the effects of potential bracken and scrub control herbicides on lowland calluna and grass heath...

Biological Conservation 32 (1985) 13-32

The Effects of Potential Bracken and Scrub Control Herbicides on Lowland Calluna and Grass Heath

Communities in East Anglia, UK

R. H. Marrs

NERC Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Cambs PE17 2LS, Great Britain

ABSTRA CT

In order to assess the effects of nine herbicides, which have a potential use for bracken and/or scrub control, on non-target lowland heath vegetation, two screening experiments were carried out at a Calluna vulgaris heath, and a grass heath dominated by Festuca ovina. The aims of the experiments were to select herbicides which caused minimal damage to the heath plant communities, and to monitor recovery from damage should any occur. No significant damage to Calluna was found where asulam,fosamine ammonium or 2,4,5-T were applied," on the grass heath these three herbicides and triclopyr caused no damage. Thus these herbicides are suitable for use for the control of bracken and/or scrub on lowland heaths. It is suggested that damage caused by other herbicides may be beneficial in some situations, for example in promoting new pioneer Calluna, or by increasing numbers of dicotyledons on the grass heath. This is almost certainly caused by gap creation and suppression of competition, but the role of increased rabbit activity in damagedplots on the grass heath may also be important.

Some of the difficulties inherent in assessing risks of herbicide use on nature reserves are noted, and briefly discussed.

I N T R O D U C T I O N

One of the main problems in managing lowland heaths in southern England is that created by succession; unless succession is restricted

13 Biol. Conserv. 0006-3207/85/$03' 30 © Elsevier Applied Science Publisfiers Ltd, England, 1985. Printed in Great Britain

14 R. H. Marrs

by positive management the heaths are invaded by 'weeds' (Harrison 1976; Marrs & Lowday, 1983, 1984), for example birch Betula spp., bracken Pteridium aquilinurn, gorse Ulex europaeus, rhododendron Rhododendron ponticum and Scots pine Pinus sylvestris. In the past succession was prevented on these lowland heaths by a combination of grazing (natural and agricultural), burning and cutting. However, over the last 40 years, and for a variety of reasons, these management practices have often been abandoned, and succession has proceeded unhindered. If these lowland heaths are to be maintained, positive management is essential; first to tackle the 'weed' problem, and second to maintain the lowland heath vegetation (Marrs & Lowday, 1983, 1984).

The 'weed' problem is a particularly difficult one, for the following reasons;

(1) the concept of 'weeds' in nature reserves is one which many conservationists find difficult to accept;

(2) herbicides must be used to control many weed species effectively; (3) as most studies of herbicide use are done in connection with

agriculture or forestry (or by the herbicide manufacturers), there is little information on the use of herbicides on semi-natural vegetation for conservation purposes.

To overcome some of these problems, the effect of a range of nine herbicides which have a potential use for bracken and scrub control has been examined on the vegetation of two lowland heath sites, a Calluna and a grass heath. The herbicides studied included asulam (currently recommended for bracken control), and ammonium sulphamate, picloram and 2,4,5-T (recommended for use against scrub--Fryer & Makepeace, 1978), as well as some more recently developed herbicides including fosamine ammonium, glyphosate, hexazinone, tebuthiuron and triclopyr, which may be suitable for bracken and/or scrub control (Niehuss & Roediger, 1974; Barring, 1978; Bjerregaard et al., 1978; Kossuth et al., 1978; Morton et al., 1978; Bovey et al., 1979, 1981; McLennan & McFadzean, 1979; Pettit, 1979; Evans, 1980; Meyer & Bovey, 1980a, b, c; Richardson, 1980; inter alia). While much is known about the effects of these herbicides on target species, there is very little information regarding their effects on the non-target plant communities likely to be found on lowland heaths. It is, of course, the effects on the non-target vegetation which are the most important consideration, where herbicides are used for conservation purposes.

