Soil water repellency in riparian systems invaded by Eucalyptus camaldulensis: a restoration perspective from the Western Cape Province, South Africa

Abstract

South African riparian systems are threatened by major alien plant invasions through the widespread replacement of native plant species by fast-growing alien species, including several Eucalyptus species. Since Eucalyptus species are known to cause soil water repellency, this study examined the occurrence of soil water repellency coupled with soil moisture and infiltration under laboratory conditions from soils collected along the Berg River which is heavily invaded by alien tree species, especially E. camaldulensis. The connection between alien clearing for restoration purposes and soil water repellency is important as it has the potential to affect the success of native vegetation recovery. The topsoil was sampled at 12 sites, under different restoration treatments, namely invaded by Eucalyptus, completely cleared, thinned and native (control) sites. The water drop penetration time (WDPT) and the critical surface tension (CST) methods were performed. Soil moisture was found to be higher in invaded and natural sites compared to completely cleared and thinned sites. Soil water repellency, measured with the WDPT test on dried samples taken at 5–10 cm depth, differed with invasion status and/or restoration condition. In invaded sites water repellency varied from wettable to severely water repellent: in thinned sites from non-repellent to strongly water repellent; in natural sites from wettable to slightly water repellent; all samples from completely cleared sites were wettable. Soil water repellency had no impact on soil infiltration rates. We conclude that the removal of invasive Eucalyptus species has the potential to restore soils to a non-repellent state, thus improving soil related ecosystem functions, which will facilitate the restoration of indigenous species, vegetation composition and structure.