Bracken Pteridium aquilinum L. Kuhn is a problematic weed world-wide, causing difficulties for managers of upland and marginal land in the UK (Marrs et al. 2000). Many attributes make control difficult but most workers identify the rhizome system as the major factor (Kirkwood & Archibald 1986; Marrs, Johnson & Le Duc 1998a; Martin 1976; Robinson 2000). Three strategies are commonly used for bracken control (Marrs, Johnson & Le Duc 1998b). One is mechanical control: fronds are damaged, usually by cutting or bruising (e.g. mechanical rolling). The aim is to damage the fronds in early summer, with maximum frond expansion and depletion of rhizome carbohydrate reserve, before translocation replenishes the reserves (Williams & Foley 1976). Cutting may be applied up to three times annually (Williams 1980).
Secondly, use of herbicide: the most successful herbicides kill frond-producing buds on the rhizome, with little immediate effect on carbohydrate reserves. Asulam [methyl (4-aminobenzenesulphonyl) carbamate] is most commonly used. On translocation from frond to rhizome it kills active and dormant buds (Veerasekaran, Kirkwood & Fletcher 1977, 1978). Asulam frequently achieves almost 100% frond reduction in the year following spraying, but in following years rapid recovery may occur and follow-up treatments are needed (Lowday & Marrs 1992). With lowered frond productivity, respiration and decomposition slowly reduce carbohydrate reserves.
Thirdly, inhibition by other vegetation: where bracken is dense, other vegetation is often degraded or may have disappeared. Bracken litter may aggravate this degradation. Following control treatment, restoration by litter disturbance and seeding or fertilization may be attempted. There is evidence that development of competitive vegetation can help in controlling bracken (Watt 1955; Lowday & Marrs 1992; Petrov & Marrs 2000).
While many methods are used to control bracken, individual experiments produce conflicting results. There is a need for an integrated national control strategy that takes account of the impact on rhizome performance. We have attempted to develop such a strategy.
developing an integrated strategy for bracken control
In developing a national control strategy we need to consider the effects of appropriate combinations of bracken control treatments in different situations, particularly with differing climates and differing vegetation restoration objectives. As bracken control and subsequent vegetation restoration is slow in the British uplands (Pakeman, Le Duc & Marrs 1997) these studies must be long-term.
We aimed to formulate such a strategy based on the results of seven experiments set up (1993–95) at four sites in different regions of the UK (Sourhope, Cheviot Hills; North Peak Environmentally Sensitive Area (ESA), Peak District National Park; Carneddau, Snowdonia National Park; Cannock Chase, Staffordshire). All areas had dense bracken but vegetation types differed (Table 1), and hence their management prescriptions. At two sites the desired outcome was grassland, and at the other sites Calluna heath. We used a standard experimental design throughout and applied the same bracken control treatments as main-plot treatments. For each site subsidiary treatments were applied at subplot level. Subsidiary treatments were chosen to test hypotheses concerning (i) vegetation end-points; (ii) vegetation restoration techniques; (iii) bracken control follow-up treatments. We examined the impact of these treatments on the rhizome system; effects on frond production have been reported elsewhere (Le Duc et al. 2000).
