Control of bracken and restoration of heathland. VII. The response of bracken rhizomes to 18 years of continued bracken control or 6 years of control followed by recovery.
The effects of six bracken (Pteridium aquilinum) control treatments applied in combination with a Calluna seeding treatment on the rhizome system of bracken at Cavenham Heath, UK, was assessed after 18 years. Bracken control included cutting once and twice yearly between 1978 and 1996, spraying the herbicide asulam at the recommended rate (4.4 kg a.i. ha-1) in two treatments (1978 and 1978 plus 1979), each followed by repeat doses in 1984 and 1990, and spraying asulam followed by cutting once yearly. After 6 years the experiment was split and bracken control was continued/reapplied on half the plots and stopped on the remainder. The effects of treatment on the numbers of active and dormant buds and the length and biomass on both frond-bearing and storage rhizomes were measured. When applied continuously over the 18 years, all treatments reduced all rhizome measures, but treatments that included cutting were more effective than ones that just used asulam. The best treatment after 10-12 years was cutting twice yearly, but after 18 years this treatment had apparently reached an equilibrium and the cutting-once-yearly treatment was superior. No treatment eradicated the bracken in 18 years of continuous treatment. When bracken control treatments were stopped after 6 years rhizome recovery started within 4 years and there was significant recovery thereafter. After 12 years of the least effective treatment, cutting once yearly, although still showing a significant reduction in rhizome biomass, bracken had recovered to 88% of the untreated plots. The most effective treatment was cutting twice yearly for 6 years, where most measures of rhizome performance remained at approximately 50% of untreated plots 12 years after cutting stopped. Regression analysis was used to investigate the relationship between frond production and rhizome characteristics. Rhizome biomass, especially of the storage rhizome component, was consistently the most important variable; frond bud numbers were less important. Simple models, developed making assumptions that the response of the rhizome to treatment is linear through time, have been shown to be invalid over the longer time period. This is because the impacts of the treatments reduce through time. These results are discussed in relation to long-term bracken control and the assessment of bracken stocks. If bracken is to be suppressed on a long-term basis a continuous control strategy is needed. It is unlikely that complete eradication will be possible with current control technology. The long-time scale found for the total recovery of the bracken from only 6 years of treatment (>12 years) means that the large areas of bracken that have been treated in the past will take some time to recover to pretreatment levels. This recovery is an unknown factor in bracken stock assessments.