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Effects of Cutting on the Growth ofCrataegus monogyna(Hawthorn) in Hedges
jem p107
14-11-95 10:25:29
Journal of Environmental Management (1995) 45, 395–410
Effects of Cutting on the Growth of Crataegus monogyna (Hawthorn) in Hedges N. R. Bannister∗ and T. A. Watt Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, U.K. Received 9 February 1995
Effects of the position and timing of cutting on shoot growth of young Crataegus monogyna plants in newly-planted hedges were studied. Total shoot length was unaffected by cutting. In general, a combination of both vertical and horizontal cutting produced a tall hedge with long, bud-tipped shoots. The timing of cut was important: a horizontal cut in summer resulted in fewer but longer shoots whereas a vertical cut in summer produced more thorn-tipped shoots. A vertical cut in winter resulted in longer shoots than one in summer and reduced the number which were thorn-tipped. Management types of farm hedges could be characterised by various growth parameters. Hand-cut hedges had numerous short shoots and many older-wood branches per unit area, whereas unmanaged ones had a greater leaf area and longer shoots. Summerflailed hedgerows were characterised by a smaller leaf area although this may have been partly due to Galium aparine L. infestation. The use of the flail on the current season’s growth did not significantly retard growth the following year. 1995 Academic Press Limited Keywords: hedgerow, management, pruning, flailing, growth.
1. Introduction 1.1. Crataegus monogyna Jacq. (hawthorn) occurs in scrub and woods up to 550 m altitude and is the commonest shrub on most soils throughout the British Isles (Clapham et al., 1981) as well as being widely planted in hedgerows. Many hedges are cut both along their sides and top. However, sometimes a hedge may be cut only along the top to develop a bush habit or only along its sides to encourage upward extension. The traditional time to cut hedges is from July onwards between hay and corn harvests and in early autumn when field work is slack and hedges are accessible (Beddall, 1950). Even with the advent of mechanised hedge trimmers and new crops such as autumnsown oilseed rape, hedges are still usually cut at the end of harvest. However, current
∗ Present address: Brenoweth, Grampound, Truro, Cornwall. TR2 4RB, U.K. 395 0301–4797/95/040395+16 $12.00/0
1995 Academic Press Limited
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advice to farmers is to trim hedges in January or February to minimise disturbance of wildlife (Farming and Wildlife Trust 1983; Agricultural Training Board, 1990). The effects of different methods of cutting hedges on hedge growth have been reviewed (Maclean, 1992) but current information is derived from experience and observational studies rather than from experimental evidence. Here we investigate the effect of the season and position of cutting on the first two years’ growth of a newlyplanted hedge. 1.2. In conventional arable farming systems a Crataegus monogyna hedge may be hardpruned annually and exposed both to agrochemicals and root disturbance. The tolerance of C. monogyna to disturbance and its hardiness and ability to recover from damage has made it the most successful and favoured hedging plant in England (Malden, 1899). Nevertheless, there has been concern about how well it can survive modern farming practices. In Huntingdonshire, many hedges, already of poor quality in 1974, had disappeared by 1984 (Countryside Commission, 1984). This disappearance was attributed to “intensive arable cultivations, continual hard trimming, plus possible occasional herbicide treatment and burning”. It was also found that “lack of maintenance led to a general decline in hedge quality” and that C. monogyna was not suited to mechanical cutting in some (unspecified) conditions. Some hedges on stock farms were in a “bad condition” due to lack of management and stock damage, whereas on some intensive, efficient farms with institutional owners, the hedges were being actively managed in a “thoughtful and caring way”. More recent surveys of British field boundaries have shown that 21% of boundaries classified as hedgerows in 1984 were classified as a different type of boundary (for example lines of trees or shrubs or relict hedgerows) when re-surveyed in 1990; suggesting less active management had taken place during the 1980s (Barr et al., 1991). The aim of this study was to determine whether different methods of hedge cutting could be distinguished in terms of their effects on hedge growth, by making observations both on young experimental hedges and selected established farm hedges over a three year period. 2. Materials and methods 2.1. The experimental site was an area of pasture on loam soil (pH 7·8) on the Wye College Estate which had been taken out of arable five years previously. The C. monogyna plants were grown from seed collected in south-east England. At the nursery, after one year’s growth, the seedlings had their leading shoot removed to encourage them to develop a bushy habit; at the start of the experiment, most had more than one dominant shoot. In January, the two year-old plants between 45 and 50 cm in height were planted in groups of five (three at the front and two in alternating positions at the back of the hedge) per plot, with 25 cm between each plant. Of the five plants in each plot, only the central front one was measured as it typified a plant cut on one side within a hedgerow. The four guard plants were cut on both sides as each plot was cut as a whole. The plots were laid out 0·5 m apart in two parallel rows 1 m apart in each of two blocks.
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Four types of cutting simulated the ways in which farmers cut hedges and they formed a 2×2 factorial structure with two positions and two seasons of cutting: (i) Control. No cutting. (ii) Vertical cut. Lateral shoots were pruned back to 10 cm from the main stem using secateurs. (iii) Horizontal cut. All leading shoots over 70 cm above the ground were cut back to that height. (iv) Both horizontal and vertical cuts. In the two years following planting, the above treatments were imposed either in summer (middle of August) or in winter (February) giving a 23 factorial structure for the eight treatments which were each replicated three times within each block. Figure 1 shows the development of the plants during the period of the study and the timing of the data collection. After a preliminary season of measurements (August 1985 and February 1986), the lengths and type (whether bud- or thorn-tipped) of all shoots of the current year’s growth and the main stem diameter at 10 cm above the ground were recorded in May in both 1986 and 1987. The height of each central plant was measured in May 1986 and in both May and August 1987, and the shoot dry weight of all the material removed was recorded in February and August 1987. The data were analysed using factorial analysis of variance in Genstat 5 (Genstat 5 Committee, 1988). Main stem diameter at the time of planting was used as a covariate in the analysis, to take account of any inter-plant differences in vigour that already existed at the time of planting. 2.2. 2.2.1. Sites The National Farmers’ Union Representatives for Kent and Leicestershire recommended farmers who would be willing to participate in the research project. Twelve hedges were chosen to cover a range of management types using the following guidelines: (i) (ii) (iii) (iv) (v)
C. monogyna was the dominant shrub species. Active management was being carried out either annually or biennially. The hedges were internal field boundaries. The length of the hedge was at least 100 m. Some were adjacent to pasture and others to arable land and, where possible, the same land use was on each side of the hedge.
Not all the chosen sites fulfilled all criteria. However, all hedges selected were hawthorn-dominant and (except for one deliberately uncut hedge) were being actively managed. All but three of the hedge sites had an annual trim and of these all except Shangton were cut by machine (Table 1). In 1987, however, because of farm staff changes, this hedge had to be cut with a flail and, in the preceding year, the hand cutting was not carried out as closely as usual. 2.2.2. Sampling Five randomly positioned transects were established in summer at right-angles across each hedge and marked with 50 cm white fibre glass rods. As hedges at Shangton and
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(a)
(b)
v
(c)
v h
70 cm
h
v10 cm 10 cm v
(d)
(e)
Figure 1. The development of C. monogyna plants during the experiment. Cutting Treatments: V=vertical trim; H=horizontal trim; VH=combined trim; (. . . .)=cutting line; (––––)=older wood of original plant; (– – –)=new shoots grown that season. (a) January 1985 and April 1985; (b) August 1985 and February 1986; (c) May 1986; (d) August 1986 and February 1987; (e) May 1987.
