The Effects of Cutting and Herbicide Treatment on Pteridium Aquilinum Encroachment - 203

Applied Vegetation Science 5: 203-212, 2002 © IAVS; Opulus Press Uppsala. - THE EFFECTS OF CUTTING AND HERBICIDE TREATMENT ON PTERIDIUM AQUILINUM ENCROACHMENT - 203

The effects of cutting and herbicide treatment on Pteridium aquilinum encroachment

Pakeman, R.J.1*; Thwaites, R.H.1; Le Duc, M.G.2 & Marrs, R.H.2

1Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK; 2Applied Vegetation Dynamics Laboratory, School of Biological Science, University of Liverpool, PO Box 147, Liverpool, L69 3GS, UK; *Corresponding author; Fax + 441224311556; E-mail [email protected]

Abstract. Pteridium aquilinum (bracken) encroachment is an Introduction important factor in the loss of certain habitats in the United Kingdom. However, no information exists as to whether pre- Pteridium aquilinum (bracken) is an invasive fern of vention of encroachment is a cost-effective strategy for considerable economic and natural heritage importance Pteridium management. Conventional methods for the control in many temperate and upland tropical regions of the of Pteridium (cutting, asulam application) were tested at one site (Levisham) to quantify their ability to prevent or delay world (Taylor 1990; Pakeman et al. 2000a). Its suppres- encroachment and to affect the vigour of the Pteridium at the sion of other vegetation, provision of suitable environ- edge of the stand. The effects of encroachment and asulam mental conditions for ticks and content of carcinogenic application on the vegetation present were monitored at a and toxic compounds are well recorded, as well as some second site (Ramsley), where techniques commonly used for positive species associations (e.g. Pakeman & Marrs moorland restoration were employed in combination with 1992a). In many regions, Pteridium continues to spread asulam application. as a result of land use change mainly forest clearance for Cutting once per year or a single application of asulam grazing or changes in grazing practices on open land delayed the advance of the Pteridium front. At Levisham, the (Taylor 1995). This encroachment is primarily via two untreated front advanced 2.7 m in 5 yr, while in the same period the cut front advanced 0.88 m and the sprayed front was routes: linear expansion from the edge of present stands 1.5 m behind its initial position. At Ramsley, the untreated front or development of sparse Pteridium into dense stands. invaded 1.8 m in 5 yr, and the sprayed front was again 1.5 m While the latter is not true encroachment, it is function- behind its starting position. Both spraying and cutting reduced ally equivalent and probably accounts for a large per- frond biomass, frond cover and rhizome biomass. Herbicide centage of perceived vegetation change (Marrs & Hicks spraying prevented the loss of Calluna vulgaris, though the 1986). Pteridium can and does reproduce through spore restoration treatments had little effect. The merits of a bal- and gametophyte production. However, little is known anced targeting of control on encroaching fronts or Pteridium about this type of spread and its success appears to be at the stand level are discussed for different situations. very limited under current climatic and ground conditions in the UK (Dyer 1990). Keywords: Asulam; Bracken; Invasive species; Vegetation Within the UK encroachment by either means re- restoration. sulted in a change of vegetation cover from other types to Pteridium of ca. 600 km2 between 1984 and 1990, and 1068 km2 between 1990 and 1998. Net changes over the Nomenclature: Stace (1991). same periods were a decline in Pteridium of ca. 400 km2 and an increase of 280 km2 respectively (Barr et al. 1993; Haines-Young et al. 2000). Despite national and Scottish statistics (Miller et al. 1989) being available little is known about linear rates of encroachment into fresh ground and many estimates are based on short time scales (Table 1). These rates may also change as a result of climate change (Gordon et al. 1999; Pakeman & Marrs 1992b; Werkman & Callaghan 1999) or land use change (Birch et al. 2000; Pakeman et al. 2000a). Even less is known about the effects that encroachment has on the associated vegetation, especially about the rate of 204 PAKEMAN, R.J. ET AL.