Effects o f herbicides on Calluna 15

This paper, therefore, concentrates on the effects of these herbicides on non-target lowland heath vegetation, with three main objectives:

(1) to investigate methods for screening herbicides for use on semi- natural vegetation,

(2) to select herbicides which cause least damage (preferably none) to the non-target heath vegetation, and

(3) to investigate the recovery of the vegetation from herbicide damage, should any occur.

Plant nomenclature follows Clapham et al. (1962).

METHODS

Experimental design

A herbicide trial was carried out at two sites, (1) a Calluna heathland (Westleton Heath NNR--gr id reference TM 455694) in the building phase (sensu Gimingham, 1972) and (2) a grass heath in the Brecklands (Cavenham Heath NNR--gr id reference TM 755694). Each experiment tested the performance of nine different herbicides against untreated control plots (Table 1). All herbicides, except tebuthiuron, were applied in water (at 300 litres ha- 1, except ammonium sulphamate, 600 litres ha- 1) using a knapsack sprayer. No wetting agents were used. Tebuthiuron pellets were applied by mixing the pellets in a 100 g sample of soil from the site, and sprinkling this mixture as evenly as possible across the plots. At each site four replicate blocks, each with ten randomly arranged treatment plots of 2 m x 2 m were used. Herbicides were applied on 12 August 1980 at Cavenham Heath, and 30 August 1980 at Westleton Heath; there was no rainfall in the succeeding 24 h.

Monitoring

Each site was surveyed immediately before herbicide treatment (August 1980), two months after treatment (October 1980), three times in the year following treatment (April, July and October 1981) and once two years after treatment (July 1982). Herbicide damage was scored visually on a 1-5 scale (Table 2). Whilst this measure is subject to recorder bias, it is suitable for conservation and amenity purposes because herbicide use

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Effects of herbicides on Calluna

TABLE 2 Scale ~ Used for Assessing the Visual Appearance of Vegetation After

Applying Herbicides

17

Scale State of vegetation Acceptability for conservation purposes

No visible damage Yes Slight discoloration of vegetation Yes Moderate discoloration of vegetation Yes Severe discoloration of vegetation Possible? Severe damage--mostly dead No Apparently completely dead No

a This scale was used in conjunction with outside reference plots, so that, for example, a plot scored 0 could not be distinguished from the reference plots.

may well be determined by the aesthetic impact of the herbicide applications. In addition to these subjective assessments, a quantitative assessment of the effects of the herbicides on the composi t ion and vigour of the heathland plants was also made.

At Westleton Calluna was the commonest species, and investigations were confined to assessing damage to 25 randomly collected shoots of Calluna from each experimental plot. In 1982, the numbers of pioneer Calluna plants established from seed in damaged patches were also counted.

At Cavenham the rooted frequencies of each species in a 50 × 50cm permanent sub-quadrat (divided into 25 l0 x 10cm areas) were determined before treatment, and two months, one year and two years after treatment. In addition, vegetation from a 20 x 20cm randomly positioned sub-quadrat was harvested from each plot at sampling times. At the grass heath site it became apparent in the autumn of 1981 that rabbits were beginning to affect the herbicide treatments differentially, and to assess this effect the numbers of rabbit droppings were counted in each plot between March 1982, and at two-monthly intervals until March 1983.

Samples of vegetation from both sites were separated into live and dead fractions, dried at 80 °C and weighed. The criterion for selection of dead material was the absence of chlorophyll pigment. The percentage of the vegetation sample which was dead was used as an index of herbicide damage.

18 R. H. Marrs

Analysis of data

All quantitative estimates of herbicide damage (except those of the amount of herbage and proportion of dead herbage in grass heaths) are expressed as the difference between the estimate at a given sampling date and the pre-treatment values, and compared to similar changes in the untreated plots. Assessment of significance was done using the Least Significant Difference (LSD) test.

