Experimental evidence on the importance of disturbance intensity for invasion of Lantana camara L. in dry rainforest–open forest ecotones in north-eastern NSW, Australia

The invasion of Lantana camara (a thicket-forming, woody weed) into dry rainforest–open forest ecotones was evaluated over a range of disturbance intensities associated with fire and livestock grazing. Three manipulation experiments were established in which wildfire (unburnt, low, and high intensity), overstorey damage (different levels of biomass removal), and nutrient addition (NPK) were used as disturbance categories in pair-wise factorial combinations across the chosen disturbance intensity range. The availability of key resources to the invading species resulting from disturbances was also measured over the established environmental gradients of light and soil fertility. Seedling establishment occurred in control treatments but growth was weak (averaging 0.85 g plant −1 year −1) and they failed to flower during the 27 months experimental period. High levels of nutrient addition (60–80 g m −2) without other forms of disturbance significantly increased growth (to 0.95 g plant −1 year −1) and germination (41.6–44.5%), while survival increased modestly (14.1–15.9 plants plot −1). Biomass increased significantly to 1.78 g plant −1 year −1 when the shrub layer was removed or burnt by a low intensity wildfire while germination and survival increased by 13.5 and 22.0% respectively over the control. Invasion increased further when the overstorey was physically removed or burnt with a high intensity wildfire as biomass increased significantly to 2.42 g plant −1 year −1, while germination and survival significantly increased by a further 35.6% and 40.7% respectively over the low intensity wildfire and shrub removal. Invasion was positively increased with disturbance intensity and increased resource availability. Light at ground level increased from 21.3 to 30.5% of ambient light when the shrub layer was damaged while it increased to 84.3% when the overstorey was damaged. A pattern of increasing plant performance with increasing intensity and number of combined disturbances was evident, although nutrient addition alone had little or no effect on plant performance, including flowering. The impact of low intensity fire on L. camara invasion was primarily produced by a response to increased light rather than increased nutrient availability while high intensity wildfire produced a secondary but minor response to increased soil fertility. Management of disturbances is critical for the long-term viability of dry rainforests and their ecotones. Shading by intact canopi