Table 1. Estimated rates of linear encroachment by Pteridium stands in the UK determined from aerial photography and ground measurement.

Site Encroachment rate (m.yr-1) Time period (yr) Source

Aerial photography Levisham Moor, North Yorkshire 0.35 19 Pakeman & Hay (1996) Ramsley Moor, Derbyshire 1.27 5 Pakeman & Hay (1996) Weeting Heath, Norfolk 0 - 1.4 26 Marrs et al. (1986) Cavenham Heath, Suffolk 0-0.9 35 Marrs et al. (1986) Ground measurement Scotland (Four heathland sites) -1.0 - 3.0 2 Miller et al. (1989) Unspecified, Scotland 0 - 0.9 5 Braid (1934) Four sites, Durham Coal Measures 0.35 - 1.25 2 Jeffreys (1917) Lakenheath Warren, Norfolk 0.43 8-18 Watt (1954) Cavenham Heath, Suffolk 0.4 3 Marrs et al. (1986)

loss of species and vegetation cover (Marrs 1987; Watt Material and Methods 1954) It has been suggested (Johnson et al. 1995) that Site descriptions Pteridium control measures targeted at encroaching fronts may be an effective way of halting the spread. Sites where the landowners were certain that This would prevent invasion of the neighbouring veg- Pteridium was encroaching were selected. Ramsley etation and prevent further loss of habitat during efforts Moor, Derbyshire (Altitude: 290 m a.s.l.; National Grid to tackle specific areas. The current practice of inten- Reference SK 289750), is in the eastern Peak District sively treating stands to control or eradicate Pteridium owned by the Peak District National Park. Here Pteridium may actually result in continuing losses to Pteridium was encroaching into a Calluna vulgaris (heather)-domi- encroachment in other areas not subject to control that nated community, where Calluna was in the mature might outweigh the gains made. No information is cur- phase (sensu Watt 1947, 1955). The second site, rently available that quantifies this. Levisham Moor, North Yorkshire (Altitude 230 m a.s.l.; The work reported in this paper was designed and National Grid Reference SE 836941), is in the centre of carried out with two purposes in mind. First to provide the North York Moors and owned by the North York information on the effects of encroachment on other Moors National Park. Here, Pteridium was invading a species present and second to test the hypothesis that mixed community dominated by Calluna and Vaccinium Pteridium control measures (treatment with herbicide or myrtillus on a steep slope, where Calluna was in the cutting) can prevent or slow its encroachment. As part mature and degenerate phases. Both sites were grazed by of this second purpose, the use of vegetation restoration sheep at relatively low stocking levels (< 0.5 sheep.ha-1). methods to reinstate vegetation after Pteridium control The approximate position of the front was marked was also tested. The treatments chosen did not represent permanently in August 1993 at Ramsley Moor and in ‘best-practice’ for eradication, but rather less intensive April 1993 at Levisham Moor. The positioning of the measures more suited to treating large areas. markers at Levisham was prior to frond emergence and As total Pteridium biomass is a more accurate meas- the position of the previous year’s fronds was used as an ure of treatment impact than frond cover, this formed approximate guide. In total, 76 m of front in four repli- part of the investigation. However, as this required cate 19 m blocks was marked at Ramsley and 112 m of ground disturbance to remove rhizome material it was front in four replicate 28-m blocks at Levisham. At both decided to tackle the study as two experiments, one to sites blocks were separated by at least 2 m. investigate the detailed effects of control on Pteridium performance, and the other the effects of Pteridium Treatment implementation encroachment or control on the vegetation present. This study forms part of a larger project where techniques for At Ramsley Moor, each block was split into two integrating Pteridium control with vegetation manage- 9 m ¥ 10 m plots separated by a 1 m guard row (Fig. 1a). ment and restoration are being developed (Le Duc et al. One randomly selected plot in each block was sprayed 2000; Pakeman et al. 2000b). with asulam (a narrow spectrum herbicide used in most control of Pteridium) from a knapsack sprayer at the recommended dose (4.4 kg active ingredient per ha) in - THE EFFECTS OF CUTTING AND HERBICIDE TREATMENT ON PTERIDIUM AQUILINUM ENCROACHMENT - 205

