decrease in the mean rate of storm arrivals (from 0.38 to 0.09 1/day) rather than to a change in the mean storm depth (practically constant at 10 mm). Using this information and typical vegetation and soil parameters, the analysis relates the vegetation properties along the transect with those of climate and soil. It is shown that differences in water balance and plant water stress between trees and grasses generate varying preferences for vegetation types along the transect, with deeper-rooted trees favored in the more mesic regions of the northern Kalahari and grasses favored in the drier zones of the southern Kalahari. The point of equal plant water stress is found at about 420 mm of rainfall during the growing season (October–April), indicating the possibility of tree-grass coexistence in the central sector of the Kalahari. These findings are consistent with patterns of vegetation distribution across the Kalahari transect.

[1]  Luca Ridolfi,et al.  Tree‐grass coexistence in Savannas: The role of spatial dynamics and climate fluctuations , 1999 .

[2]  Luca Ridolfi,et al.  Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: II. Probabilistic soil moisture dynamics , 2001 .

[3]  David S. G. Thomas,et al.  The Kalahari environment , 1992 .

[4]  Robert J. Scholes,et al.  Trends in savanna structure and composition along an aridity gradient in the Kalahari , 2002 .

[5]  Robert C. Maggio,et al.  Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland , 1988 .

[6]  R. Scholes,et al.  The Kalahari Transect: Research on Global Change and Sustainable Development in Southern Africa , 1997 .

[7]  Francesco Laio,et al.  Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: IV. Discussion of real cases , 2001 .

[8]  S. Crimp,et al.  THE CLIMATE OF THE KALAHARI TRANSECT , 1998 .

[9]  R. Scholes,et al.  An African Savanna: African savannas: an overview , 1993 .

[10]  G. Hornberger,et al.  Empirical equations for some soil hydraulic properties , 1978 .

[11]  S. Sullivan.  Book review: Savanna details with a paradigm shift: review of "An African savanna". Scholes, R.J. and Walker, B.H. Cambridge University Press, Cambridge , 1994 .

[12]  P. Tyson.  Climatic change and variability in southern Africa , 1986 .

[13]  V. Isham,et al.  Probabilistic modelling of water balance at a point: the role of climate, soil and vegetation , 1999, Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[14]  Luca Ridolfi,et al.  Plants in water-controlled ecosystems: active role in hydrologic processes and response to water stress: III. Vegetation water stress , 2001 .