Better left alone: trying to control pasture grasses in untended rainforest plantings incurs multiple costs and delivers few benefits.
Abstract
Rainforest revegetation projects often deliver suboptimal outcomes due to the recolonization of invasive pasture grasses, but little is known about the effects of grass reinvasion on the survival and growth of established saplings. Even less is known about the costs and benefits of controlling pasture grasses once they have reinvaded. 2. To address these knowledge gaps, we implemented a split-plot grass control experiment in a 2-year old subtropical rainforest restoration planting in South East Queensland, Australia, that was reinvaded by the exotic pasture grass Chloris gayana. 3. Grass removal involved brush cutting around saplings, spraying herbicide and then laying 1 m2 jute matting. The costs of implementing the treatment were recorded, and the survival, growth and physiological stress of treated and control saplings were monitored for 1 year. 4. Non-target herbicide application reduced survival by 6.5% in treated saplings, affecting mainly smaller plants that were below the grass canopy at the onset of the experiment. Beyond this direct herbicide effect, smaller treated saplings were also more stressed (lower chlorophyll fluorescence) and had substantially lower survival after 1 year than untreated saplings of the same size. There was limited evidence that removing grass increased growth rates, even for saplings that were already taller than the grass canopy at the start of the experiment. 5. While the growth benefits of controlling grass may become more apparent with time, our results suggest that grass removal is not an effective management strategy in untended plantings due to the heightened risk of sapling mortality, coupled with the considerable labour and material costs.