Cestersover were only just over 50 cm in length, only three transects were used at these sites. Lateral growth and production were measured from 0·25 m2 square quadrats held vertically at chest height, one on either side of each transect. Thus, the position of the quadrat in relation to the top of the hedge varied depending on the height of the hedge. Each year, the quadrat was moved along the hedge so that the samples were not affected by the previous sampling procedure. The number of secondary branches coming from the main stem out to the edge of
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T 1. Hedge management at each site 1985 Site Whitehouse Scords A Scords P Debden Althorp Adams Cestersover Bowden Shangton Paddocks Wye I Wye II
1986
1987
Shape
Type
Time
Type
Time
Type
Time
Box A A Box Box Chamfered Box Chamfered Chamfered A A A
Flail Flail Flail Flail Flail Flail Flail Flail Hand Flail None Flail
Sep 84 Aug 84 Feb 85 Nov 84 Jul 84 Nov 84 Sep 84 Jan 86 Sep 84 Nov 84 na Sep 84
Flail Flail Flail Flail Flail Flail Flail Flail Hand None None None
Sep 85 Aug 85 Feb 86 Dec 85 Aug 85 Aug 85 Oct 85 Sep 85 Sep 85 na na na
Flail — Flail — — — — — Flail Flail None None
Sep 86 — Feb 87 — — — — — Sep 86 Dec 86 na na
na=not applicable.
the hedge was counted within each quadrat. Only branches which were at least half within the quadrat were counted. Three of these branches were chosen at random, pruned from the point where they grew out from the main vertical stem and placed in polythene bags for measurement. In the laboratory, the branches were split into their component parts: new shoots (the current season’s shoot growth), older wood (the previous season’s woody growth), and their respective leaves. Leaf area was measured, using an area measurement system (Delta-T Devices Ltd, Cambridge, U.K.). The length of each branch was measured and the type of terminal bud (thorn, bud or pruned) was noted. In addition to the above growth sample, a separate sample of new shoots, the biomass sample, was made to estimate dry matter production. Twelve new shoots were chosen randomly from within an adjacent 0·25 m2. They were sealed in polythene bags and kept in a cold store at 3°C, until measured in the laboratory. Their length and the type of terminal bud was recorded. The total dry matter of all the shoots was obtained by weighing after oven drying at 80°C for 36 hours. 2.2.3. Multivariate analysis The values from the three (or five) quadrats on either side of the hedge on each year were averaged for each variable. The results for all years from both sides of the hedges were included and both the full set and subsets of discriminant variables were analysed. Potential discriminant variables were: orientation, presence of a ditch, number and length of bud-tipped and thorn-tipped new shoots and older wood, mean leaf area of new shoots and older wood, number of branches and new shoot biomass per 10 cm shoot length. In total, there were 60 samples from the 12 hedges. Four analyses on the range of variables were carried out: A: All management types, hedge features and sites; B: As A, excluding handcut and topcut hedges; C: As B, excluding numbers of new shoots and older wood twigs; D: As B, excluding lengths of new shoots and older wood twigs.
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T 2. Allocation of group codes for the management types Code 1 2 3 4 (none) 5 6 7
Management Flailed Flailed Flailed Not flailed
Time Summer Autumn Winter Summer
Hand cut Top cut Unmanaged
Autumn Winter na
na=not applicable.
Management was defined as “flailed” or “not flailed” and also classified by time of operation: summer (July or August), autumn (September) or winter (October–February). Together with an “uncut” category, this gives seven groups. Within the “not flailed” group (4–6) were those hedges which were either hand-cut or top-cut (Table 2). The data were examined using the SAS Canonical Discriminant Analysis procedure, CANDISC (Klecka, 1980; SAS Institute Inc., 1985). Examination of the raw data showed that there was considerable variation in the orders of magnitude between the different variables. Therefore, the data were transformed to logarithms so that the effect of outliers was reduced. All means are presented per 0·25 m2, back-transformed from logarithms. 3. Results 3.1. 3.1.1. Total shoot length The data for the mean length and number of all shoot types were multiplied together to give an estimate of the total length of shoots produced (data not presented). The position and timing of cutting had no significant effect on the total length of shoots produced, but they did affect its components: mean shoot length and the number of shoots per plant. 3.1.2. Mean shoot length The main effect of cutting position was significant in both May 1986 and May 1987 (P=0·028, P=0·004, respectively, Table 3a). In 1986, this effect was largely accounted for by the interaction between vertical and horizontal cutting. Vertical cutting increased shoot length when the plants were also cut horizontally, but decreased it when the plants were not cut horizontally (Table 3b(i), P=0·025). When the data for bud- and thorn-tipped shoots were analysed separately, the bud-tipped shoots were found to be responsible for this effect (Table 3b(ii), P=0·037). Analysis of all shoot types in 1987 showed that, in general, a horizontal cut resulted in longer shoots (14·3 cm vs. 11·3 cm, P=0·001) and, again, bud-tipped shoots were largely responsible for this (18·3 cm vs. 16·0 cm, P=0·037). The effect of timing of cut was not significant for all shoot types. However, the
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T 3(a). Mean shoot length of current year’s growth per plant (cm) (i) May 1986 All Cut Control Vertical Horizontal Both SED
Bud
Thorn
Winter
Summer
Winter
Summer
Winter
Summer
7·8 7·8 4·4 10·7
9·1 7·5 8·4 9·0
9·0 9·3 5·3 11·6
10·1 8·7 9·4 10·4
3·4 5·3 2·7 2.7
4·4 4·3 3·5 3·4
1·82
1·94
1·66
All
Bud
Thorn
(ii) May 1987
Cut Control Vertical Horizontal Both SED
Winter
Summer
Winter
Summer
Winter
Summer
10·7 11·5 11·9 15·8
12·2 10·9 16·3 13·4
16·8 16·3 15·1 18·2
15·5 15·4 20·5 19·4
5·6 7·8 7·2 7·2
8·0 6·3 7·1 7·2
1·74
2·12
1·40
interaction of the vertical cut with timing was nearly significant in 1986 when the mean length of all shoot types after vertical cutting in winter was longer than when not cut vertically, whereas, in contrast, a vertical cut in summer did not affect mean shoot length (P=0·053, Table 3b(iii)). In 1987, when the plants were cut vertically in winter, there was a slight increase in the mean length of all shoot types, compared to when plants were not cut vertically. However, when the plants were cut vertically in summer, there was a decrease in the mean shoot length (P=0·016, Table 3b(iv)). When the mean lengths of the two types of shoot were analysed separately, different effects were found. In 1987, the interaction of horizontal cut with timing was significant for bud-tipped shoots (P=0·043, Table 3b(v)). Horizontal cutting in summer greatly increased the length of bud-tipped shoots, whereas such cutting in winter was ineffective. 3.1.3. Mean number of shoots per plant The effect of position of cutting was not significant for all shoots in either year of the experiment (Table 4a). However, in 1987, the main effect of horizontal cutting was to reduce the mean number of thorn-tipped shoots from 22 to 14 (P=0·037). In contrast, the number of bud-tipped shoots was not significantly affected by either position or timing of cut. However, the interaction of vertical cut with timing (P= 0·045) was significant for all types of shoot numbers and this was mainly due to thorntipped shoots. A vertical cut carried out in winter reduced thorn-tipped shoot numbers. However, when the vertical cut was carried out in summer, thorn-tipped shoot numbers in the following year increased (Table 4b, P=0·007).