RESULTS

Effects of herbicides on Calluna

Ammonium sulphamate, glyphosate, picloram and triclopyr produced the worst appearance (Table 3). Ammonium sulphamate was the worst treatment initially as most of the Calluna was killed within two months of treatment. Picloram also killed most of the treated Cailuna, but this effect was not noticed until 14 months after spraying (October 1981). Both glyphosate and triclopyr damaged the Calluna; symptoms included a reddening of the shoots, some shoots being killed. However, after two years

TABLE 3 Assessment of the Visual Appearance of Calluna After Herbicide Application

(0, foliage undamaged; 5, Calluna dead.)

Effect Treatment Sampling-date

October April July October July 1980 1981 1981 1981 1982

Untreated 1.0 1.2 0 0 0 Least Asulam 0.2 1.2 0 0.5 0

effect Hexazinone 1.5 1.5 0-7 0-5 0 Fosamine ammonium 1.0 1.5 2.0 1-5 0 2,4,5-T 0.7 1.7 0-5 0.5 0 Tebuthiuron 2.2 0.7 0-7 0.5 0

Moderate Glyphosate 3-0 3.5 3.0 3-5 0.3 damage Triclopyr 3-7 4.2 3.7 3.2 2.3

Severe Picloram 1.7 3-7 4.0 5.0 5.0 damage Ammonium sulphamate 5.0 5.0 5.0 5.0 5.0

Effects of herbicides on Calluna 19

the visual appearance of glyphosate and triclopyr plots had improved, because of the production of new shoots. No signs of major visible damage were found in plots treated with asulam, hexazinone, fosamine ammonium, 2,4,5-T and tebuthiuron during the two years following treatment; in these treatments flowers were produced at the end of both the 1981 and 1982 seasons.

When a quantitative assessment of herbicide effects was made, broadly similar results were found (Table 4). There was a significant increase in the proportion of dead Calluna shoots in plots treated with glyphosate, triclopyr, picloram and ammonium sulphamate within two months of treatment (October 1980). During the spring following treatment (April 1981) Calluna treated with hexazinone and tebuthiuron also showed an increase in dead tissue compared to the untreated plots. However, during the growing season of 1981, plots treated with these two herbicides and glyphosate recovered. At the end of 1981 only triclopyr, picloram and ammonium sulphamate had significantly greater amounts of dead material than untreated plots. Two years after treatment, the triclopyr- treated plots had recovered, leaving only the picloram and ammonium sulphamate plots containing significantly greater amounts of dead Calluna than untreated plots.

TABLE 4 Damage to Calluna Vegetation by Herbicides Applied in August 1980

(The °/o change in dead tissue from pre-treatment levels in each treatment is presented. transformed to angles.)

Effect Treatment Sampling date


No Untreated I l-0 7-3 3.9 1.2 3. l significant Asulam 7.8 2.5 5.6 I. l 3.9 difference Fosamine ammonium 20.7 29. l 7.4 4.4 0.2 from 2,4,5-T 24.5 16.2 9.2 19.9 5.0 untreated Hexazinone 22.3 ~ 9-5 17-0 2.2 plots Tebuthiuron 27.2 I 46.4 ' 15.6 [ 5.5 6.9 Significant Glyphosate 42.8 58.0 27.4 I 13.0 18.0 damage Triclopyr 46.0 53.9 28.2 24.1 I 13-4

Picloram 49.6 50.4 50-4 56.1 51.8 Ammonium sulphamate 78-2 78.2 78-2 78.2 70.5 LSD (p < 0.05) 16-4 24-7 12.7 20-7 17.6

20 R. H. Marrs

The densities of pioneer Calluna plants found in each treatment (Table 5) showed that only in plots which have had major damage, i.e. the four worst herbicide treatments, has there been invasion of new Calluna from seed. Indeed the two worst herbicides, ammonium sulphamate and picloram, had the highest densities of pioneer Calluna. This invasion by new Calluna plants has probably been caused by the creation of gaps by the herbicide treatments.

