vanWijk - WATER RESOURCES RESEARCH, VOL. 38, NO. 9, 1179,

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Tree-grass competition in space and time: Insights from a simple cellular automata model based on ecohydrological dynamics Mark T. van Wijk 1 Physical Geography and Soil Science, Institute of Biodiversity and Ecosystem Dynamics (IBED), Universiteit van Amsterdam, Amsterdam, Netherlands Ignacio Rodriguez-Iturbe Department of Civil and Environmental Engineering and Princeton Environmental Institute, Princeton University, Princeton, New Jersey, USA Received 13 July 2001; revised 1 February 2002; accepted 1 February 2002; published 28 September 2002. [ 1 ] Water is a key resource in determining the composition and structure of savanna ecosystems. In this study we present a simple cellular automata model in which death and reproduction chances of trees and grasses are based on the dynamical description of plant water stress by a probabilistic ecohydrological point model, using the parameterization for a Texas savanna. The results show that the model behavior, despite its simplicity, can be linked to ecological reality: the model yields a dynamic tree-grass coexistence driven by the annual rainfall, and the space-time behavior shows that both random and clustered tree distributions for periods up to 100 years can be observed. Both temporal and spatial model output display fractal characteristics suggesting the possibility of a self- organized critical dynamics. Thus power law behavior is observed in both the spectral density function and the cluster size distribution. The presence of spatial fractal characteristic opens avenues for more thorough model testing. INDEX TERMS: 1851 Hydrology: Plant ecology; 1869 Hydrology: Stochastic processes; 1866 Hydrology: Soil moisture; 1812 Hydrology: Drought; KEYWORDS: cellular automata, drought, plant ecology, competition, ecohydrology Citation: van Wijk, M. T., and I. Rodriguez-Iturbe, Tree-grass competition in space and time: Insights from a simple cellular automata model based on ecohydrological dynamics, Water Resour. Res. , 38 (9), 1179, doi:10.1029/2001WR000768, 2002. 1. Introduction [ 2 ] Savannas are one of the major biomes of the world, and one of its characteristics the coexistence of grasses and trees [ Scholes and Archer , 1997; Bourliere and Hadley , 1970]. Soil water availability is generally considered critical for determining the development of woody vegetation [ Fensham and Holman , 1999]. Classical models suggest that water is the key resource in determining tree densities, although other important variables like fire and grazing can maintain the population levels below their climatic equili- brium [ Fensham and Holman , 1999; Belsky , 1990]. In the case of southern Texas, changes in livestock grazing and fire regimes appear to be important forces behind the increase of woody plant abundance. Nevertheless, vegeta- tion changes in this region are highly punctuated by variations in precipitation. Thus the decrease in total woody plant cover in the 1941–1960 drought was followed by a threefold to eightfold increase during the above normal annual rainfall period between 1960 and 1983 [ Archer et al. , 1988].
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This note was uploaded on 07/09/2011 for the course CWR 5104C taught by Professor Miralles during the Spring '11 term at FIU.

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vanWijk - WATER RESOURCES RESEARCH, VOL. 38, NO. 9, 1179,

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