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T 3(b). Interactions between the two positions of cutting and between position and time of cutting on the mean shoot length per plant of the current year’s growth (cm)
(i) All shoots May 1986 Horizontal − Horizontal + SED (ii) Bud shoots May 1986 Horizontal − Horizontal + SED (iii) All shoots May 1986 Winter Summer SED (iv) All shoots May 1987 Winter Summer SED (v) Bud shoots May 1987 Winter Summer SED
Vertical −
Vertical +
8·4 6·4
7·6 9·8
Vertical −
1·15
9·5 7·3 Vertical −
9·0 11·0 1·22
6·1 8·3 Vertical −
Vertical +
Vertical + 9·2 8·3
1·15
11·3 14·2
Vertical + 13·6 12·2
1·10 Horizontal −
Horizontal +
16·5 15·5
16·6 20·0 1·34
3.1.4. Stem diameter There were no significant treatment effects. In May 1986, the mean stem diameter was 11·0 mm (SD=0·86), while by May 1987, it had increased to 14·2 mm (SD=1·69). 3.1.5. Plant height There was no significant effect of season of cutting on plant height (Table 5a), but horizontal cutting had a highly significant effect on height in both years (May 1986, 84·3 cm vs. 70·7 cm and May 1987, 112·5 cm vs. 97 cm, both P<0·001). However, by August 1987, the effect of horizontal cutting was no longer significant. The interaction between vertical and horizontal cutting was just significant in May 1986. The absence of a horizontal cut alone increased height as expected, but, in the presence of a vertical cut, this increase was greater (P=0·049, Table 5b). 3.1.6. Dry weight of material removed When the hedge was cut in winter, only small amounts of material were removed with combined horizontal and vertical cutting, although more than either type of cutting removed separately. However, when cutting took place in summer, much more material
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T 4(a). Mean number of shoots per plant (i) May 1986 All Cut Control Vertical Horizontal Both SED
Bud
Thorn
Winter
Summer
Winter
Summer
Winter
Summer
12·7 17·0 10·6 15·8
17·7 24·3 12·5 22·8
8·7 7·4 6·5 10·7
12·5 16·6 8·0 14·2
5·2 9·6 4·1 5·2
5·2 7·5 4·5 8·8
8·17
4·45
4·99
All
Bud
Thorn
(ii) May 1987
Cut Control Vertical Horizontal Both SED
Winter
Summer
Winter
Summer
Winter
Summer
49·6 26·8 28·9 30·2
42·5 55·9 31·2 47·5
24·8 17·2 16·8 23·8
20·8 25·2 19·2 22·5
30·2 15·0 13·7 15·8
16·1 29·0 11·8 15·5
12·37
6·00
7.74
T 4(b). Interaction between vertical cutting and time of cut on the mean number of thorn-tipped shoots per plant in May 1987
Winter Summer SED
Vertical −
Vertical +
20 15
10 27 4·9
T 5(a). Mean plant height (cm) May 1986 Cut Control Vertical Horizontal Both SED
May 1987
August 1987
Winter
Summer
Winter
Summer
Winter
Summer
76·1 85·8 73·7 75·2
83·2 92·3 70·2 63·8
103·8 111·1 91·8 99·8
112·2 122·8 100·2 96·2
133·9 143·6 115·4 134·0
141·2 163·6 147·2 138·7
5·83
8·40
13·29
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T 5(b). Interaction between the two types of cutting on mean plant height (cm) in May 1986
Horizontal − Horizontal + SED
Vertical −
Vertical +
79·6 71·9
89·0 69·5 3·69
T 6. Dry weight (g) of shoot material removed in February and August 1987 February
Dry weight Log (x+1)
August
Horizontal
Vertical
Both
Horizontal
Vertical
Both
0·90 0·57
0·73 0·49
4·38 1·48
41·23 3·68
17·50 2·76
33·12 3·32
SE mean (log transformation)=0·408.
T 7. Summary of results of discriminant analyses. Run A: All management types, hedge features and sites. Run B: As A, excluding handcut and topcut hedges. Run C: As B, excluding numbers of new shoots and older wood twigs. Run D: As B, excluding lengths of new shoots and older wood twigs Run
% variation accounted for by function, followed by the most important variables with their standardised canonical coefficients First
Second
A
57 Length of bud-tipped shoots 1·1907 Number of branches −1·1772
25 Number of thorn-tipped new shoots 0·8879
B
57 Shoot leaf area 0·8111
26 Number of thorn-tipped new shoots −0·9224 length of older wood thorn-tipped shoots 1·0771
C
69 Leaf area of new shoots 0·8353
22 Length of older wood thorn-tipped shoots 0·9025
D
62 Shoot leaf area 0·8092
27 Number of thorn-tipped shoots 0·9563
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5 5 5
More thorn-tipped shoots
4
Hand cut autumn
5
3
7 7
7 7
7 7
2 5
3
Axis 2
1 2 1 7
0 –1
2 1
7 7 3 7 73 7 3
1
1 33 2 22 21 2 3 3 1 23 3 1 32 3 1 1 2 22 33 3
–2 1
–3 –4
6 6
23 3
Top cut winter
–5 –10 –9
–8
–7
–6
More branches
–5 –4 –3 Axis 1
–2
–1
0
1
Longer bud-tipped shoots
Figure 2. Canonical discriminant analysis. Run A: All management types, hedge features and sites. Management codes in Table 2.
was removed especially from the horizontal cut (with or without a vertical cut) compared with the vertical cut alone (Table 6, timing×cut interaction, P=0·031). 3.2. The percentage of variation accounted for by the first two functions and the coefficients of the most important discriminant variables in each analysis are presented in Table 7. 3.2.1. A: All management types, hedge features and sites The two groups of handcut and topcut hedges exerted a strong effect on the dispersion of the other samples (Figure 2). These two groups represented six samples (on two sites). The handcut hedge (5) was characterised by a large number of thorn-tipped new shoots (49·4) with both shorter bud-tipped new shoots (7·4 cm) and a greater number of branches (22·2). The topcut hedge (6) was characterised by many old branches (8·2), shorter bud-tipped new shoots (4·5 cm) and fewer thorn-tipped new shoots (1·3). The rest of the observations were grouped at the opposite end of the first axis (i.e. with longer bud-tipped new shoots on fewer branches).
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3 Longer older-wood thorn-tipped shoots
2 2
2 3
2
3
3 3
2
3
1
2 27
3 3 27
2
Axis 2
2 1
2
0
3 1
2 2 3
2
3
73
7 73
2 1 1
3 7
1
3
3
3 3
More thorn-tipped shoots
1
–1 7
1
7 7 1 3
–2
1 7
1 7 7
–3 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 Axis 1 Larger leaf area Figure 3. Canonical discriminant analysis. Run B: As A, excluding handcut and topcut hedges. Management codes in Table 2.
These two groups were removed from the data set to maximise the dispersion of the remaining samples. 3.2.2. B: As A, excluding handcut (autumn) and topcoat (winter) hedges Hedgerows which were uncut (7) and winter flailed (3) were characterised by a large leaf area (992 cm2 and 898 cm2, respectively) than summer (1) and autumn (2) flailed hedgerows (446 cm2 and 544 cm2, respectively) (Figure 3). However, summer-flailed samples usually had high levels of Galium aparine infestation. Both uncut (7) and summer-flailed (1) samples had a greater number of thorn-tipped shoots than the summer-flailed samples (30·0 and 18·2, respectively) and shorter, old wood, thorn shoots (2·0 cm and 2·5 cm). Correlations between the discriminant variables were examined. Length measurements were strongly negatively correlated with the corresponding number, for example the number of older wood bud-tipped shoots per 0·25 m2 and their mean length. To eliminate these strong correlations, the data were re-analysed twice, once without the length measurements and once without the numbers.
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3 7 7
2
1
7 7 7
3 1
1
1
3 7
1
2 7 2 1
3
Axis 2
3
3
1
0
3 2 2
2
1
1 1
7
2 2 2
1
7 7 3
2
73 7
3 3
Longer older-wood thorn-tipped shoots
3 3
–1
2
3 2
2 2 3 3
3
3
–2
2 3
–3 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 Axis 1
0.0
0.5
1.0
1.5
Larger leaf area Figure 4. Canonical discriminant analysis. Run C: As B, excluding numbers of new shoots and older wood twigs. Management codes in Table 2.
3.2.3. C: As B, excluding numbers of new shoots and older wood twigs The plot of the canonical discriminant analysis is rather dispersed but does show a pattern (Figure 4). The unmanaged (7) samples were characterised by the large leaf area of the new shoots (992 cm2) and short thorn-tipped new shoots (2·0 cm). Summerflailed samples (1) had a small leaf area of the shoots (446 cm2) with medium length thorn-tipped twigs of older wood (2·5 cm). The autumn (2) and winter flailed (3) samples were characterised by the longest thorn-tipped older wood twigs (3·7 cm and 3·0 cm, respectively).
3.2.4. D: As B, excluding lengths of new shoots and older wood twigs Analysis of the data excluding lengths of shoots gave an almost identical pattern to that found in C above but with “less thorn-tipped shoots” replacing “longer olderwood thorn-tipped shoots” as an important discriminatory variable.