TABLE 5 Densities of Pioneer Calluna Plants in Herbicide

Treated Plots in 1982 (numbers m - 2)

Herbicide treatment Density

Picloram 7.9 Ammonium sulphamate 6.5 Giyphosate 4-2 Triclopyr 0.4 Other herbicides 0 LSD (p < 0.05) 4-05

It should be noted that the quantitative assessment of the effect of picloram indicates damage within two months, whereas no apparent visual damage was found. This means that there was either a bias in recording the visual damage in spite of care being taken to avoid this, or the quantitative assessment is more sensitive.

Effects of herbicides on grass heaths

Ammonium sulphamate, hexazinone and tebuthiuron produced the worst appearance (Table 6), with most of the vegetation being killed, and the creation of many bare patches within two months. Slight, but acceptable, visible damage was found in all other herbicide treatments (asulam, fosamine ammonium, picloram and triclopyr). In general this minor damage was confined to scorching, and a change in coloration (usually a blue tinge was produced) and there appeared to be no detrimental effect on grass flowering. The appearance of the plots treated with asulam, fosamine ammonium, picloram, 2,4,5-T and triclopyr recovered to pre-treatment levels in the year after spraying (1981), but an

Effects o[ herbicides on Calluna 21

TABLE 6 Assessment of the Visual Appearance of the Grass Heath After Herbicide Application

(0, foliage undamaged; 5, complete death of vegetation.)



Untreated 1.5 0.5 0-7 0.7 0.5

Least Asulam 2.5 0.5 0.7 0.7 0.5 effect 2,4,5-T 1.7 1.0 1.0 0.5 0.7

Picloram 2.0 0.7 0 0 0.2 Triclopyr 2.5 1-0 1.0 0.5 0.2 Fosamine ammonium 3.0 1.7 0.5 0.5 0 Glyphosate 2.2 1.5 2.2 2-2 1-0

Severe Ammonium sulphamate 4-2 3.5 3.2 3-2 1.7 damage Hexazinone 4.5 4.7 3.7 3.2 2.5

Tebuthiuron 4.7 4.7 5.0 5.0 5-0

adverse appearance persisted in glyphosate (slight effect), and ammonium sulphamate, hexazinone- and tebuthiuron-treated plots (severe effect) during 1981. Two years after spraying the appearance in these damaged plots improved, except for the tebuthiuron plots where the visual appearance remained poor throughout.

Several other changes were noted from the quantitative assessment of vegetation damage.

First, several herbicides affected the diversity of dicotyledons; however, no significant effects on monocotyledons were found. Two months after treatment a significant reduction in dicotyledons (1.5-2.5 species per treatment) was found in plots treated with ammonium sulphamate, hexazinone and picloram. Indeed picloram almost completely eliminated dicotyledons, and this effect persisted for at least two years. In contrast, the initial reduction in dicotyledons in ammonium sulphamate and hexazinone treatments was reversed in the year after spraying, with a mean of 2.5 and 3.5 species colonising these treatments respectively. For example, in the four replicate plots, seven new species had colonised the ammonium sulphamate treatment and five the hexazinone treatment (Table 7). In the second year after treatment a further seven and eight new species were found in these treatments respectively (Table 7).

22 R. H. Marrs

TABLE 7 New Species which Colonised the Ammonium Sulphamate and Hexazinone

Treatment One and Two Years After Treatment

Ammonium sulphamate Hexazinone

One year after treatment ( 1981)

Two years after treatment (1982)

Arrhenatherum elatius Bromus mollis A renaria serpyllifolia Myosotis ramosissima Potentilla reptans Stellaria media Veronica arvensis

Holcus lanatus Centaurium erythraea Cerastium fontanum Ornithopus perpusillus Potentilla reptans Senecio jacobea Trifolium repens

Arenaria serpyllifol& Centaurium erythraea Cerastium holosteoides Filago minima Geranium molle

A rrhenatherum elatius A ira praecox Bromus mollis Holcus lanatus Luzula campestris Myosotis ramosissima Ornithopus perpusillus Stellaria media

TABLE 8 Damage to Grass Heath Vegetation by Herbicides Applied in August 1980

(The % of dead material in each treatment, transformed to angles, is presented.)



