Restoration thinning promotes resprouting and recruitment in an Australian floodplain forest

Thickening of woody vegetation has degraded numerous ecosystems globally. In forests, woody thickening often follows gap‐creating disturbance that removes mature trees and promotes the dense recruitment of woody resprouts or seedlings. Restoration thinning seeks to reduce tree densities in thickened forests to hasten recovery of stand structure for habitat and other conservation outcomes. However, as restoration thinning involves gap‐creating disturbance, it may stimulate further recruitment. We investigated recruitment responses to restoration thinning in thickened river red gum (Eucalyptus camaldulensis Dehnh.) forests on the Murray River floodplain in semiarid south‐eastern Australia by implementing mechanical thinning at various intensities. The average distance between retained trees (up to 15 m) was intended to be insufficient to stimulate further recruitment, and herbicide was applied to cut stumps to reduce resprouting. We recorded seedling abundance annually for 5 years and resprout abundance after 5 years. Resprouting occurred at all levels of thinning intensity. On average, almost one third of the trees affected by thinning resprouted with over three resprout stems per resprouting tree. Thinning that reduced tree densities to below about 400 per hectare also increased seedling abundance by up to 7500 additional seedlings per hectare in some plots in the fifth year of the study when establishment conditions were favorable. These results demonstrate that effective recruitment controls must be identified prior to implementing restoration thinning programs. Without effective recruitment controls, restoration thinning may stimulate more stems than were removed by thinning, and therefore be an ineffective conservation intervention.