Fig. 1. An example of the layout of one of the four blocks at (a) Ramsley Moor and (b) Levisham Moor showing the position of quadrats relative to the front of the block (0, 4 or 8 m) and typical initial positions of the actual front relative to the baseline (0 m) set out.

August 1993. At each of three distances (0, 4 and 8 m) change in position relative to the position of the front in from the marked front edge, four 1 m ¥ 1 m quadrats 1993. Differences in position were analysed for each were marked, each separated from the others and the year using a randomized block ANOVA. The rate of sides of the plot by 1 m. In October 1993 each quadrat advance was calculated from 1994 (after the direct was subject to a factorial combination of two treat- effect of the asulam treatment) to 1998 and tested with ments, ± addition of Calluna seed in the form of locally the same model. harvested brash (500 g.m-2) and ± litter disturbance (raking and removal). These treatments were selected as Pteridium biomass measurement (Levisham Moor) they are known to encourage the regeneration and establishment of Calluna (Lowday & Marrs 1992a; Putwain Pteridium biomass was sampled with a 0.25 m ¥ 1 m & Rae 1988). quadrat with the long edge perpendicular to the Pteridium At Levisham Moor each block was split into three front (Fig. 1b). Three samples per plot were removed at 8 m ¥ 10 m plots separated by a 2-m guard row (Fig. 1b). a random position, one each at 0, 4 and 8 m behind the One randomly selected plot in each block was sprayed initial position of the front. No samples were taken from with asulam as at Ramsley Moor in August 1993. A adjacent positions throughout the experiment, such that at second randomly selected plot was cut every year in least 0.25 m separated any two samples. Ground distur- August with a brush cutter at ca. 10 cm above the ground to bance over the experiment totalled 30% of the available mimic the effects of a tractor-mounted cutter set at a height material at each position (0, 4 and 8 m) behind the front. to minimize direct effects on any ground vegetation. This could have slowed growth, but as the measure of Where months are specified April refers to the last encroachment was based on the whole front, the effects week in April, August to the first week in August (and of removing 9.4% of the rhizome in 3 m ¥ 1 m bands occasionally to the last week of July) and October to the each year were probably minimal. Frond biomass was fourth week of that month. sampled by removing all above-ground material present in the first week of August prior to treatment. The Encroachment (Levisham and Ramsley Moors) material was dried at 80 ∞C and weighed. Rhizome material was sampled by removing all the soil within the The Pteridium front within each plot was divided quadrat, sorting and removing rhizome material and into 1-m segments. The position of the most advanced replacing the soil. Rhizome material was washed, sorted frond within each segment relative and perpendicular to into frond-bearing (short shoots) and storage (long the marked baseline was measured in August every shoots) tissue and dried at 80 ∞C before weighing year. The mean position of the front in each plot was (Pakeman & Marrs 1994a). Rhizome material was ini- taken as the mean of all segments within that plot. The tially sampled in April, August and October from 1993 advance or retreat of the front was calculated as a to 1995. A final sampling of fronds and rhizomes was 206 PAKEMAN, R.J. ET AL. carried out in August and October 1998 respectively. Vegetation change (Ramsley Moor) The effects of treatment on Pteridium biomass was analysed by calculating the change in rhizome biomass Pteridium cover and the cover of all ground vegeta- between April 1993 and 1995 (the longest possible tion and other cover categories (e.g. litter and bare