No Untreated 54.1 59.9 significant Asulam 63.2 58.1 difference Fosamine ammonium 60.3 56.5 from 2,4,5-T 57.6 63.8 untreated Triclopyr 62.6 64.5 plots Picloram 62-8 61.1

Glyphosate 67-8 66.8 Significant Hexazinone 77-0 74.4 damage Ammonium sulphamate 71.3 78.2

Tebuthiuron 72-4 78.3 LSD (p < 0-05) 10-6 13.9

54'6 50-9 45"9 56"4 52'5 54-0 53"1 53"9 47-6 53"9 51'0 51"3 49"4 54' 1 52-4 55' 1 53.0 52"8 51.1 53.3 43"1 59' 1 59.1 49"5 54.2 58'3 56-8 78'4 85.5 I 69"5 11.6 9.6 24-5


Second, there has been an increase in the amount of dead material in the sward after application of several herbicides (Table 8). A significant increase in dead material was found initially where ammonium sulphamate, glyphosate, hexazinone and tebuthiuron were applied, but after one year all treatments, except tebuthiuron, had recovered. Greater amounts of dead material were found in tebuthiuron plots two years after treatment, but this difference fell just short of significance, indicating that recovery was occurring.

Third, there has been a selective change in the balance of species in the grass heath (Table 9). Festuca was reduced drastically by ammonium sulphamate, hexazinone and tebuthiuron in the year after treatment, but after two years only tebuthiuron showed a significant reduction. Poa

TABLE 9 Significant Changes in Rooted Frequency of Four Common Species After Herbicide

Application to Grass Heath Vegetation (Only herbicide treatments that differ significantly from untreated plots are shown)

Species Treatment Reduction in mean rooted frequency after herbicide treatment

( '~ transformed to angles)

2 months 1 year 2 years a[R'r after after

Festuca spp.

P o a pratensis

Rumex acetosella

Crepis capillaris

Untreated 2.9 Ammonium sulphamate 48.0 Hexazinone 62.8 Tebuthiuron 60.5 LSD (p < 0.05) 24-1

Untreated 0 Ammonium sulphamate 29.0 Hexazinone 47.9 LSD (p < 0.05) 30.7

Untreated 1.8 Glyphosate 40.2 Tebuthiuron 38.4 LSD (p < 0.05) 37-1

Untreated 2.9 Ammonium sulphamate 29.6 LSD (p < 0-05) 26-8

0 0 43.8 14.4 37.9 0 57.2 41.8 27.6 27.8

No significant differences

2.2 No 20'2 significant 42.4 differences 36-0

No significant differences

24 R . H . M a r r s

pratensis initially was reduced significantly by ammonium sulphamate and hexazinone, but this effect did not persist. Rumex acetosella was reduced in plots treated with glyphosate and tebuthiuron but recovered after two years, and Crepis capillaris, although reduced after two months where ammonium sulphamate was applied, recovered after one year. Other species have also been affected by these herbicide treatments (Table 10). However, these species were not present in all plots, and where they did occur it was often at low frequencies; this means that it is not possible to make an adequate assessment of the significance of the herbicide effects on these species.

TABLE I0 Effects of Herbicide Treatments on Species Present at Low Frequency Before Treatment

in the Year After Treatment (+ , increase; 0, no effect; - , decrease.)

