Changes in the rhizome system of bracken subjected to long-term experimental treatment.
Bracken Pteridium aquilinum is a serious weed of upland and marginal land. Its extensive rhizome system and large carbohydrate reserves make control difficult. This paper reports the results of seven long-term experiments, established in four diverse UK locations, to test control and vegetation restoration treatments. Samples were obtained from 580 rhizome pits between 1998 and 2000. Total dry mass per unit area (M, a measure of performance) and ratio of frond-bearing to total rhizome dry mass (R, a relative measure of investment in frond production) were measured. The range of means for M in untreated bracken was 1.8-5.1 kg m-2, greater than that reported by others (1.2-3.0 kg m-2). Measured values for R were 0.24-0.42, again differing from other reports (0.10-0.38), probably due to our policy of avoiding advancing bracken fronts. Randomization tests were used to check for non-independence of sampling units. They confirmed the general adequacy of the ANOVA results; there was little detectable interference between plots. Two regions contained matching pairs of experiments, thus variation within region was tested and differences were found in one region. Five bracken control treatments (cutting, herbicide application and combinations) were employed at all but one experiment. Cutting once or twice per year generally gave the best results, reducing M by c. 60% after ≤5 years. Some subtreatments designed mainly for vegetation restoration, notably surface disturbance, also affected M. R was reduced by herbicide treatment, for example from 0.30 to 0.16. Such low values of R are typical of invading bracken and are thought to represent rejuvenated and invasive rhizomes. Follow-up treatments are needed in such situations. Despite large differences between untreated M at matching experiments on Cannock Chase (Staffordshire), treatment effects were similar at both sites (cutting twice per year or cutting together with herbicide application were best). At Sourhope (Cheviot Hills) a different pattern of response was obtained, with cutting alone giving better results than treatments involving herbicide. The ratio of rhizome to frond dry mass per unit area was 10:1 in good conditions for growth, but the proportionate size of rhizomes could be reduced in poor growing conditions such as wet summers. Synthesis and applications. In order to develop a national control strategy the following must be considered: rhizome mass differs between sites and in response to control treatments; cutting twice per year is generally most effective; where cutting is impossible, herbicide treatment should be applied. Weather may affect rhizome mass, with wet years being detrimental. This raises the possibility that bracken may increase under the drier conditions that may occur with global warming.