Abstract
Tree–grass coexistence is broadly observed in tropical savannas. Recent studies indicate that, in arid savannas, such coexistence is stable and related to water availability. The role of different factors (from niche separation to demographic structure) has been explored. Nevertheless, spatial mechanisms of water–vegetation interactions have been rarely taken into account, despite their well-known importance for vegetation distribution. Here, we introduce a spatial model including tree and grass biomass dynamics, together with soil and surface water dynamics. We consider two water–vegetation feedbacks. Grasses increase water infiltration into the soil, while tree shadow limits evaporation, and both mechanisms increase soil water availability, leading to positive feedbacks. The infiltration feedback can also lead to spatial pattern formation. Despite the fact that trees and grasses compete for the same resource, namely water, we observe stable coexistence as a possible model outcome. The system displays a complex behavior, with multiple stable states and possible catastrophic shifts between states, e.g., patterned grassland, bare soil and forest. In our model, coexistence is always linked with multi-stability and spatial pattern formation, driven by grass infiltration feedback. Given such complex model solutions, we expect that, under real conditions, heterogeneities and disturbances, acting on the multi-stable states, may further foster coexistence.
Highlights
Herbaceous and woody plant coexistence is observed in many terrestrial ecosystems
Given the importance of spatial resource-driven vegetation dynamics, here we study their effects for the possible outcomes of tree–grass competition for soil water in arid savannas
We studied the effect of spatial dynamics and water–vegetation feedbacks on tree–grass coexistence in arid savannas
Summary
Herbaceous and woody plant coexistence is observed in many terrestrial ecosystems. Tropical savannas are the most common example. Savannas occupy about 20 % of the Earth land surface and they are observed in a large range of mean annual precipitation (e.g., in Africa, approximately between 100 and 1,500 mm, Lehmann et al 2011). They are broadly defined as systems where woody and herbaceous vegetation coexist, competing mainly for the same resource, namely soil water. Most of the models explaining savannas as deterministic stable coexistence assume different root depths between trees and grasses, separating tree and grass niches (e.g., van Langevelde et al 2003; Walker and Noy-Meir 1982; Walter 1971).
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