Species

H o l c u s

l a n a t u s 0 + - + + + + - 0 +

L u z u l a

c a m p e s t r i s + - 0 0 + + 0 + + +

H i e r a c i u r n

p i l o s e l l a + - + - + + - - 0 -

O r n i t h o p u s

p e r p u s i l l u s + 0 - + . . . . 0 0

S e n e c i o

. j a c o b a e a 0 + 0 + + - 0 0 - +

Fourth, there was an increased rabbit activity, measured by numbers of droppings, on plots with most severe vegetation damage, i.e. in ammonium sulphamate, glyphosate, hexazinone and especially tebuthiuron treatments (Table 11), compared to treatments where there was little increase in dead material; asulam, fosamine ammonium, picloram, 2,4,5-T and triclopyr. This increase was not noticed until the end of 1982, but it is clear that recovery from damage in the affected treatments may be complicated by this increased rabbit activity.


TABLE 11 Rabbit Droppings as an Index of Rabbit Activity on Herbicide Treated Plots

(Mean numbers of droppings (numbers m - 2) found in March 1982, and the total amount found up to March 1983 in each treatment are shown; all data have been transformed to square root prior to analysis of variance and detransforms are shown in parentheses.)

Herbicide March Pooled March Pooled

treatment 1982 means 1982-83 means ~

Group 1 Picloram 0 1.87 Untreated 1.37 2- 59 Asulam 1.37 1.44 3.65 4.06 (16-5) Triclopyr 1-37 (2.1) 4.40 Fosamine ammonium 2.00 6.00 2,4,5-T 2.51 5.90

Group 2 Glyphosate 4.67 7.45 Ammonium sulphamate 5.03 6.53 8.59 8.17 (66.7) Hexazinone 7.61 (42.6) 8.49 Tebuthiuron 8.83 19-70 19.70 (388)

LSD (p <0.05) 4.15 6.95

a Values shown in the pooled means column are significantly different (p < 0.05).

Nevertheless, four herbicides, asulam, fosamine ammonium, 2,4,5-T and triclopyr were shown to cause little damage to the grass heath vegetation on all four counts.

DISCUSSION

One of the main uses of herbicides for conservation and amenity purposes is to control weeds invading plagioclimax vegetation. Lowland heaths are one example of an ecosystem where ingress of weeds, bracken and/or birch scrub is a problem, and urgent action to control these weed species is required (Harrison, 1976; Marrs & Lowday, 1983, 1984). However, before herbicides (or indeed any other management treatments) are used to manage these habitats, an assessment of their impact on the ecosystem to be treated should be made. The experiments reported in this paper were a preliminary attempt to assess methods for screening herbicides for use in semi-natural vegetation and the impacts of a range of herbicides that may have a potential use for the control of bracken or birch on the non- target vegetation in two lowland heaths, a Calluna and a grass heath.

26 R. H. Marrs

Methods for screening herbicides in semi-natural vegetation

The results from this study have demonstrated the need for a detailed quantitative approach to the assessment of herbicide damage. Although the results from the visual assessment show a broad agreement with the quantitative methods, there were some treatments where damaging effects could have been missed if only a cursory visual inspection was made. For example, damage to Calluna by picloram was not apparent visually after two months, yet there had already been a significant increase in dead Calluna tissue; moreover, in the grass heath treated with picloram no adverse visual damage was found but all, or almost all, of the dicotyledons had been eliminated. This loss of dicotyledons may have gone unnoticed without a detailed inspection of grass tussocks. It is therefore essential to use a quantitative method where possible. Indeed, with hindsight, it is obvious that the methods used in the grass heath experiment were not adequate to separate the effects of herbicides on species of low abundance from background fluctuations in their numbers. Asulam, for example, appeared to depress the abundance of Holcus lanatus, which was present at very low levels (Table 10), and this herbicide has been reported as effective for the selective control of Holcus in pastures (Kirkham et al., 1982). Therefore, more detailed studies are required in grasslands, larger plots should be used, and more detailed methods of assessing species abundance should be considered.