ground) comparison using a pre-treatment measure), the change were visually estimated in the central 0.5 m ¥ 0.5 m part in rhizome biomass between October 1993 and 1998 of each quadrat at Ramsley Moor in October each year. (the longest possible contrast) and the change in frond Changes in vegetation cover through time were ana- biomass from August 1993 to August 1998. Compari- lysed using orthogonal polynomial time contrasts son of treatment effects on the rhizome after 5 years (VORTHPOL; Anon. 1996). The resultant derived slope should be comparable, as this approximates to the low- parameters (mean, linear and quadratic terms) were est point in rhizome reserves observed after spraying analysed in a split-split plot analysis of variance (proce- (Pakeman & Marrs 1994a). April and October measures dure ANOVA, Genstat 5 release 3.2; Anon. 1996) with of biomass are not comparable as rhizome reserves are treatment as the main plot, distance behind the edge of used over winter (Pakeman & Marrs 1994a). Treatment the Pteridium front as the sub-plot factor and litter will have had little (cutting) or no (asulam) effect on disturbance and seed addition as sub-sub plot factors. rhizome biomass by October 1993 (Pakeman & Marrs 1994a). These changes were analysed in a randomized block ANOVA with initial distance from the position of Results the Pteridium front in August 1993 as a covariate. The covariate was necessary because of the ‘ragged’ nature of Encroachment (Levisham and Ramsley Moors) the actual front. As some sample locations never had Pteridium present these were removed from the analysis. At both sites, Pteridium continued to encroach into the adjacent vegetation throughout the period of measurement (Fig. 2), except in 1997 at Levisham when no change in position from 1996 was measured. Mean invasion rates at the sites were 0.36 m.yr-1 at Ramsley Moor and 0.55 m.yr-1 at Levisham Moor. The effect of cutting once per year at Levisham Moor was to slow the rate of advance of the Pteridium front to approximately a third, 0.18 m.yr-1 (Fig. 2b), of the untreated front. Though the relative positions of the untreated and cut fronts did not differ significantly, the difference in rate of advance was significant at the 5% level. At both Levisham and Ramsley Moors the initial effect of treatment with asulam was to set the position of the leading fronds in 1994 relative to 1993 back by 3.6 m and 4.9 m respectively. At both sites the difference between the position of the untreated front and the sprayed fronts was always significant except in 1997 at Levisham. Pteridium recovered from asulam treatment at both sites to sustain a continued advancement of the front, though by 1998 the leading fronds had still not reached the starting point of the front in 1993. The rate of invasion (post-1994 in the case of asulam treated plots) was not significantly different between untreated and sprayed Pteridium at either site. At Levisham, an en- Fig. 2. The effects of control measures on the mean (± 1 croachment rate of 0.53 m.yr-1 was equivalent to that of s.e.) position (m) of the bracken front (1993 to 1998) the untreated. The considerably higher encroachment rate relative to its 1993 position. Effects of (a) asulam (––) at Ramsley Moor (0.87 m.yr-1) would mean that the and no treatment (------) at Ramsley Moor and (b) asulam (––), cutting once per year (---◆---) and no treatment (-- sprayed Pteridium would catch up the untreated Pteridium ----) at Levisham Moor. Significant differences of posi- within 12 yr. However, as the sprayed Pteridium at this tion within year are indicated by * = 0.05 ≥ P > 0.01 and ** site has yet to recover to its untreated extent, this predic- = 0.01 ≥ P > 0.001. tion may not apply beyond that point (Fig. 2a). - THE EFFECTS OF CUTTING AND HERBICIDE TREATMENT ON PTERIDIUM AQUILINUM ENCROACHMENT - 207

