Experimental manipulation of restoration barriers in abandoned eucalypt plantations.
Expansion of the nature conservation estate in northeastern New South Wales, Australia, has captured weed-infested timber plantations amid a mosaic of high conservation value lands. We adopted a state-and-transition approach to test the hypothesis that restoration barriers restrict the natural regeneration of native species in Eucalyptus grandis plantations infested by Lantana camara in Bongil Bongil National Park, New South Wales. Plantation tree thinning and weed control were applied in factorial combination at three sites (totaling to 4.5 ha). Topsoil chemistry responses to these interventions were attributable to the "ash bed" effect, with temporary increases in topsoil pHW and nitrate, particularly where canopy reduction was greatest. Other soil changes were minor, indicating that thinning and burning did not risk soil degradation. Plant species richness and functional group representation in the regenerating understorey were improved by the interventions. Regeneration of native potential canopy trees, understorey trees, shrubs and woody climbers, and perennial forbs all increased with canopy retention. Grass cover dominated the regeneration where canopy cover was less than 50%. In the absence of weed control, the cover of introduced shrubs increased with reduction in canopy cover, as did the rate of understorey regeneration generally. These responses indicate that thinning and weed control can reinstate succession, leading to structurally and compositionally diverse forest. Given the abundance of native woody regeneration under retained canopy, the lantana understorey was more important in inhibiting native regeneration. The experimental approach will promote efficient use of resources across the remaining 200 ha of low conservation value plantations in this national park.