A further problem is that, in these two experiments, the herbicide treatments were applied at one application rate, at one time of year, and in one season; variation in all of these could influence the result. However, the screening trial is an artificial situation, as the damage to the non-target vegetation has been deliberately overestimated, because the application rates used were higher than would normally be achieved in practice. This is because there was no interception of the herbicide by the target species.

Possibly the most serious criticism of the approach used here is that only the structure of the vegetation (in terms of species composition and amounts of dead material) were used as indices of herbicide damage. No account has been taken of the possible damaging effects on the functioning of the ecosystem. Whilst it is reasonable to suppose that the structure of the vegetation would reflect damaging effects on ecosystem processes, it is possible that it does not. Thus for a full risk assessment, the effects of the herbicides on photosynthesis, transfer of energy, toxicity to the fauna, and decomposition should be studied under field conditions.


Some evidence is already available on toxicology and decomposition in soil (Table 12). Most of the herbicides studied here have a fairly low toxicity to wildlife, and with the exception of picloram and tebuthiuron have a short half-life in the soil. There should, therefore, be no build up of toxic residues in the ecosystem, especially as the herbicide should only need to be used at infrequent intervals.

In spite of these criticisms these two experiments provide a useful initial selection of herbicides suitable for general use for the control of bracken and scrub on lowland heaths for conservation purposes. Certainly the methods used are better than the trial and error approaches used hitherto, although a full analysis of herbicide effects on all ecosystem processes would be preferred to improve the precision of the assessment of risks of herbicide use. Extension of this type of work to other habitats would also be useful.

TABLE 12 The Mammalian Toxicity and Persistence in Soil of Herbicides used for Bracken and

Scrub Control (Data from McEwen & Stephenson, 1979: Mullison et al., 1979)

(a) Mammalian toxicity (rates acute oral LDs0 - mg kg- 1)

Herbicides Fosamine

Least ammonium (product) toxic Picloram

Asulam Glyphosate (product) Ammonium sulphamate Triclopyr (product) 2,4,5-T

Most toxic

(b) Persistence in soil

Least Fosamine ammonium persistent Asulam

2,4,5-T Triclopyr Glyphosate Hexazinone

Most Tebuthiuron persistent Picloram

24 400 8 200

> 8 000 4 900 3 900 2 485

300

Other substances

Epsom salts > 3 000

DDT 113-118 Strychnine 1-30

½ life 6-10 days ½ life 6-14 days

persists 7-28 days ½ life 46 days ½ life < 60 days ½ life 4 5 months ½ life 12-15 months persists up to 24 months

28 R. H. Marrs

Selection .of herbicides for use on lowland heaths

Calluna heaths Of the nine herbicides tested no significant increase in dead Calluna tissue was found with asulam, 2,4,5-T and fosamine ammonium. All other herbicides produced a significant increase in dead tissue, but after one growing season all treatments had recovered except plots treated with ammonium sulphamate, picloram and triclopyr; the triclopyr plots had recovered after two years. In this experiment the ester formulation of triclopyr was applied at a rate slightly higher than that recommended (2-9 kg a.i. ha- 1 as opposed to 0.9-1.9 kg a.i. ha- ~ recommended), but the manufacturers note that at 1-44 kg a.i. ha- ~ Betula pubescens is only moderately susceptible, so higher rates than recommended may be needed in practice. In view of the damage to Calluna by application of the triclopyr ester formulation at 2-9 kg a.i. ha- ~, and reports of a complete kill of Calluna with 4 kg a.i. ha- 1 of the salt formulation, but no damage at lower rates (1 kg a.i. ha- ~ salt formulation) (Turner et al., 1980), there may be scope for further investigation of the response of Calluna to a range of low application rates of triclopyr.

