Understory shrubs improve soil infiltrability in overgrazed Mediterranean oak woodlands, but have little impact on ungrazed woodlands

Understory woody plants are increasing in open Mediterranean oak open woodlands as a result of a number of factors, including land abandonment and/or less active management. This change in vegetation cover has many implications—such as increased fire hazard, changes in plant and animal biodiversity, reduced ability of oak trees to regenerate, and altered soil infiltrability. Until now little work has been done to assess the effects of understory woody plants on soil infiltrability in these ecosystems. The goal of this study was to explore those effects in areas that have experienced heavy grazing vs. areas that have been protected from grazing. We found that (1) In heavily grazed areas, soil infiltrability was dramatically lower than in areas protected from grazing—except in soils under shrubs, where infiltrability was comparable;(2) In areas protected from grazing, there was little difference in soil infiltrability between shrub-covered areas and open areas; and (3) In grazing-protected areas, soil infiltrability progressively increased over a 23-year period of test measurements. Our results suggest that where open oak woodlands are heavily grazed, shrub patches may provide benefits in terms of improved soil health, likely because shrubs protect the soil from trampling (reducing soil compaction). There may be some benefit in managing open oak woodlands to maintain heterogeneous vegetation cover—a mosaic of herbaceous and woody patches. •In heavily grazed areas, soil infiltrability was much lower except under shrubs.•In ungrazed areas, there was no difference in soil infiltrability by cover type.•Soil infiltrability progressively increased with duration of protection from grazing.•In grazed woodlands, shrub patches provide soil health benefits.•Oak woodlands should be managed for heterogeneous vegetation cover.