Biomass changes (Levisham Moor) Table 2. Probability values (and degrees of freedom) derived from ANCOVA of the effects of Pteridium control treatment Untreated Pteridium had a mean net increase of at Levisham Moor on changes in Pteridium rhizome biomass (g.m-2) between April 1993 - April 1995 and October 1993 - frond biomass of 85 g.m-2 and a mean net increase of October 1998 and frond biomass (August 1993 to August rhizome biomass of 234 g.m-2 over the whole plot from 1998). Distance from the initial front position in August 1993 1993 to 1998 (Fig. 3). In comparison, there was a was used as the covariate. significant reduction in the biomass of fronds as a result of both treatments, with a greater reduction in frond April August October biomass after cutting. Spraying resulted in a small de- 1993-1995 1993-1998 1993-1998 crease in rhizome biomass that was significantly differ- Fronds - < 0.001 (2; 19) - ent from the increase in the untreated plots (Table 2). Frond-bearing rhizome ns - 0.004 (2; 22) This overall decrease was made up of a small increase in Storage rhizome ns - 0.035 (2; 22) frond-bearing rhizomes and a large decrease in the Total rhizome ns - 0.010 (2; 22) amount of storage rhizomes. The reduction in rhizome biomass by the end of the experiment after cutting was considerably larger than that of a single spraying (Fig. with time was seen with litter disturbance (Table 3). 3). These significant changes in rhizome biomass were However, there was a significant effect (linear) of only evident after 5 yr of treatment as the changes by Pteridium control treatment and cover varied consider- 1995 were small compared with the variation within ably with distance from the front (Fig. 4b). There was a treatments. small loss of Calluna at 8 m in the untreated plots and a small gain at 8 m in the treated plots. After 5 yr there was Vegetation change (Ramsley Moor) no difference in the cover of Calluna at 4 m. However, at 0 m there was a loss of Calluna cover in the untreated Only a small number of species were common enough plots as the Pteridium front encroached, whereas in the across the experiment to show any treatment response treated plots there was an initial reduction (a result of (Table 3). Agrostis capillaris had a significant interac- the disturbance treatment) and a subsequent increase tion between Pteridium control treatment and distance with time. Of the other species present only Deschamp- (Fig. 4a). There was no increase in cover in the untreated sia flexuosa had any treatment effect, a significant plots, no increase at 0 m in the treated plots but an interaction of Pteridium control and litter disturbance. increasingly fast development of cover was evident at In 1998 it had a cover on litter-disturbed quadrats in both 4 m and 8 m in the sprayed plots. treated plots of 37.1% compared to 25.1% in other Contrary to expectations, there was no response of quadrats. It was also present at higher cover values and Calluna vulgaris to either the addition of seed or litter was increasing faster in the sprayed plots though this disturbance, except that a significant quadratic effect effect was not significant (mean cover P = 0.067, linear change P = 0.056). Pteridium had the typical dynamics expected (Fig. 4c). Its cover remained relatively constant at 4 m and 8 m in the untreated plots, while at 0 m it showed a slow increase in cover through time as expected with its encroachment through space. Where sprayed, cover was reduced considerably in 1994 and a subsequent gradual increase with time at all distances from the front, though recovery was more complete after 5 years further away from the actual front. Pteridium litter had a considerable response to treatment, with a relatively small increase in cover in the untreated stands (Fig. 4d) but a considerable decrease in treated stands (59.2 to 16.0%). This pattern did not vary depending on distance, but the rate was affected by Fig. 3. The effects of treatment on changes in mean Pteridium treatment. Litter disturbance slightly increased the rate biomass (g.m-2) at Levisham Moor from 1993 to 1998 across all distances from the front. ‘F-b’ and ‘Sto’ refer to of Pteridium litter loss (significant interactions between Frondbearing and Storage rhizome tissue respectively. litter disturbance and both treatment and distance) and = no treatment; = asulam; = cut once per year. Different adding seed as cut brash also reduced the contribution of letters indicate significant differences (P £ 0.05). Pteridium litter to the ground cover. 208 PAKEMAN, R.J. ET AL.