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4. Discussion 4.1. A combination of both vertical and horizontal cutting produced a tall hedge with long, bud-tipped shoots. A vertical cut in summer produced a thornier hedge than a vertical cut in winter, whereas a horizontal cut in summer resulted in longer shoots than one in winter. Thus, the time of cut will determine whether emphasis is given to a thornier hedge, possibly desirable for stock-keeping, or to one with longer bud-tipped shoots with the potential for more flower and fruit production. Although, as in work with fruit tree pruning (Mika, 1986), cutting did not affect the total length of shoots produced, a vertical cut produced a taller hedge. Possibly, the removal of lateral shoots provides more resources for the leading dominant shoots. In fruit trees, the removal of the apices of leading shoots removes the inhibition of lateral buds, resulting in extension growth (Mika, 1986). Horizontal cutting of a hedge appears to have a similar effect which is increased by a combined horizontal and vertical cut, possibly because the remaining intact shoots have more resources and can therefore grow more vigorously. A horizontal cut reduced the number of thorn-tipped shoots produced the following year as compared with no cut. This might be explained by the direction of assimilates both to the rapidly extending bud shoots (which replace those removed) and to those laterals released from inhibition. These laterals may also exert a strong influence over the thorn initials, thus preventing shoot extension. However, the length of thorn-tipped shoots never exceeded 8 cm which may be a genetically-determined maximum. Thorniness is one of the characteristics of juvenility (Kramer and Kozlowski, 1979). Thorns have the potential to elongate into shoots with nodes, leaves and lateral thorns. Spines, thorns and prickles are also a defensive mechanism against browsing by herbivores. It has been found that removing leaves of bramble plants (Rubus fruticosus L.), under experimental conditions, increased the density of prickles while browsing by deer increased the prickliness of plants growing in woodland (Bazely et al., 1991). The increase in thorn-tipped shoot numbers following vertical cutting in summer may be a similar response. A vertical trim in winter resulted in fewer thorn-tipped shoots the following year, whereas one in summer increased their number. This is probably related to weaker apical dominance in summer (Edmond et al., 1975) and also to the removal of the dominant shoot apices of the laterals. Hackett (1985) found that woody plants retain juvenile characteristics preserved at the base of plants in ontogenetically young tissue while maturation occurs in the periphery of the plant in ontogenetically older but chronologically younger tissues. Therefore, a vertical cut which removes apical lateral shoots from the relatively juvenile base of the plant may be expected to stimulate thorn elongation. A vertical trim in winter resulted in longer shoots than a corresponding one in the summer. This is in agreement with many of the studies on fruit trees reviewed by Mika (1986) and is probably related to the strong apical dominance being exerted by the remaining growing points in winter. Maggs (1965) also found that apple trees pruned during winter (dormant pruning) produced fewer but longer shoots, whereas a summer pruning produced numerous shorter shoots. This work was carried out on juvenile plants in a vegetative growth phase. Repeated removal of photosynthetically active material from juvenile plants over a long period might reduce the plants’ ability to mature and lay down woody tissue, thereby reducing
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their vigour. In apple trees, summer pruning for three years has dwarfing effects on the trees when compared with unpruned trees (Mika, 1986). There are numerous examples of trees which have been regularly coppiced for several centuries (Rackham, 1980). Flailing shrubs is not as severe an act as coppicing. Whilst it might be expected to stimulate vegetative growth initially, perhaps such growth might be at the expense of secondary growth of woody material, resulting in a stunted plant in the long term? 4.2. 4.2.1. Effects of hand cutting The hand-cut hedge had numerous branches carrying many thorn-tipped new shoots and short bud-tipped ones. It was very thick and dense, but with all the growth concentrated on the outer edges. Vertical summer pruning of young hedges results in numerous short shoots because of weak apical dominance but hand-cutting, which can be more accurate than a mechanical cut, removes only the current season’s shoots from where they branch from the old wood. This pruning produces short old wood with a much more calloused end where new shoots arise. This form of pruning resembles pollarding where buds tend to be concentrated around present or former branches and can proliferate (Mitchell, 1989). The increased number of thorn-tipped shoots may also be a response to continued pruning (Bazely et al., 1991). 4.2.2. Effects of top-cutting The top-cut hedge was characterised by many old branches, shorter bud-tipped shoots and fewer thorn-tipped shoots. It had been intensively sheep-grazed, and, in the second winter of the study, it was cut back using a flail which reduced its growth. Apical dominance was exerted over the thorn-tipped shoots, but it seems that the top-cut removed such a large proportion of the accumulated reserves of the hedge that it was unable to produce much new growth in the following year. 4.2.3. Effects of flailing compared with top-cutting Uncut and winter flailed hedges were characterised by a larger leaf area per unit area than both a summer or autumn flailed ones. The unmanaged hedges had longer old wood shoots and fewer thorn-tipped shoots because apical dominance suppresses thorntipped shoots. Thorns are characteristic of juvenility (Kramer and Kozlowski, 1979) and so there were few thorns on the ontogenetically older wood of these un-trimmed shrubs. In contrast, the summer-flailed samples exhibited the effects of pruning when apical dominance is weak, with shorter old wood twigs and numerous thorn-tipped shoots. Work at Silsoe College has shown that the more severe the damage, the further back along the branch shoot growth occurs, resulting in a “bushier” hedge (Semple et al., 1994). 4.2.4. Effects of flailing as identified by shoot lengths The leaf area of the shoots exerted the strongest influence over the dispersion of the management classes. However the summer-flailed hedges were dominated by longer
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thorn-tipped older wood, with uncut ones having much shorter older wood. This could be explained by the partitioning of assimilates, with the summer cut under weak apical dominance directing growth into the thorn-tipped shoots. In unmanaged hedges, growth is directed into other areas such as secondary growth, larger leaf area and the production of reproductive parts. In conclusion, since severe cutting decreased hawthorn shoot growth the following year, hawthorn hedges are probably best rejuvenated by gapping up with young plants. In addition, although an annual flail did not retard subsequent growth, such management needs to be assessed for longer than three years studied here (Whelon, 1994). Our study suggests that if a dense, thorny, compact hedge is required, a late summer cut, either by hand or with the careful use of a flail, is preferable to a winter one. However, care should be taken to delay this until birds have fledged and, in general, a winter cut is more desirable than a late summer one from the point of view of wildlife conservation. This work was funded by a postgraduate studentship to N. R. Bannister from the Ministry of Agriculture, Fisheries and Food and Studley College Trust provided additional financial assistance. We thank Dr G. P. Buckley, Mr P. B. Dodd, Dr K. J. Kirby, Professor W. W. Schwabe and Mrs S. E. Wright for helpful discussion; Dr R. E. Kempson and Mr J. Batt for technical advice; Mr B. Wilson and Mr D. E. Bannister for the figures and the landowners for allowing field work to be carried out on their hedgerows.
References Agricultural Training Board (1990). Environmental Handbook for Agricultural Trainers. ATB, Rhone Poulenc and Nature Conservancy Council, National Agricultural Centre, Stoneleigh, Warwickshire. Sheet 26. Barr, C., Howard, D., Bunce, R., Gillespie, M. and Hallam, C. (1991). Changes in hedgerows in Britain between 1984 and 1990. Report, Institute of Terrestrial Ecology, Merlewood. Bazely, D. R., Myers, J. H. and Burke da Silva, K. (1991). The response of numbers of bramble prickles to herbivory and depressed resource availability. Oikos 61, 327–336. Beddall, J. L. (1950). Hedges for Farm and Garden, 68 pp. London: Faber and Faber. Clapham, A. R., Tutin, T. G. and Warburg, E. F. (1981). Excursion flora of the British Isles, 3rd edn. Cambridge: Cambridge University Press. Countryside Commission (1984). Agricultural Landscapes: a second look. CCP 168, 20 pp. Cheltenham. Edmond, J. B., Senn, T. L., Andrews, F. S. and Halfacre, R. G. (1975). Fundamentals of Horticulture. 4th ed. 560 pp. New York: McGraw-Hill Book Co. Farming and Wildlife Trust (1983). A hedgerow code of practice. 6 pp. Sandy Bedfordshire. Genstat 5 Committee (1988). Genstat 5 Reference Manual. Oxford Science Publications. 749 pp. Oxford: Clarendon Press. Hackett, W. P. (1985). Juvenility, maturation and rejuvenation in woody plants. Horticulture Review 7, 109–155. Klecka, W. R. (1980). Discriminant Analysis. Sage University Paper no. 19. London: Sage Publications. Kramer, P. J. and Kozlowski, T. T. (1979). Physiology of woody plants. 811 pp. London: Academic Press. Maclean, M. (1992). New hedges for the countryside. 276 pp. Ipswich Farming Press. Maggs, D. H. (1965). Dormant and summer pruning compared by pruning young apple trees once on a succession of dates. Journal of Horticultural Science 40, 249–265. Malden, W. H. (1899). Hedges and hedge-making. Journal of the Royal Agricultural Society 10, 87–117. Mika, A. (1986). Physiological responses of fruit trees to pruning. Horticultural Review 8, 337–378. Mitchell, P. L. (1989). Repollarding large neglected pollards; a review of current practice and results. Arboricultural Journal 13, 125–142. Rackham, O. (1980). Ancient woodland: its history, vegetation and uses in England. 402 pp. London: Edward Arnold. SAS Institute Inc. (1985). Users’ guide: Statistics. Version 5. Cary N. C. SAS Institute Inc., pp. 155–169. Semple, D. A., Dyson, J. and Godwin, R. J. (1994). Effect of mechanised cutting on the short term regrowth of hawthorn hedgerows. In Field margins: integrating agriculture and conservation. (N. Boatman, ed.) pp. 235–240. BCPC Monograph no. 58. Farnham: British Crop Protection Council. Whelon, D. (1994). The Hedgerow Incentive Scheme. In Hedgerow Management and Nature Conservation. (T. A. Watt and G. P. Buckley, eds) pp. 137–145. Wye: Wye College Press.
