Individual study: The efficacy of bracken Pteridium aquilinum control by aerial spraying with asulam at moorland sites in North Yorkshire, England
Pakeman R.J., Small J., Le Duc M.G. & Marrs R.H. (2005) Recovery of moorland vegetation after aerial spraying of bracken (Pteridium aquilinum (L.) Kuhn) with Asulam. Restoration Ecology, 13
Bracken Pteridium aquilinum is a major weed of seminatural vegetation in the UK and it is potentially invasive, often colonizing large areas of heath, upland pasture and moorland. Attempts at large-scale bracken control have been undertaken with the objective being to restore the former vegetation, for example heather Calluna-dominated moorland or heath. The use of aerial spraying with asulam herbicide has become a common application method, especially in upland areas where access by vehicles is difficult. The cost of such bracken control is often grant aided as part of agri-environment schemes, but the long-term efficacy of aerial spraying had not been assessed. This study undertook such an assessment with two main aims: to assess the long-term effectiveness of bracken control by aerial spraying; and to assess its effectiveness in facilitating reinstatement of the main target vegetation i.e. moorland dominated by heather Calluna vulgaris.
Study sites: Twenty-two upland sites in North Yorkshire (northern England) surveyed in 1990/1991 and 1994 were resurveyed in August 2002. The localities (and number of sites at each) were: Hutton-le Hole (6) Lastingham (3) Rosedale (2) Arden Great Moor (3) Bridestones (2) Levisham Moor (2) Hawnby Moor (2) Glaisdale (2).
The sites had been selected to cover a range of times since a single aerial spraying of asulam had been applied with the purpose of controlling bracken and restoring a heather Calluna vulgaris-dominated plant community. Prior to spraying all sites were covered by dense bracken Pteridium aquilinum with adjacent areas of Calluna–dominated sheep-grazed moorland. Four unsprayed sites adjacent to sprayed areas were included for comparison. Since the first surveys, two of the four unsprayed sites had been subjected to an aerial application of asulam and three previously treated sites had been resprayed. Two sites had been fenced to prevent sheep grazing.
Monitoring: At all sites 15, 1 × 1 m quadrats were positioned systematically along transects through the sprayed areas. The number of transects (two to four) and the distance between samples were determined by the size and shape of each stand. Cover (%) of all species of higher plants, bryophytes and lichens, bare ground, sheep droppings, bracken litter, and litter from other sources was estimated visually within each quadrat. This method was adopted as part of a nationwide study of the efficacy of bracken spraying. The survey in 1994 was carried out in an identical fashion, but the survey in 1990/1991 differed in being based on point quadrats
Spraying success: Surveys showed that a single application of asulam was effective in eradicating bracken (i.e. to levels of less than 1% cover) on a third of sites. In contrast at 10% of sites, bracken had regenerated more or less completely achieving over 80% cover. On the remainder it persisted in patches (occurring in over 20% of quadrats), often at high density. Data analysis indicated that over half the sprayed sites had seen good recovery of moorland vegetation being classed as upland heathland vegetation (the desired restoration target) within the UK National Vegetation Classification. This shift in composition meant that the 13 sites previously dominated by dense bracken were now classified as having heathland vegetation.
Reasons for variability of results: There were a number of potential factors identified as influencing vegetation change following spraying. These included initial species composition (no data for most sites), site altitude and aspect, and management such as grazing and follow-up treatment. Low altitude sites tended to be grassier, whereas higher altitude sites had high bilberry Vaccinium myrtillus cover. This may have been due to initial species present beneath the bracken or also a feature of the grazing pressure (the latter was estimated through dung counts). Grassland vegetation is more likely to develop under high grazing pressures than vegetation dominated by dwarf shrubs
Conclusions: This study shows that aerial spraying of asulam can be an effective bracken-control and vegetation-restoration strategy. However, it can also fail completely, either in the short term because of poor application and take up by bracken, or in the longer term because the bracken regenerates unhindered by any follow-up management. The most common result was a partial success, where bracken had been removed from the majority of the area it previously covered but still persisted often in dense patches on parts of treated areas. Intensive follow-up treatment by spraying any emerging fronds, is therefore recommended.
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