Influence of invasive macrophytes on channel morphology and hydrology in an open tropical lowland stream, and potential control by riparian shading

1. The catchments of many tropical lowland streams in far north Queensland have been extensively cleared for the cultivation of sugar cane to the extent where very little of the native riparian vegetation remains. Stream channels are often choked by a matrix of introduced pasture grass (Brachiaria mutica, or para grass) and accumulated sediment from cropland erosion. 2. Detailed transects across Bamboo Creek, a fourth order cane-land stream, revealed an estimated sediment load of 20000 t km(-1). This has resulted in an estimated 85% reduction in the predicted bankful discharge of the original stream channel. Channel capacity has been reduced from 2.3 times to 0.3 times the predicted Q(50) flood discharge of 140 m(3) s(-1). 3. Shade cloth treatments of 50% and 90% across the stream were used to mimic the effect of shading by riparian vegetation. Three months of shading resulted in a substantial reduction in the height and standing biomass of para grass in both shade treatments, compared to open plots (0% shade). The most dramatic effect was in the 90% treatment, where a mean reduction of 63% in height and 52% in total biomass was recorded. This was despite high net primary production of para grass in the open plots of 2.8 g dry wt m(-2) day(-1), which resulted in a overall increase of 11% and 28% in plant height and total biomass, respectively. 4. These data suggest that restoration of native riparian vegetation will be an effective long-term means of controlling invasive macrophytes in disturbed cane-land stream channels. Reduction of excessive macrophyte growth and the mobilisation of accumulated sediment are essential to the restoration of natural hydrological and ecological processes.