Modelling tree-grass coexistence in water-limited ecosystems

Tree-grass coexistence is commonly observed in arid and semi-arid ecosystems. Many of these ecosystems are undergoing a shift from grassland to tree dominance or the opposite. Therefore, exploring the mechanism of tree-grass coexistence and studying how ecosystems respond to environmental change are critical to understanding such shift. We construct a tree-grass-water model by considering the Walter’s two layer hypothesis, i.e., the grass species only access to the shallow soil water, while the tree can uptake both the shallow and deep soil water. Results show that the tree-grass coexistence region increases with increasing their niche separation. Hence, the vertical niche separation between trees and grass with respect to limiting water resources might be one of mechanisms for tree-grass coexistence. Besides, both tree and grass display positive responses to precipitation, but surprisingly, there exists a catastrophe that the mean vegetation density first rapidly increases, but then quickly decreases as precipitation increases in the tree-grass-water model with infiltration feedback. Most likely, this catastrophe is due primarily to the periodical fluctuation of vegetation density and water caused by its intrinsic infiltration feedback. Overall, our study provides new insights into the dynamics of tree-grass in arid and semi-arid ecosystems.