14-11-95 10:25:29
Journal of Environmental Management (1995) 45, 395–410
Effects of Cutting on the Growth of Crataegus monogyna (Hawthorn) in Hedges N. R. Bannister∗ and T. A. Watt Wye College, University of London, Wye, Ashford, Kent, TN25 5AH, U.K. Received 9 February 1995
Effects of the position and timing of cutting on shoot growth of young Crataegus monogyna plants in newly-planted hedges were studied. Total shoot length was unaffected by cutting. In general, a combination of both vertical and horizontal cutting produced a tall hedge with long, bud-tipped shoots. The timing of cut was important: a horizontal cut in summer resulted in fewer but longer shoots whereas a vertical cut in summer produced more thorn-tipped shoots. A vertical cut in winter resulted in longer shoots than one in summer and reduced the number which were thorn-tipped. Management types of farm hedges could be characterised by various growth parameters. Hand-cut hedges had numerous short shoots and many older-wood branches per unit area, whereas unmanaged ones had a greater leaf area and longer shoots. Summerflailed hedgerows were characterised by a smaller leaf area although this may have been partly due to Galium aparine L. infestation. The use of the flail on the current season’s growth did not significantly retard growth the following year. 1995 Academic Press Limited Keywords: hedgerow, management, pruning, flailing, growth.
1. Introduction 1.1. Crataegus monogyna Jacq. (hawthorn) occurs in scrub and woods up to 550 m altitude and is the commonest shrub on most soils throughout the British Isles (Clapham et al., 1981) as well as being widely planted in hedgerows. Many hedges are cut both along their sides and top. However, sometimes a hedge may be cut only along the top to develop a bush habit or only along its sides to encourage upward extension. The traditional time to cut hedges is from July onwards between hay and corn harvests and in early autumn when field work is slack and hedges are accessible (Beddall, 1950). Even with the advent of mechanised hedge trimmers and new crops such as autumnsown oilseed rape, hedges are still usually cut at the end of harvest. However, current
∗ Present address: Brenoweth, Grampound, Truro, Cornwall. TR2 4RB, U.K. 395 0301–4797/95/040395+16 $12.00/0
1995 Academic Press Limited
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advice to farmers is to trim hedges in January or February to minimise disturbance of wildlife (Farming and Wildlife Trust 1983; Agricultural Training Board, 1990). The effects of different methods of cutting hedges on hedge growth have been reviewed (Maclean, 1992) but current information is derived from experience and observational studies rather than from experimental evidence. Here we investigate the effect of the season and position of cutting on the first two years’ growth of a newlyplanted hedge. 1.2. In conventional arable farming systems a Crataegus monogyna hedge may be hardpruned annually and exposed both to agrochemicals and root disturbance. The tolerance of C. monogyna to disturbance and its hardiness and ability to recover from damage has made it the most successful and favoured hedging plant in England (Malden, 1899). Nevertheless, there has been concern about how well it can survive modern farming practices. In Huntingdonshire, many hedges, already of poor quality in 1974, had disappeared by 1984 (Countryside Commission, 1984). This disappearance was attributed to “intensive arable cultivations, continual hard trimming, plus possible occasional herbicide treatment and burning”. It was also found that “lack of maintenance led to a general decline in hedge quality” and that C. monogyna was not suited to mechanical cutting in some (unspecified) conditions. Some hedges on stock farms were in a “bad condition” due to lack of management and stock damage, whereas on some intensive, efficient farms with institutional owners, the hedges were being actively managed in a “thoughtful and caring way”. More recent surveys of British field boundaries have shown that 21% of boundaries classified as hedgerows in 1984 were classified as a different type of boundary (for example lines of trees or shrubs or relict hedgerows) when re-surveyed in 1990; suggesting less active management had taken place during the 1980s (Barr et al., 1991). The aim of this study was to determine whether different methods of hedge cutting could be distinguished in terms of their effects on hedge growth, by making observations both on young experimental hedges and selected established farm hedges over a three year period. 2. Materials and methods 2.1. The experimental site was an area of pasture on loam soil (pH 7·8) on the Wye College Estate which had been taken out of arable five years previously. The C. monogyna plants were grown from seed collected in south-east England. At the nursery, after one year’s growth, the seedlings had their leading shoot removed to encourage them to develop a bushy habit; at the start of the experiment, most had more than one dominant shoot. In January, the two year-old plants between 45 and 50 cm in height were planted in groups of five (three at the front and two in alternating positions at the back of the hedge) per plot, with 25 cm between each plant. Of the five plants in each plot, only the central front one was measured as it typified a plant cut on one side within a hedgerow. The four guard plants were cut on both sides as each plot was cut as a whole. The plots were laid out 0·5 m apart in two parallel rows 1 m apart in each of two blocks.
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Four types of cutting simulated the ways in which farmers cut hedges and they formed a 2×2 factorial structure with two positions and two seasons of cutting: (i) Control. No cutting. (ii) Vertical cut. Lateral shoots were pruned back to 10 cm from the main stem using secateurs. (iii) Horizontal cut. All leading shoots over 70 cm above the ground were cut back to that height. (iv) Both horizontal and vertical cuts. In the two years following planting, the above treatments were imposed either in summer (middle of August) or in winter (February) giving a 23 factorial structure for the eight treatments which were each replicated three times within each block. Figure 1 shows the development of the plants during the period of the study and the timing of the data collection. After a preliminary season of measurements (August 1985 and February 1986), the lengths and type (whether bud- or thorn-tipped) of all shoots of the current year’s growth and the main stem diameter at 10 cm above the ground were recorded in May in both 1986 and 1987. The height of each central plant was measured in May 1986 and in both May and August 1987, and the shoot dry weight of all the material removed was recorded in February and August 1987. The data were analysed using factorial analysis of variance in Genstat 5 (Genstat 5 Committee, 1988). Main stem diameter at the time of planting was used as a covariate in the analysis, to take account of any inter-plant differences in vigour that already existed at the time of planting. 2.2. 2.2.1. Sites The National Farmers’ Union Representatives for Kent and Leicestershire recommended farmers who would be willing to participate in the research project. Twelve hedges were chosen to cover a range of management types using the following guidelines: (i) (ii) (iii) (iv) (v)
C. monogyna was the dominant shrub species. Active management was being carried out either annually or biennially. The hedges were internal field boundaries. The length of the hedge was at least 100 m. Some were adjacent to pasture and others to arable land and, where possible, the same land use was on each side of the hedge.