Table 3. ANOVA of the slope parameters derived from orthogonal polynomial time contrasts at Ramsley Moor. All single treatment comparisons and selected two-way interactions are shown. Treatment codes: A = asulam application; D = distance from 1993 position of the bracken front (0, 4, or 8 m); L = litter disturbance; S = addition of Calluna vulgaris seed. Only significant differences (P < 0.05) are listed and the effect of treatment or increasing distance from the front within single treatment comparisons is indicated by ‘+’ or ’-’ after the probability; ~ indicates a non-linear effect of distance.

Species/cover type Slope A D A ¥ DL S A ¥ LD ¥ LA ¥ S parameters (1; 3) (2; 12) (2; 12) (1,56) (1,56) (1; 56) (2; 56) (1; 56)

Agrostis capillaris Mean 0.033~ 0.014 0.013 Linear 0.027~ 0.026 Quadratic 0.049~ 0.023 Calluna vulgaris Mean <.001- Linear 0.005+ 0.009 Quadratic 0.025 0.049+ 0.006 Deschampsia flexuosa Mean 0.034 Pteridium aquilinum Mean 0.008- 0.009+ Linear 0.005 Quadratic 0.007+ 0.070+ 0.005 Pteridium litter Mean 0.037-<.001+ Linear 0.006-<.001~ 0.037 <.001+ 0.030 0.027 0.021 Other litter Mean 0.020+ Linear 0.009-<.001- Total moss Mean 0.046 Droppings Mean 0.005+<.001+ 0.002 0.015+ 0.007- 0.035 0.019 0.020 Linear 0.025~ Quadratic 0.006- 0.002- 0.002 0.003- 0.008+ 0.010 0.013 0.022 Total vegetation cover Mean <.001- Linear 0.016+<.001~ 0.015 Vascular plant cover Mean <.001- Linear 0.017+<.001~ 0.014 Ericaceous plants Mean <.001- 0.017 Linear 0.002+ 0.015 Quadratic 0.028+ 0.011 Total grass cover Mean 0.045+ 0.029+ 0.004 Linear 0.042+ 0.049

One further cover type had significant changes in The response of the total vegetation grouping was cover with time. Pteridium control substantially in- very similar to that of the total grass grouping (Table 3), creased the presence of sheep droppings, particularly on except for a larger amplitude of change (Fig. 4f). Where quadrats where the litter had been disturbed but no no control had been carried out there was a gradual loss brashings had been added. The sheep appeared to prefer of cover, whereas where control had been carried out resting in the open areas left after control rather than in there was an overall increase with time. Litter distur- the Calluna or untreated Pteridium. Their increased bance increased the final overall percentage cover on presence may have accounted for the occasional occur- the treated plots. rence in the plots of Cerastium fontanum, Poa annua and Trifolium repens which possibly colonized from seeds present in the sheep droppings. As the presence of some species varied across the experiment, the effect of the treatments was also tested on a number of composite cover classes. Some did not respond to treatment (total moss cover, total forb cover), while others had the same response as their major compo- nent species (total ericaceous cover and Calluna). Total grass cover had a clear response to Pteridium control treatment and to litter disturbance (Fig. 4e). There was little change in grass cover in untreated plots but a gradual increase in cover on treated plots, especially where the litter had been disturbed, such that in these quadrats these species comprised 45% of the ground cover. - THE EFFECTS OF CUTTING AND HERBICIDE TREATMENT ON PTERIDIUM AQUILINUM ENCROACHMENT - 209

Fig. 4. The effects of selected treatment combinations on the major components of the ground cover, (a) Agrostis capillaris; (b) Calluna vulgaris; (c) Pteridium aquilinum; (d) Pteridium litter; (e) total grass cover; (f) total vegetation cover at Ramsley Moor, 1993 to 1998. Treatments denoted by A = asulam; N = no treatment; S = added Calluna vulgaris seed, L = litter disturbance, measured at 0, 4 and 8 m behind the initial starting position of the bracken front. Where only one part of the numbering is presented all of the possible combinations are represented by that label. 210 PAKEMAN, R.J. ET AL.

