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Individual study: Post-burning herbicide and 'bashing' control of bracken Pteridium aquilinum to restore heathland habitat for the heath fritillary Mellicta athalia at Halse Combe, Exmoor, Somerset, England

Published source details

McCracken M., Bulman C., Camp P. & Bourn N. (2005) Heath fritillary habitat management: a three-year experimental study at Halse Combe, Exmoor. Butterfly Conservation, Wareham, Dorset, UK. Report no. S05-44 (added by: Brook S. 2005). Heath fritillary habitat management: a three-year experimental study at Halse Combe, Exmoor, 5-44

Summary

The heath fritillary butterfly Mellicta athalia, is a UK Biodiversity Action Plan priority species due to its rarity and recent decline. In Britain it is now found in only three counties in southwest England - Cornwall, Devon and Somerset (the latter on Exmoor), and two counties in the southeast, Kent and Essex.

Within Exmoor the heath fritillary occupies early successional heathland habitats dominated by heather Calluna vulgaris, bilberry Vaccinium myrtillus and bracken Pteridium aquilinum. Its primary larval foodplant is common cow-wheat Melampyrum pratense which tends to be absent where heathland vegetation is older. Favoured breeding areas are sheltered heathland valleys (known locally as combes, up to about 400 m a.s.l) with a high proportion of bilberry and common cow-wheat. The heath fritillary has a dynamic distribution characterised by local extinctions, colonisations and recolonisations. It is a relatively sedentary species however, rarely travelling further than 150 m, therefore small strips of unsuitable habitat can be major barriers to the species' dispersal.

Exmoor is a major stronghold for the heath fritillary, at one time containing 50% of the UK's population. However, in recent years it has experienced a severe decline in range and population size on Exmoor, from 29 colonies in 1989 to 15 in 1999/2000. Major factors implicated in this decline was a lack of appropriate management, including a reduction in grazing which led to a loss of early successional stages of heathland vegetation and a decline in common cow-wheat.

In an attempt to reinstate suitable conditions for the heath fritillary, various management options were considered. Grazing and burning are commonly used in the management and restoration of heathland but burning results in a large input of potash to the soil, causing vigorous bracken growth. This would result in unfavourable conditions for common cow-wheat and therefore heath fritillary, and also unfavourable conditions for other desired plants and invertebrates characteristic of open heathland. An experiment was therefore undertaken to test for the most successful method in controlling bracken growth after burning.

Study area: In 1996, Halse Combe (a large area of heathland) held the largest population of heath fritillary within Exmoor but by 2001 they became extirpated from this site. This was due to a lack of appropriate management, including a decline in livestock grazing, resulting in the invasion of scrub and dense stands of bracken Pteridium aquilinum, thus reducing habitat quality for the butterfly. Halse Combe was therefore selected as the study site to test methods of habitat restoration and management. This site is owned by the National Trust, who were partners with Butterfly Conservation in the experiment and carried out the treatments and heathland restoration.

Initial burning: The former heath fritillary flight area at Halse Combe (covering approximately 10 ha) was burnt in March 2002. Following this the site was divided into 18, 40 x 20 m plots and treatments were assigned to each plot as follows:

Control - six plots were left untreated after the initial burn.

Herbicide treatment - six plots were treated with Asulox (a selective herbicide) using a knapsack sprayer in May 2002. These were not treated again throughout the study period. Application rate of Asulox was 9.6 l/ha.

Mechanical treatment - six plots were cleared of scrub by hand in the first year after burning. Following this 'bracken bashing' took place on an annual basis (2002-2005) in May. This entailed driving an Aebi (a specialist vehicle for steep and difficult terrain), quad bike and/or tipper or tractor and swipe (where possible). In areas inaccessible to vehicles, bracken was cut by hand. Bracken bashing controls bracken growth by breaking and weakening the stems.

Vegetation monitoring: Thirty 1 m² quadrats were laid at random throughout each plot. Vegetation composition and structure was recorded within each quadrat during the heath fritillary peak flight period from 2003-2005, to assess bracken density and vigour, cow-wheat density and cover of other vegetation.

The effects of the treatments on non-target species (other invertebrates) were not investigated.

Effect of herbicide on vegetation structure & composition: Spraying with Asulox was the most effective method of bracken control after a burn. Bracken was less dense in the sprayed plots than in the control and bashed plots, particularly soon after the treatment took place (Fig. 1, attached). In 2003 for example, the control plots had on average of 26 bracken fronds per quadrat (1 m²), compared to an average of 13 for the bashed plots and 2 in the sprayed plots. Bracken density did increase in 2005 but some bracken is beneficial to the heath fritillary as it creates sheltered microclimates on exposed sites.

Cow-wheat density was also greatest on the sprayed plots (Fig. 2, attached) and although cow-wheat was initially adversely affected by the herbicide, it increased as the study progressed to on average 1.5 spikes per m² in 2005 - this was the only treatment to have greater than one spike per m².

Bilberry cover increased in the sprayed plots, which was a favourable consequence as cow-wheat is a hemi-parasite of bilberry. Cow-wheat density was positively correlated with bilberry cover. The sprayed plots also had the greatest overall vegetation cover (excluding bracken) by 2005.

Effect of mechanical treatment on vegetation structure & composition: The bashed plots had the lowest densities of bracken by 2005 (an average of six fronds per 1 m² quadrat) but bilberry cover also declined and consequently it was considered likely that cow-wheat would decline over time in conjunction with bilberry. It is thought, however, that this method of treatment would be more beneficial if carried out less frequently than on an annual basis as it would then not be so detrimental to bilberry. This is currently being investigated.

There were no apparent problems with soil compaction following the use of vehicles to carry out the bracken bashing, a problem probably avoided due to the relatively lightweight equipment used. However, soil disturbance probably had an effect in the mechanically treated plots due to the steepness of the slopes, the peaty soils and turning of the vehicles. These effects have not been quantified.

Control plots: The control plots overall contained the greatest densities (accounting for 60% of the canopy cover in 2004 and 30% in 2005) and heights (> 1 m tall i.e. twice the height of the other treatments in all years) of bracken. No common cow-wheat was found in dense bracken areas within the plots (>40 stems/m²). These plots also contained the least amount of all other vegetation in all years.

Heath fritillary population: Monitoring of the fritillary population took place throughout the study by undertaking timed counts during the peak flight period. Heath fritillaries recolonised the study site in 2004 and by 2005 it supported a large fritillary population with an estimated peak of around 965 individuals.

Conclusions: Rotational burning is a very important management technique for the heath fritillary as it creates and maintains the early successional heathland habitat favoured by common cow-wheat, the larval foodplant. However, the shade offered by low density bracken stands appears to provide a microclimate favourable for cow-wheat (the majority of cow-wheat occurred in areas with 5-15 bracken fronds per square metre at heights of 15-50 cm).

From the control plots it is evident that without management, bracken will soon dominate, thus some form of bracken control is required post-burning. Bracken control by spraying with the herbicide, Asulox, was found to be an effective means of control in this study, and was more efficient than 'bracken bashing'. Although not an intrinsic part of this study, grazing is also considered important to implement after bracken control has taken place, as it helps maintain early successional habitat by controlling invasion by woody plants and undesirable herbs such as stinging nettle Urtica dioica.


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