Not all the chosen sites fulfilled all criteria. However, all hedges selected were hawthorn-dominant and (except for one deliberately uncut hedge) were being actively managed. All but three of the hedge sites had an annual trim and of these all except Shangton were cut by machine (Table 1). In 1987, however, because of farm staff changes, this hedge had to be cut with a flail and, in the preceding year, the hand cutting was not carried out as closely as usual. 2.2.2. Sampling Five randomly positioned transects were established in summer at right-angles across each hedge and marked with 50 cm white fibre glass rods. As hedges at Shangton and
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(a)
(b)
v
(c)
v h
70 cm
h
v10 cm 10 cm v
(d)
(e)
Figure 1. The development of C. monogyna plants during the experiment. Cutting Treatments: V=vertical trim; H=horizontal trim; VH=combined trim; (. . . .)=cutting line; (––––)=older wood of original plant; (– – –)=new shoots grown that season. (a) January 1985 and April 1985; (b) August 1985 and February 1986; (c) May 1986; (d) August 1986 and February 1987; (e) May 1987.
Cestersover were only just over 50 cm in length, only three transects were used at these sites. Lateral growth and production were measured from 0·25 m2 square quadrats held vertically at chest height, one on either side of each transect. Thus, the position of the quadrat in relation to the top of the hedge varied depending on the height of the hedge. Each year, the quadrat was moved along the hedge so that the samples were not affected by the previous sampling procedure. The number of secondary branches coming from the main stem out to the edge of
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T 1. Hedge management at each site 1985 Site Whitehouse Scords A Scords P Debden Althorp Adams Cestersover Bowden Shangton Paddocks Wye I Wye II
1986
1987
Shape
Type
Time
Type
Time
Type
Time
Box A A Box Box Chamfered Box Chamfered Chamfered A A A
Flail Flail Flail Flail Flail Flail Flail Flail Hand Flail None Flail
Sep 84 Aug 84 Feb 85 Nov 84 Jul 84 Nov 84 Sep 84 Jan 86 Sep 84 Nov 84 na Sep 84
Flail Flail Flail Flail Flail Flail Flail Flail Hand None None None
Sep 85 Aug 85 Feb 86 Dec 85 Aug 85 Aug 85 Oct 85 Sep 85 Sep 85 na na na
Flail — Flail — — — — — Flail Flail None None
Sep 86 — Feb 87 — — — — — Sep 86 Dec 86 na na
na=not applicable.
the hedge was counted within each quadrat. Only branches which were at least half within the quadrat were counted. Three of these branches were chosen at random, pruned from the point where they grew out from the main vertical stem and placed in polythene bags for measurement. In the laboratory, the branches were split into their component parts: new shoots (the current season’s shoot growth), older wood (the previous season’s woody growth), and their respective leaves. Leaf area was measured, using an area measurement system (Delta-T Devices Ltd, Cambridge, U.K.). The length of each branch was measured and the type of terminal bud (thorn, bud or pruned) was noted. In addition to the above growth sample, a separate sample of new shoots, the biomass sample, was made to estimate dry matter production. Twelve new shoots were chosen randomly from within an adjacent 0·25 m2. They were sealed in polythene bags and kept in a cold store at 3°C, until measured in the laboratory. Their length and the type of terminal bud was recorded. The total dry matter of all the shoots was obtained by weighing after oven drying at 80°C for 36 hours. 2.2.3. Multivariate analysis The values from the three (or five) quadrats on either side of the hedge on each year were averaged for each variable. The results for all years from both sides of the hedges were included and both the full set and subsets of discriminant variables were analysed. Potential discriminant variables were: orientation, presence of a ditch, number and length of bud-tipped and thorn-tipped new shoots and older wood, mean leaf area of new shoots and older wood, number of branches and new shoot biomass per 10 cm shoot length. In total, there were 60 samples from the 12 hedges. Four analyses on the range of variables were carried out: A: All management types, hedge features and sites; B: As A, excluding handcut and topcut hedges; C: As B, excluding numbers of new shoots and older wood twigs; D: As B, excluding lengths of new shoots and older wood twigs.
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T 2. Allocation of group codes for the management types Code 1 2 3 4 (none) 5 6 7
Management Flailed Flailed Flailed Not flailed
Time Summer Autumn Winter Summer
Hand cut Top cut Unmanaged
Autumn Winter na
na=not applicable.
Management was defined as “flailed” or “not flailed” and also classified by time of operation: summer (July or August), autumn (September) or winter (October–February). Together with an “uncut” category, this gives seven groups. Within the “not flailed” group (4–6) were those hedges which were either hand-cut or top-cut (Table 2). The data were examined using the SAS Canonical Discriminant Analysis procedure, CANDISC (Klecka, 1980; SAS Institute Inc., 1985). Examination of the raw data showed that there was considerable variation in the orders of magnitude between the different variables. Therefore, the data were transformed to logarithms so that the effect of outliers was reduced. All means are presented per 0·25 m2, back-transformed from logarithms. 3. Results 3.1. 3.1.1. Total shoot length The data for the mean length and number of all shoot types were multiplied together to give an estimate of the total length of shoots produced (data not presented). The position and timing of cutting had no significant effect on the total length of shoots produced, but they did affect its components: mean shoot length and the number of shoots per plant. 3.1.2. Mean shoot length The main effect of cutting position was significant in both May 1986 and May 1987 (P=0·028, P=0·004, respectively, Table 3a). In 1986, this effect was largely accounted for by the interaction between vertical and horizontal cutting. Vertical cutting increased shoot length when the plants were also cut horizontally, but decreased it when the plants were not cut horizontally (Table 3b(i), P=0·025). When the data for bud- and thorn-tipped shoots were analysed separately, the bud-tipped shoots were found to be responsible for this effect (Table 3b(ii), P=0·037). Analysis of all shoot types in 1987 showed that, in general, a horizontal cut resulted in longer shoots (14·3 cm vs. 11·3 cm, P=0·001) and, again, bud-tipped shoots were largely responsible for this (18·3 cm vs. 16·0 cm, P=0·037). The effect of timing of cut was not significant for all shoot types. However, the
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T 3(a). Mean shoot length of current year’s growth per plant (cm) (i) May 1986 All Cut Control Vertical Horizontal Both SED
Bud
Thorn
Winter
Summer
Winter
Summer
Winter
Summer
7·8 7·8 4·4 10·7
9·1 7·5 8·4 9·0
9·0 9·3 5·3 11·6
10·1 8·7 9·4 10·4
3·4 5·3 2·7 2.7
4·4 4·3 3·5 3·4
1·82
1·94
1·66
All
Bud
Thorn
(ii) May 1987
Cut Control Vertical Horizontal Both SED
Winter
Summer
Winter
Summer
Winter
Summer
10·7 11·5 11·9 15·8
12·2 10·9 16·3 13·4
16·8 16·3 15·1 18·2
15·5 15·4 20·5 19·4
5·6 7·8 7·2 7·2
8·0 6·3 7·1 7·2
1·74
2·12
1·40
interaction of the vertical cut with timing was nearly significant in 1986 when the mean length of all shoot types after vertical cutting in winter was longer than when not cut vertically, whereas, in contrast, a vertical cut in summer did not affect mean shoot length (P=0·053, Table 3b(iii)). In 1987, when the plants were cut vertically in winter, there was a slight increase in the mean length of all shoot types, compared to when plants were not cut vertically. However, when the plants were cut vertically in summer, there was a decrease in the mean shoot length (P=0·016, Table 3b(iv)). When the mean lengths of the two types of shoot were analysed separately, different effects were found. In 1987, the interaction of horizontal cut with timing was significant for bud-tipped shoots (P=0·043, Table 3b(v)). Horizontal cutting in summer greatly increased the length of bud-tipped shoots, whereas such cutting in winter was ineffective. 3.1.3. Mean number of shoots per plant The effect of position of cutting was not significant for all shoots in either year of the experiment (Table 4a). However, in 1987, the main effect of horizontal cutting was to reduce the mean number of thorn-tipped shoots from 22 to 14 (P=0·037). In contrast, the number of bud-tipped shoots was not significantly affected by either position or timing of cut. However, the interaction of vertical cut with timing (P= 0·045) was significant for all types of shoot numbers and this was mainly due to thorntipped shoots. A vertical cut carried out in winter reduced thorn-tipped shoot numbers. However, when the vertical cut was carried out in summer, thorn-tipped shoot numbers in the following year increased (Table 4b, P=0·007).