Grass heaths Asulam, fosamine ammonium, 2,4,5-T and triclopyr had least effect on grass heath ~¢egetation. All other herbicides caused some damage to the plant community, at least initially, for example:

(1) ammonium sulphamate, hexazinone and tebuthiuron caused most visual damage;

(2) ammonium sulphamate, hexazinone and picloram reduced numbers of dicotyledons, although plots treated with ammonium sulphamate and hexazinone recovered, and indeed showed an increase in species numbers in the two years after spraying;

(3) plots treated with ammonium sulphamate, glyphosate, hexazinone and tebuthiuron showed a significant increase in dead vegetation, although all except tebuthiuron-treated plots recovered after one year;

(4) ammonium sulphamate, glyphosate, hexazinone and tebuthiuron reduced some of the main species selectively.

Recommendations

As asulam, fosamine ammonium and 2,4,5-T caused no significant damage to Calluna or grass heath, and triclopyr did not damage grass

E[lk, cts o[herbicides on Calluna 29

heaths, these selective herbicides should be suitable for use by foliar spraying in lowland heath habitats. Asulam is effective against bracken, and fosamine ammonium, 2,4,5-T and triclopyr are suitable for scrub control. In view of the problems currently associated with the use of 2,4,5-T, both fosamine ammonium and triclopyr appear to be suitable alternatives.

It is, however, also possible to achieve selectivity between target and non-target plants by a suitable choice of application method. Herbicides can be applied directly to target plants, where there is no herbicide contact with the non-target vegetation, for example using a paint brush to cut stumps, a rope-wick applicator, crop roguing glove or modified chain saw (Peevy, 1972a, b; Jones & Morgan, 1978; Kossuth et al. , 1978; Evans, 1980; Christensen, 1984). These direct methods, if used properly, should reduce the risk to the non-target ground vegetation almost to zero. Thus, herbicides initially rejected in the screening trials may be used for conservation purposes, if they can be applied by a direct application method.

Recovery of heath vegetation after damage

Although the main object of this study was to select herbicides which do no damage to lowland heath vegetation, the processes of vegetation recovery after herbicide damage was also interesting from a conservation viewpoint. Calluna, for example, was completely killed by ammonium sulphamate and picloram, and some small gaps were created where glyphosate and picloram were applied. In all of these treatments the vegetation recovered by the invasion of pioneer Cal luna from seed. At the grass heath two herbicides, hexazinone and tebuthiuron, increased numbers of dicotyledons in the two years after treatment, presumably because of gap creation, and suppression of competition (Grime, 1973, 1979; Grubb, 1977; Harper, 1977; Bakker et al., 1980). However, recovery on the grass heath was complicated by the increased rabbit grazing activity in these treatments. It is possible that the ingress of new species is because of the interaction between the initial disturbance by herbicide action followed up by an increased grazing pressure, and not because of herbicide treatment alone.

Although it is generally undesirable to damage vegetation on nature reserves, there may be some situations where herbicide damage would be beneficial. Herbicides could be used, for example: (1) to encourage cyclic regeneration on heathlands where burning, or even cutting, was not

30 R. H. Marrs

possible; (2) to encourage small areas of diverse vegetation in grasslands; (3) to create rabbit latrines for demonstration purposes. It is envisaged that this type of controversial work would be done on a very small scale (a few m 2 at most).

Before embarking on such a programme, it is strongly advised that experimental trials should be done first, to assess likely effects, and to gain experience and confidence in herbicide practice.

A C K N O W L E D G E M E N T S

This work was funded in part by the Nature Conservancy Council as part of its programme of research into nature conservation. I would also like to thank the NCC East Anglian Region for permission to work at Cavenham and Westleton Heaths, the Chipman Chemical Company, Dow Chemical Company, Du Pont (UK) Ltd, and Elanco Products Ltd for the provision of herbicides, J. D. Cheesman and G. B. Nevison for assistance in the field and laboratory, J. O. Mountford for confirming species identification, and M. D. Hooper, F. T. Last and J. N. R. Jeffers, who made valuable comments on the manuscript.

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32 R. H. Marrs

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the effects of potential bracken and scrub control herbicides on lowland calluna and grass heath...

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