Discussion Controlling encroachment and reinstating vegetation

Encroachment and loss of vegetation Both methods of control (cutting and asulam application) succeeded in reducing Pteridium encroachment The rate of Pteridium encroachment in the untreated into neighbouring vegetation. In terms of removal of plots at both sites fell within the range of rates measured fronds to allow persistence of other vegetation, both elsewhere within Great Britain (Table 1). The maximal were effective (Fig. 2b). The effects of asulam treatment encroachment in a single year at Levisham Moor (1.4 were more immediate, but after 6 years of treatment, m) also matched the maximal rates seen in other studies. frond biomass on the cut once a year plots was lower The mean rate of encroachment (Fig. 2) was higher at than on the sprayed once plots. However, the long-term Levisham Moor (0.55 m.yr-1) than at Ramsley Moor success of Pteridium control treatments is dependent on (0.36 m.yr-1), which may reflect either the higher alti- depleting the rhizome reserves, cutting appeared to be tude at Ramsley, the more vigorous Calluna there or an more successful than herbicide application over the 6 unmeasured soil water, nutrient supply or management years, though both restricted the increase in rhizome difference. However, it is difficult to attribute reasons biomass observed in the untreated plots between 1993 for this difference, especially as Ramsley Moor had the and 1998 (Fig. 3). higher rate of encroachment from analysis of aerial The asulam application was carried out at both sites, photographs (Table 1). but the response differed between them. At Levisham Without treatment Pteridium always encroached on Moor the sprayed Pteridium front appeared not to be neighbouring Calluna-dominated vegetation, although catching up with the unsprayed front, whereas at Ramsley at Levisham the front was static for 2 years. Continued Moor the encroachment rate of the sprayed front was expansion appears to be the case in all field studies, considerably faster than the unsprayed front after its though at small scales static and sometimes retreating initial setback. However, it is difficult to extrapolate fronts, have been observed (Miller et al. 1989; Watt from rates where it is recovering on ground it already 1955). Under field conditions, it would appear that occupies to encroachment rates into fresh territory. The Pteridium is usually the superior competitor to Calluna. difference may be the result of small differences in Extending the results of mesocosm studies (e.g. Gordon initial treatment success, which can have considerable et al. 1999 where Calluna appears to be a stronger effects on the rate of recovery (Williams 1980). competitor for nitrogen and water) to field situations Management aimed at the restoration of Calluna may not be possible without taking into account the cover within treated areas was not particularly success- structure and light climate of the encroachment zone. ful, though there was some indication that litter distur- The linear encroachment of Pteridium was accompa- bance increased Calluna cover where it had not previ- nied by an increase in frond and rhizome biomass across ously been present (Table 2). However, spraying pre- the three sampled distances behind the front at Levisham vented loss of Calluna cover at the boundary between (Fig. 3). At Ramsley Moor, there was an increase in the two vegetation types. Spraying increased the total Pteridium cover, and hence shading, at the invading cover of the grass species present, especially if com- edge (0 m) of the stand (Fig. 4c). This shows that the bined with litter disturbance. The primary species to process of encroachment consists of both new tissue benefit were Agrostis capillaris and Deschampsia (rhizome and frond) being produced at the invading flexuosa. The ability of these species to persist under the edge and an increase in Pteridium biomass and cover in encroaching front for longer than Calluna allowed them the area immediately behind the edge. Comparison of to expand rapidly, both vegetatively and by seed, when biomass and cover measures perpendicular to the front the Pteridium cover and litter were removed. This may suggest that the zone where Pteridium biomass and not be a desirable outcome where the primary objective cover is below that typical of the centre of a large stand