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T 3(b). Interactions between the two positions of cutting and between position and time of cutting on the mean shoot length per plant of the current year’s growth (cm)
(i) All shoots May 1986 Horizontal − Horizontal + SED (ii) Bud shoots May 1986 Horizontal − Horizontal + SED (iii) All shoots May 1986 Winter Summer SED (iv) All shoots May 1987 Winter Summer SED (v) Bud shoots May 1987 Winter Summer SED
Vertical −
Vertical +
8·4 6·4
7·6 9·8
Vertical −
1·15
9·5 7·3 Vertical −
9·0 11·0 1·22
6·1 8·3 Vertical −
Vertical +
Vertical + 9·2 8·3
1·15
11·3 14·2
Vertical + 13·6 12·2
1·10 Horizontal −
Horizontal +
16·5 15·5
16·6 20·0 1·34
3.1.4. Stem diameter There were no significant treatment effects. In May 1986, the mean stem diameter was 11·0 mm (SD=0·86), while by May 1987, it had increased to 14·2 mm (SD=1·69). 3.1.5. Plant height There was no significant effect of season of cutting on plant height (Table 5a), but horizontal cutting had a highly significant effect on height in both years (May 1986, 84·3 cm vs. 70·7 cm and May 1987, 112·5 cm vs. 97 cm, both P<0·001). However, by August 1987, the effect of horizontal cutting was no longer significant. The interaction between vertical and horizontal cutting was just significant in May 1986. The absence of a horizontal cut alone increased height as expected, but, in the presence of a vertical cut, this increase was greater (P=0·049, Table 5b). 3.1.6. Dry weight of material removed When the hedge was cut in winter, only small amounts of material were removed with combined horizontal and vertical cutting, although more than either type of cutting removed separately. However, when cutting took place in summer, much more material
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T 4(a). Mean number of shoots per plant (i) May 1986 All Cut Control Vertical Horizontal Both SED
Bud
Thorn
Winter
Summer
Winter
Summer
Winter
Summer
12·7 17·0 10·6 15·8
17·7 24·3 12·5 22·8
8·7 7·4 6·5 10·7
12·5 16·6 8·0 14·2
5·2 9·6 4·1 5·2
5·2 7·5 4·5 8·8
8·17
4·45
4·99
All
Bud
Thorn
(ii) May 1987
Cut Control Vertical Horizontal Both SED
Winter
Summer
Winter
Summer
Winter
Summer
49·6 26·8 28·9 30·2
42·5 55·9 31·2 47·5
24·8 17·2 16·8 23·8
20·8 25·2 19·2 22·5
30·2 15·0 13·7 15·8
16·1 29·0 11·8 15·5
12·37
6·00
7.74
T 4(b). Interaction between vertical cutting and time of cut on the mean number of thorn-tipped shoots per plant in May 1987
Winter Summer SED
Vertical −
Vertical +
20 15
10 27 4·9
T 5(a). Mean plant height (cm) May 1986 Cut Control Vertical Horizontal Both SED
May 1987
August 1987
Winter
Summer
Winter
Summer
Winter
Summer
76·1 85·8 73·7 75·2
83·2 92·3 70·2 63·8
103·8 111·1 91·8 99·8
112·2 122·8 100·2 96·2
133·9 143·6 115·4 134·0
141·2 163·6 147·2 138·7
5·83
8·40
13·29
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T 5(b). Interaction between the two types of cutting on mean plant height (cm) in May 1986
Horizontal − Horizontal + SED
Vertical −
Vertical +
79·6 71·9
89·0 69·5 3·69
T 6. Dry weight (g) of shoot material removed in February and August 1987 February
Dry weight Log (x+1)
August
Horizontal
Vertical
Both
Horizontal
Vertical
Both
0·90 0·57
0·73 0·49
4·38 1·48
41·23 3·68
17·50 2·76
33·12 3·32
SE mean (log transformation)=0·408.
T 7. Summary of results of discriminant analyses. Run A: All management types, hedge features and sites. Run B: As A, excluding handcut and topcut hedges. Run C: As B, excluding numbers of new shoots and older wood twigs. Run D: As B, excluding lengths of new shoots and older wood twigs Run
% variation accounted for by function, followed by the most important variables with their standardised canonical coefficients First
Second
A
57 Length of bud-tipped shoots 1·1907 Number of branches −1·1772
25 Number of thorn-tipped new shoots 0·8879
B
57 Shoot leaf area 0·8111
26 Number of thorn-tipped new shoots −0·9224 length of older wood thorn-tipped shoots 1·0771
C
69 Leaf area of new shoots 0·8353
22 Length of older wood thorn-tipped shoots 0·9025
D
62 Shoot leaf area 0·8092
27 Number of thorn-tipped shoots 0·9563
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5 5 5
More thorn-tipped shoots
4
Hand cut autumn
5
3
7 7
7 7
7 7
2 5
3
Axis 2
1 2 1 7
0 –1
2 1
7 7 3 7 73 7 3
1
1 33 2 22 21 2 3 3 1 23 3 1 32 3 1 1 2 22 33 3
–2 1
–3 –4
6 6
23 3
Top cut winter
–5 –10 –9
–8
–7
–6
More branches
–5 –4 –3 Axis 1
–2
–1
0
1
Longer bud-tipped shoots
Figure 2. Canonical discriminant analysis. Run A: All management types, hedge features and sites. Management codes in Table 2.
was removed especially from the horizontal cut (with or without a vertical cut) compared with the vertical cut alone (Table 6, timing×cut interaction, P=0·031). 3.2. The percentage of variation accounted for by the first two functions and the coefficients of the most important discriminant variables in each analysis are presented in Table 7. 3.2.1. A: All management types, hedge features and sites The two groups of handcut and topcut hedges exerted a strong effect on the dispersion of the other samples (Figure 2). These two groups represented six samples (on two sites). The handcut hedge (5) was characterised by a large number of thorn-tipped new shoots (49·4) with both shorter bud-tipped new shoots (7·4 cm) and a greater number of branches (22·2). The topcut hedge (6) was characterised by many old branches (8·2), shorter bud-tipped new shoots (4·5 cm) and fewer thorn-tipped new shoots (1·3). The rest of the observations were grouped at the opposite end of the first axis (i.e. with longer bud-tipped new shoots on fewer branches).
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3 Longer older-wood thorn-tipped shoots
2 2
2 3
2
3
3 3
2
3
1
2 27
3 3 27
2
Axis 2
2 1
2
0
3 1
2 2 3
2
3
73
7 73
2 1 1
3 7
1
3
3
3 3
More thorn-tipped shoots
1
–1 7
1
7 7 1 3
–2
1 7
1 7 7
–3 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 Axis 1 Larger leaf area Figure 3. Canonical discriminant analysis. Run B: As A, excluding handcut and topcut hedges. Management codes in Table 2.
These two groups were removed from the data set to maximise the dispersion of the remaining samples. 3.2.2. B: As A, excluding handcut (autumn) and topcoat (winter) hedges Hedgerows which were uncut (7) and winter flailed (3) were characterised by a large leaf area (992 cm2 and 898 cm2, respectively) than summer (1) and autumn (2) flailed hedgerows (446 cm2 and 544 cm2, respectively) (Figure 3). However, summer-flailed samples usually had high levels of Galium aparine infestation. Both uncut (7) and summer-flailed (1) samples had a greater number of thorn-tipped shoots than the summer-flailed samples (30·0 and 18·2, respectively) and shorter, old wood, thorn shoots (2·0 cm and 2·5 cm). Correlations between the discriminant variables were examined. Length measurements were strongly negatively correlated with the corresponding number, for example the number of older wood bud-tipped shoots per 0·25 m2 and their mean length. To eliminate these strong correlations, the data were re-analysed twice, once without the length measurements and once without the numbers.
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3 7 7
2
1
7 7 7
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2 7 2 1
3
Axis 2
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0
3 2 2
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Longer older-wood thorn-tipped shoots
3 3
–1
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–2
2 3
–3 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 Axis 1
0.0
0.5
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1.5
Larger leaf area Figure 4. Canonical discriminant analysis. Run C: As B, excluding numbers of new shoots and older wood twigs. Management codes in Table 2.
