Study

Changes in the rhizome system of bracken subjected to long-term experimental treatment

  • Published source details Le Duc M.G., Pakeman R.J. & Marrs R.H. (2003) Changes in the rhizome system of bracken subjected to long-term experimental treatment. Journal of Applied Ecology, 40.

Summary

Bracken Pteridium aquilinum can be a problematic invasive weed, causing difficulties for managers of upland and marginal land in the UK. Many attributes make bracken control difficult but most workers identify the persistence of the rhizome system as the major factor. The two main methods of current control are cutting and herbicide application. Cutting reduces frond length but does not impact frond density; herbicide reduces frond density significantly but length is maintained. The frond-bearing fraction of the rhizome system might reflect this difference. Therefore two aspects of response to these treatments were examined: total rhizome dry mass per unit area; and the ratio of frond-bearing to total dry mass.

Study areas: Seven experiments were established in four different locations in the UK: Sourhope Estate (Cheviot Hills), North Peak ESA (Derbyshire), Carneddau Estate (Snowdonia National Park, North Wales) and Cannock Chase (Staffordshire). Two pairs of experiments (Sourhope 1 and 2; Cannock 1 and 2) were matched, with identical treatment history except that Sourhope 2 (1994) was started 1 year after Sourhope 1 (1993).

Main treatments: The bracken control treatments were applied to main plots and split plots, within replicated blocks. Treatments were site-specific, allocated randomly and included untreated controls. Treatments were: i) untreated control; ii) cut once per year; iii) cut twice per year; iv) cut year 1, sprayed with herbicide (asulam) year 2; v) sprayed once only; vi) sprayed year 1, cut year 2.

Generally, single cuts took place in June, second cuts and herbicide application in August. Follow-up treatments were included in some experiments to assess standard bracken control procedures.

Vegetation restoration and follow-up: As there were site vegetation differences, treatments varied. Sourhope 1 and 2: grass seeding to improve sward quality and prevent bare patches (subtreatment experimental level). Sourhope 1 and 2: weed-wiping, standard for follow-up control, compared between two experiments (Sourhope 1 wiped, Sourhope 2 not wiped). Carneddau: spot-spraying (± spot spray) for follow-up bracken control (subsubtreatment level). Carneddau: fertilizer, alone and combined with grass seeding (three treatments levels, including control, at subtreatment level). North Peak: two methods of heather Calluna vulgaris seeding (brash or litter, three treatment levels including control) to restore heath (subsubtreatment level). North Peak: ESA prescription rate grazing (0.5 sheep/ha) vs. no grazing (two treatments at subtreatment level). Cannock Chase 1 and 2: weed-wiping for follow-up bracken control (comparison between Cannock 1 with and 2 without). Cannock Chase 1 and 2: fertilizer application with and without litter disturbance, by harrowing (three treatment levels, including control, at subtreatment level). Cannock Chase 3: Calluna seeding by brash (two treatment levels including control, at subsubtreatment level). Cannock Chase 3: litter disturbance by burning (two treatment levels including control, at subtreatment level).

The grass seed mix included sheep's fescue Festuca ovina, common bent-grass Agrostis capillaris, smooth meadow-grass Poa pratensis and common sorrel Rumex acetosa. Calluna treatments included an Agrostis castellana nurse crop at North Peak and A.capillaris at Cannock 3.

Rhizome sampling: Rhizome sampling took place in October, at Cannock in 1998, Sourhope in 1999 and North Peak in 2000 (scheduled for 1999, abandoned due to waterlogged soil). Carneddau was sampled in 1998, followed in 1999 by a sample of 50% of plots, chosen at random, and the complementary 50% in 2000; this was to attempt a sampling quality check over time.

Two randomly located samples were taken per subplot, for split-plot experiments, and one per subsubplot in others. Samples comprised the rhizome content of a 0.5 × 0.5 m pit with depth according to rhizome presence. Quality control of pit size was achieved by requiring that a 0.495 × 0.495 m quadrat should pass to the bottom on completion. Median pit depths were: Sourhope 1, 24 cm (range 16–38 cm); Sourhope 2, 40 cm (25–47 cm); Carneddau, 35 cm (25–50 cm); Cannock 1, 48 cm (15–80 cm); Cannock 2, 63 cm (25–100 cm). A total of 580 pits were dug.

Rhizome samples were washed with water to remove soil, dried at 80 °C and weighed to give a total rhizome dry mass (kg/m²), then separated into storage and frond-bearing fractions. Both fractions were reweighed. A small portion of fragments of indeterminate type, were discarded.

The ratio of frond-bearing to total dry mass was reduced by herbicide treatment, e.g. from 0.30 to 0.16. Such low values of this ratio 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 rhizome dry mass at matching experiments on Cannock Chase, treatment effects were similar at both sites. Here, cutting twice per year or cutting with herbicide application, were best at controlling bracken. In contrast, at Sourhope cutting alone gave 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.

Conclusions: Bracken rhizome mass differs between sites and in response to control treatments. Weather may affect rhizome mass, with wet years being detrimental.
Of the cutting and herbicide treatments tested, cutting twice per year is generally most effective. However, constraints due to terrain where cutting is impossible or impractical, may leave herbicide treatment as the only feasible control option.


Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://blackwellpublishing.com/submit.asp?ref=0021-8901

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