is to restore a heather-dominated vegetation after con- extends from the edge of the stand to between 7 and 14 trolling the Pteridium, and they may prevent the suc- m back into it (Miller et al. 1989; Watt 1942). cessful establishment of introduced Calluna seeds. Also noticeable during the period of study was the gradual decline in Calluna cover at all distances from An effective strategy? the edge of the untreated fronts at Ramsley Moor (Fig. 4b). At 0 m this decline was 68 % to 48 %, the decline at It appears feasible to prevent or delay the encroach- 4 m and 8 m were proportionately higher, 14 % to 7 % ment of Pteridium into neighbouring vegetation using and 5 % to 2 % respectively. The encroachment of the conventional, and not highly intensive, techniques ap- Pteridium front at this site had a measurable effect on plied to a narrow zone along the edge of the stand. The the vegetation present between 1993 and 1998. choice of technique used will depend greatly on the - THE EFFECTS OF CUTTING AND HERBICIDE TREATMENT ON PTERIDIUM AQUILINUM ENCROACHMENT - 211 resources available. However, it should be stressed that Acknowledgements. The authors would like to acknowledge cutting is well known to favour grass species over the continuing funding of the Department of the Environment, ericaceous species (Pakeman & Marrs 1994b), and so is Food and Rural Affairs. Access to the site was by the kind not appropriate where the intention is to restore a Calluna- permission of Sue Rees, previously of the North York Moors National Park, and Helen Buckingham, Peak District National dominated community. Asulam could be replaced by Park. Thanks are due to Alan Frost, Emma Hay and Jim Small the cheaper glyphosate, though the application of this for assistance with the fieldwork. herbicide needs greater care (e.g. weed-wiping) to prevent deleterious effects on other species present (Petrov & Marrs 2000). References Both approaches to control could be improved in their efficacy by following best practice (Pakeman et al. Anon. 1996. Genstat 5 for Windows, Release 3.2. Lawes 2000a). If cutting were carried out twice per year then Agricultural Trust, Rothamstead, UK. the effects, particularly on frond cover, are more imme- Barr, C.J., Bunce, R.G.H., Clarke, R.T., Fuller, R.M., Furse, diate (Lowday & Marrs 1992b), and the faster removal M.T., Gillespie, M.K., Groom, G.B., Hallam, C.J., Hornung, M., Howard, D.C. & Ness, M.J. 1993. Country- of rhizome reserves (Pakeman & Marrs 1994a) may side Survey 1990: Main Report. Countryside 1990 Series: possibly result in the retreat of the front. Best practice Vol. 2, Department of the Environment, London, UK. for asulam application would mean following up the Birch, C.P.D., Vuichard, N. & Werkman, B. 2000. Modelling initial application – by helicopter, tractor or all terrain the effects of patch size on vegetation dynamics: bracken vehicle (ATV) – in following years with hand held (Pteridium aquilinum) under grazing. Ann. Bot. (Lond.) sprayers worked from an ATV and spot guns (Robinson 85 Suppl. B: 63-76. 1995). This approach continues to deplete the popula- Braid, K.W. 1934. Bracken as a colonist. Scot. J. Agric. 17: tion of frond buds such that regeneration is very much 59-70. slower or prevented. However, following best practice Dyer, A.F. 1990. Does bracken spread by spores? In: Thomson, requires a greater investment of resources and hence J.A. & Smith, R.T. (eds.) Bracken biology and management, pp. 35-42. Australian Institute of Agricultural Sci- less area can be treated, but it may be the only option to ence Occasional Publication No. 40, Sydney, AU. completely prevent encroachment. Gordon, C., Woodin, S.J., Alexander, I.J. & Mullins, C.E. The approach of preventing or delaying Pteridium 1999. Effects of increased temperature, drought and nitro- encroachment appears feasible. However, its imple- gen supply on two upland perennials of contrasting func- mentation in favour of an approach based solely on the tional type: Calluna vulgaris and Pteridium aquilinum. treatment of individual areas may well be dependent on New Phytol. 142: 243-258. the characteristics of the sites involved and the aims of Haines-Young, R.H. et al. 2000. Accounting for Nature: As- the land manager. Where Pteridium is invading pro- sessing Habitats in the UK Countryside. 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