Effect of shrubs on soil saturated hydraulic conductivity depends on the grazing regime in a semi-arid shrub-encroached grassland

Abstract

Encroachment of woody plants has a positive effect on hydrological functions (e.g., infiltration), but few studies have examined the effects of shrub patches on infiltration under different grazing scenarios. We examined differences in characteristics of plants and soils and saturated hydraulic conductivity between shrubs and their associated interspaces in two grazing regimes at three sites in a semi-arid grassland encroached by Caragana microphylla, a leguminous N-fixing shrub. The results showed that the shrub patches had greater plant height, cover, and biomass compared to interspaces, and the shrubs positively affected the characteristics of soils such as porosity and water holding capacity, whereas grazing had a strong opposite effect. The effect of the relative interaction intensity (RII) of shrubs on the saturated hydraulic conductivity of the soil was greater under grazed (0.60) than ungrazed conditions (-0.01), and the negative effect of grazing was mainly achieved in interspaces. Generally, structural equation modeling revealed positive indirect effects of shrubs on saturated hydraulic conductivity via a greater root mass, with 3.5 times of that in interspaces and a lower bulk density (90% of that in interspaces) under conditions of overgrazing. Our findings highlight the critical role that shrubs play in maintaining infiltration under overgrazing conditions and suggest that they might partially offset the damaging effects of overgrazing on infiltration. Thus, shrub encroachment might be an adaptive mechanism of grassland ecosystems to encounter intensive human induced disturbance such as overgrazing.