Table 1. Site vegetation description. The National Vegetation Classification (NVC; Rodwell 1991a,b, 1992) descriptions represent the pretreated condition; figures in parentheses were obtained with bracken left out of calculations. Fit (= goodness-of-fit) was given by TABLEFIT version 1·0 (Hill 1996), and is: > 80 very good, > 70 good, > 60 fair, < 60 poor
|Location||Sourhope Estate||Carneddau Estate||North Peak ESA||Cannock Chase|
|Vegetation description||Acid grassland with moderate grazing. There are conifer plantations and wet areas near both sites||Acid grassland with high grazing (sheep and ponies). Up- slope from the experiment the vegetation changes, first to Ulex then Calluna heath||Antecedent Calluna/ Erica heath altered by bracken invasion. Grazing levels low. Patches of pine woodland nearby, heath more distant||Has extensive areas of Calluna heath with patches of grass heath, large pine plantations, ancient oak woodland and birch scrub, all bracken infested. No grazing|
|Bracken litter status||Moderate litter disappearing rapidly after treatment||Moderate litter disappearing rapidly after treatment||Extensive litter, often in discrete heaps that remain after treatments||Extensive, often continuous, litter beds remaining after treatment|
|Site differences within location||Sourhope 2 more base-rich than Sourhope 1, but prone to development of bare patches and invasion by other weeds. Suffers from trampling|| || ||Cannock 1 adjacent to oak–birch woodland; Cannock 2 has some pine invasion; Cannock 3 in danger of pine and birch invasion. Cannock 1 and 3 suffer from trampling|
|Site NVC class||Sourhope 1: U4e (U4e), fit 75 (77); Sourhope 2: U4a (U4a), fit 61 (64)||U4a (U4a), fit 74 (79)||U20 (H18), fit 77 (83)||Cannock 1: W16 (U2b), fit 78 (67); Cannock 2: W16 (U2b), fit 61 (55); Cannock 3: U2 (U2), fit 63 (62)|
problems in studying rhizomes
The rhizome system comprises a massive underground network, with growth and senescence causing fragmentation and temporal change. Confusion over nomenclature exists in the literature. We define a bracken patch as an area of fronds comprising single or multiple genets and/or single or multiple plants. A genet has constant genetic make-up but may comprise many individuals (clones), i.e. independent physiological units (Daniels 1985). A plant is a single genet and a physiologically independent unit. These definitions concur with Birch (2002). Watt (1940) suggested the actual size of a bracken plant was usually quite small in comparison with its potential, rarely exceeding 20 m from rhizome apex to the senescent end. It has also been suggested that the extent of a single genet (perhaps many plants) may be considerable (Sheffield et al. 1993).
Lateral transfer of nutrients occurs across short distances (Tyson, Sheffield & Callaghan 1999) but can only occur within single plants. When sampling rhizomes it is possible only to obtain a broad view at patch scale; it is impossible to obtain plant-scale information without an archaeological approach.
The rhizome system presents problems for experimental design. Lateral translocation leads to non-independence of experimental units, increasing potential for type II errors. Increased plot size reduces this effect, and randomization techniques can be used to test the robustness of statistical results (Manly 1997).
The bracken rhizome is dichotomously dividing, comprising stem tissue of systematically varying internode length (length between branches; Watt 1940; Whitehead & Digby 1997). Watt defined three readily identifiable forms: long, intermediate and short shoots, depending on internode lengths of 15–40 cm, 2–15 cm and 0·5–2 cm, respectively. Although short shoots arise from long and intermediate ones, there is evidence that they can revert back again (Watt 1940). Short shoots are principally the frond-bearing components (Watt 1940), commonly bearing petiole scars and found near the soil surface. The main function of long and intermediate shoots is carbohydrate storage but they are also important in foraging, with an occasional minor role in frond bearing (Whitehead & Digby 1997). As the function of intermediate and long shoots appears more or less identical we do not differentiate between them. We considered two categories, the short shoots and the long shoots (including intermediate shoots), and refer to these using the functional terms ‘frond-bearing’ and ‘storage’ rhizomes (Williams & Foley 1976).
Williams & Foley (1976) monitored carbohydrates within fronds and rhizomes throughout the year, and suggested that rhizome reserves were maximum in early autumn. Thus, we made measurements in October.
Various measures have been used to assess the impact of control treatments on the rhizome, including length, diameter, internode length, number of buds, carbohydrate concentrations and biomass. However, two separate studies in Breckland, UK, have shown that the best predictor of frond performance, for a range of sites, was total rhizome biomass (Marrs, Johnson & Le Duc 1998a). In a small number of cases bud counts were significant, but length (m m−2) was not. We therefore confined our assessment to the effects on rhizome biomass
The impact on frond production differs between the two main methods of control, cutting and herbicide application. Cutting maintains frond density but length is reduced; herbicide reduces density significantly but length is maintained (Le Duc et al. 2000). The frond-bearing fraction of the rhizome system must reflect this difference. Therefore two aspects of response were examined: (i) total rhizome dry mass per unit area (M) and (ii) the ratio of frond-bearing to total dry mass (R). We also discuss the effects on rhizomes in relation to earlier work on fronds (Le Duc et al. 2000). Nomenclature follows Stace (1991).