Hydrologic reinforcement induced by contrasting woody species during summer and winter

Aims: Vegetation can improve slope stability by transpiration-induced suction (hydrologic reinforcement). However, hydrologic reinforcement varies with seasons, especially under temperate climates. This study aims to quantify and compare the hydrologic reinforcement provided by contrasting species during winter and summer.

Methods: One deciduous (Corylus avellana) and two evergreens (Ilex aquifolium and Ulex europaeus) were planted in 1-m soil columns. Soil columns were irrigated, left for evapotranspiration and then subjected to extreme wetting events during both summer and winter. Soil water content, matric suction and strength were measured down the soil profile. Plant water status and growth (above- and below-ground) were also recorded.

Results: The tested species showed differing abilities to remove water, induce suction and hence influence soil strength. During summer, only Ulex europaeus provided a soil strength gain (up to six-fold the value at saturation) along the entire depth-profile inducing high suction (e.g. 70 kPa), largely maintained after wetting events in deeper soil (0.7 m). During winter, the evergreen species could remove water but at slower rates compared to summer.

Conclusions: Evergreens could slowly induce suction and hence potentially stabilise slopes during winter. However, there were large differences between the two evergreens because of different growth rate and resource use.