Studies on the conservation of lowland Calluna heaths. II. Regeneration of Calluna, and its relation to bracken infestation

Summary

This study investigated methods of regenerating heather Calluna vulgaris on a heathland in the Brecklands of eastern England. Heather is often the dominant and characteristic ericaceous shrub found on lowland heaths, and in nature conservation terms, management to maintain an open heather heathland with associated flora and fauna is desirable to ensure the survival of many species of conservation concern.

The treatments tested included disturbing the ground flora by cutting alone and in combination with burning and rotavating. Reducing bracken Pteridium aquilinum density by fosamine application was also trialed - in the UK bracken (a tall fern that grows in dense stands) commonly invades heathlands and outcompetes underlying plants.

Study site: Methods to encourage heather regeneration in combination with bracken control were tested on degenerate Calluna heathland at Knettishall Heath Country Park (OS grid ref: TL 985805) in Suffolk, southeast England. There was substantial invasion from silver birch Betula pendula and downy birch B.pubescens and bracken, and heather was mainly in the late mature and degenerate phases (estimated, by counting the annual growth rings, to be 17 years old in 1982).

Experimental design and treatments: In January 1980, four blocks, each containing four 6 m x 4 m plots separated by 1 m pathways, were established in areas of invading birch and bracken. The birch trees were removed by cutting to ground level, and painting the cut surfaces within 10 min with 2,4,5-T (Trioxone-50, 2.5% active ingredient in diesel).

Four main treatments were applied randomly within each block:

i) no treatment - ground vegetation left untouched

ii) cutting - ground vegetation cut using a brush clearer and cut material removed

iii) burning - as treatment (ii), but plant debris and litter burnt

iv) rotavating - as treatment (ii), but ground surface rotavated to incorporate plant debris and litter into the surface mineral soil


These plots were divided into two 4 m x 3 m sub-plots and bracken control treatment applied randomly to one. Initially bracken was sprayed in August 1980 with fosamine at a rate of 4.8 kg a.i./ha in 300 l water/ha +0.25% of a non-ionic wetter (Agral), using a motorized mist blower. Spraying was done in good dry weather, and there was no rain in the 24 h after spraying. This treatment was not very effective, therefore the bracken was subsequently cut three times a year between 1983 and 1985.

Vegetation recording: In early July within the central 2 m x 2 m area of each sub-plot the following was recorded: i) the numbers of bracken fronds each year between 1980 and 1985; ii) recruitment, mortality and survivorship of marked (loose collar of coloured wire) Calluna plants between 1980 and 1982, and tree seedlings between 1980 and 1985; iii) the standing crop of Calluna and wavy hair-grass Deschampsia flexuosa harvested from randomly-chosen 0.25 m² sub-quadrats between 1983 and 1985; iv) the cover of the main vegetation species in untreated plots estimated visually and by point quadrats in1980 and 1985.

Vegetation change in undisturbed plots: The vegetation in 1980 was old heather under a sparse bracken canopy, with <1% D.flexuosa cover. Many heather bushes were collapsing and forming gaps, with no regeneration. By 1985 bracken density had increased from 4.5 to 24 fronds/m². Bare ground, bracken litter and D.flexuosa cover also increased, whilst old heather plants had decreased correspondingly. There was a little pioneer and building phase heather present in 1985, but cover was very low (<0.5% and <2% respectively).

Bracken treatment: By 1985 bracken treatment resulted in increased bare ground, bracken litter and D.flexuosa, compared to untreated plots. In one sub-plot D. flexuosa had reached 75% cover. Old heather declined. Disturbing the ground vegetation had no effect on bracken frond density. Spraying halved frond densities from 17.1 to 8.9 fronds m² in the following year (1981), but densities recovered to pretreatment levels by 1982.

Heather regeneration: Regeneration was entirely from seed and was much greater where disturbance treatments were applied (cutting alone, or in combination with burning or rotavating) but with no significant difference between disturbance treatments. Seedling establishment was reduced in some plots where bracken density was >20 fronds m², and heather standing crops were reduced by 50% at frond densities >11 fronds m².

Tree seedlings: Birch and Scots pine Pinus sylvestris seedling invasion was mainly confined to the 3 years after disturbance, being reduced as the heather increased. Tree seedling numbers tended to be reduced in higher densities of bracken, but Scots pine persisted in greater densities (28 fronds/m²) than birch (20 fronds /m²). In 1985, birch seedlings were found only in the bracken control treatment, but Scots pine seedlings were found in both sub-treatments.

Conclusions: Where the predominantly mature and degenerate heather was left undisturbed, very little regenerated, even where bracken density was reduced. Disturbing the ground vegetation by cutting alone, or in combination with burning or rotavating, increased establishment of pioneer heather. There was no additional benefit in burning or rotavating found. All new pioneer heather established from seed, with no plants resprouting after the disturbance treatment. The poor regeneration of old rootstocks was expected, as regeneration declines with age. Results suggest that at this site management for heather regeneration should be confined to areas with a bracken density of <11 fronds/m².


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