Large shrubs partly compensate negative effects of grazing on hydrological function in a semi-arid savanna

Semiarid woodlands and savannas are globally important biomes that provide ecosystem goods and services such as habitat for biota and sinks for carbon, support millions of people that rely primarily on pastoralism, and supply livelihoods for about a third of the global human population. Savannas, however, are prone to degradation by overgrazing, and encroachment by woody plants, reducing their capacity to produce forage that pastoral enterprises depend on. We examined the impacts of livestock grazing and woody encroachment on soil hydrological processes, hypothesizing that heavy grazing by livestock would reduce hydrological function, whereas woody plants would increase hydrological function, therefore, partially offsetting any negative effects of overgrazing by livestock. Understanding the major drivers of soil hydrology in savanna ecosystems is important because water is a critical, yet limited resource in savannas. We found that livestock grazing reduced the early (sorptivity) and late (steady-state infiltration) stages of infiltration under both ponding and tension, and attributed this to a reduction in porosity caused by livestock trampling. Steady-state infiltration and sorptivity under ponding were greater under the canopies of woody shrubs than in open areas, partly compensating for any negative effect of grazing. Structural equation modeling revealed a direct positive effect of shrub height on hydrological functions, and an indirect effect via increases in litter cover. Our results suggest that woody plants can play important roles in driving hydrological function in savannas, counteracting the suppressive effect of livestock overgrazing on infiltration processes. Management strategies in semiarid savannas should aim to reduce trampling by livestock and retain large woody plants in order to maintain hydrological function.