Control of bracken and the restoration of heathland. V. Effects of bracken control treatments on the rhizome and its relationship with frond performance.
The effect of three weed control treatments on the rhizome system of bracken (Pteridium aquilinum) was assessed in field trials at Cavenham and Weeting Heaths in Breckland (UK). Control treatments included cutting once and twice yearly between 1978 and 1990, and spraying asulam at the recommended rate (4.4 kg/ha) in 1978 and 1985. In addition, the effect of continuous treatment was compared with plots in which bracken control ceased after 6 years. The effects on rhizome biomass and length, numbers of active and dormant buds, and starch and soluble carbohydrate concentrations were measured. Cutting twice yearly was the most effective treatment, but rhizomes persisted even after 12 years of continuous treatment. Cutting once yearly and applying asulam resulted in intermediate reductions in all rhizome variables. A relatively rapid increase in all evaluated variables of rhizome performance was observed when bracken control was ceased. At Cavenham Heath, remission for 4 years increased the starch levels in rhizomes, and after 6 years increased all rhizome variables evaluated. At Weeting Heath all variables were significantly increased 5 years after control treatments ceased. Regression analyses showed that the majority of the variation in summer frond performance (0.39-0.75) could be explained by rhizome biomass. At both sites, the amounts of starch and carbohydrate in the rhizome, and at Weeting the above-ground grass biomass and total bud number, improved the fit of some equations. Some earlier published data for Weeting Heath were used to estimate rates of rhizome depletion and recovery. It was suggested that it would take (a) a further 8-34 years to eradicate bracken depending on the treatments used, and (b) between 9 years (for the cut once yearly and asulam treatments) and 62 years (for the cut twice yearly treatment) for the bracken rhizome system to recover its starting biomass. Moreover, a large frond biomass would occur before the rhizome system recovered completely. The rates obtained from Weeting were used to predict the outcome of the Cavenham experiment and a reasonable fit was achieved.