3.2.3. C: As B, excluding numbers of new shoots and older wood twigs The plot of the canonical discriminant analysis is rather dispersed but does show a pattern (Figure 4). The unmanaged (7) samples were characterised by the large leaf area of the new shoots (992 cm2) and short thorn-tipped new shoots (2·0 cm). Summerflailed samples (1) had a small leaf area of the shoots (446 cm2) with medium length thorn-tipped twigs of older wood (2·5 cm). The autumn (2) and winter flailed (3) samples were characterised by the longest thorn-tipped older wood twigs (3·7 cm and 3·0 cm, respectively).
3.2.4. D: As B, excluding lengths of new shoots and older wood twigs Analysis of the data excluding lengths of shoots gave an almost identical pattern to that found in C above but with “less thorn-tipped shoots” replacing “longer olderwood thorn-tipped shoots” as an important discriminatory variable.
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4. Discussion 4.1. A combination of both vertical and horizontal cutting produced a tall hedge with long, bud-tipped shoots. A vertical cut in summer produced a thornier hedge than a vertical cut in winter, whereas a horizontal cut in summer resulted in longer shoots than one in winter. Thus, the time of cut will determine whether emphasis is given to a thornier hedge, possibly desirable for stock-keeping, or to one with longer bud-tipped shoots with the potential for more flower and fruit production. Although, as in work with fruit tree pruning (Mika, 1986), cutting did not affect the total length of shoots produced, a vertical cut produced a taller hedge. Possibly, the removal of lateral shoots provides more resources for the leading dominant shoots. In fruit trees, the removal of the apices of leading shoots removes the inhibition of lateral buds, resulting in extension growth (Mika, 1986). Horizontal cutting of a hedge appears to have a similar effect which is increased by a combined horizontal and vertical cut, possibly because the remaining intact shoots have more resources and can therefore grow more vigorously. A horizontal cut reduced the number of thorn-tipped shoots produced the following year as compared with no cut. This might be explained by the direction of assimilates both to the rapidly extending bud shoots (which replace those removed) and to those laterals released from inhibition. These laterals may also exert a strong influence over the thorn initials, thus preventing shoot extension. However, the length of thorn-tipped shoots never exceeded 8 cm which may be a genetically-determined maximum. Thorniness is one of the characteristics of juvenility (Kramer and Kozlowski, 1979). Thorns have the potential to elongate into shoots with nodes, leaves and lateral thorns. Spines, thorns and prickles are also a defensive mechanism against browsing by herbivores. It has been found that removing leaves of bramble plants (Rubus fruticosus L.), under experimental conditions, increased the density of prickles while browsing by deer increased the prickliness of plants growing in woodland (Bazely et al., 1991). The increase in thorn-tipped shoot numbers following vertical cutting in summer may be a similar response. A vertical trim in winter resulted in fewer thorn-tipped shoots the following year, whereas one in summer increased their number. This is probably related to weaker apical dominance in summer (Edmond et al., 1975) and also to the removal of the dominant shoot apices of the laterals. Hackett (1985) found that woody plants retain juvenile characteristics preserved at the base of plants in ontogenetically young tissue while maturation occurs in the periphery of the plant in ontogenetically older but chronologically younger tissues. Therefore, a vertical cut which removes apical lateral shoots from the relatively juvenile base of the plant may be expected to stimulate thorn elongation. A vertical trim in winter resulted in longer shoots than a corresponding one in the summer. This is in agreement with many of the studies on fruit trees reviewed by Mika (1986) and is probably related to the strong apical dominance being exerted by the remaining growing points in winter. Maggs (1965) also found that apple trees pruned during winter (dormant pruning) produced fewer but longer shoots, whereas a summer pruning produced numerous shorter shoots. This work was carried out on juvenile plants in a vegetative growth phase. Repeated removal of photosynthetically active material from juvenile plants over a long period might reduce the plants’ ability to mature and lay down woody tissue, thereby reducing
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their vigour. In apple trees, summer pruning for three years has dwarfing effects on the trees when compared with unpruned trees (Mika, 1986). There are numerous examples of trees which have been regularly coppiced for several centuries (Rackham, 1980). Flailing shrubs is not as severe an act as coppicing. Whilst it might be expected to stimulate vegetative growth initially, perhaps such growth might be at the expense of secondary growth of woody material, resulting in a stunted plant in the long term? 4.2. 4.2.1. Effects of hand cutting The hand-cut hedge had numerous branches carrying many thorn-tipped new shoots and short bud-tipped ones. It was very thick and dense, but with all the growth concentrated on the outer edges. Vertical summer pruning of young hedges results in numerous short shoots because of weak apical dominance but hand-cutting, which can be more accurate than a mechanical cut, removes only the current season’s shoots from where they branch from the old wood. This pruning produces short old wood with a much more calloused end where new shoots arise. This form of pruning resembles pollarding where buds tend to be concentrated around present or former branches and can proliferate (Mitchell, 1989). The increased number of thorn-tipped shoots may also be a response to continued pruning (Bazely et al., 1991). 4.2.2. Effects of top-cutting The top-cut hedge was characterised by many old branches, shorter bud-tipped shoots and fewer thorn-tipped shoots. It had been intensively sheep-grazed, and, in the second winter of the study, it was cut back using a flail which reduced its growth. Apical dominance was exerted over the thorn-tipped shoots, but it seems that the top-cut removed such a large proportion of the accumulated reserves of the hedge that it was unable to produce much new growth in the following year. 4.2.3. Effects of flailing compared with top-cutting Uncut and winter flailed hedges were characterised by a larger leaf area per unit area than both a summer or autumn flailed ones. The unmanaged hedges had longer old wood shoots and fewer thorn-tipped shoots because apical dominance suppresses thorntipped shoots. Thorns are characteristic of juvenility (Kramer and Kozlowski, 1979) and so there were few thorns on the ontogenetically older wood of these un-trimmed shrubs. In contrast, the summer-flailed samples exhibited the effects of pruning when apical dominance is weak, with shorter old wood twigs and numerous thorn-tipped shoots. Work at Silsoe College has shown that the more severe the damage, the further back along the branch shoot growth occurs, resulting in a “bushier” hedge (Semple et al., 1994). 4.2.4. Effects of flailing as identified by shoot lengths The leaf area of the shoots exerted the strongest influence over the dispersion of the management classes. However the summer-flailed hedges were dominated by longer
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thorn-tipped older wood, with uncut ones having much shorter older wood. This could be explained by the partitioning of assimilates, with the summer cut under weak apical dominance directing growth into the thorn-tipped shoots. In unmanaged hedges, growth is directed into other areas such as secondary growth, larger leaf area and the production of reproductive parts. In conclusion, since severe cutting decreased hawthorn shoot growth the following year, hawthorn hedges are probably best rejuvenated by gapping up with young plants. In addition, although an annual flail did not retard subsequent growth, such management needs to be assessed for longer than three years studied here (Whelon, 1994). Our study suggests that if a dense, thorny, compact hedge is required, a late summer cut, either by hand or with the careful use of a flail, is preferable to a winter one. However, care should be taken to delay this until birds have fledged and, in general, a winter cut is more desirable than a late summer one from the point of view of wildlife conservation. This work was funded by a postgraduate studentship to N. R. Bannister from the Ministry of Agriculture, Fisheries and Food and Studley College Trust provided additional financial assistance. We thank Dr G. P. Buckley, Mr P. B. Dodd, Dr K. J. Kirby, Professor W. W. Schwabe and Mrs S. E. Wright for helpful discussion; Dr R. E. Kempson and Mr J. Batt for technical advice; Mr B. Wilson and Mr D. E. Bannister for the figures and the landowners for allowing field work to be carried out on their hedgerows.
References Agricultural Training Board (1990). Environmental Handbook for Agricultural Trainers. ATB, Rhone Poulenc and Nature Conservancy Council, National Agricultural Centre, Stoneleigh, Warwickshire. Sheet 26. Barr, C., Howard, D., Bunce, R., Gillespie, M. and Hallam, C. (1991). Changes in hedgerows in Britain between 1984 and 1990. Report, Institute of Terrestrial Ecology, Merlewood. Bazely, D. R., Myers, J. H. and Burke da Silva, K. (1991). The response of numbers of bramble prickles to herbivory and depressed resource availability. Oikos 61, 327–336. Beddall, J. L. (1950). Hedges for Farm and Garden, 68 pp. London: Faber and Faber. Clapham, A. R., Tutin, T. G. and Warburg, E. F. (1981). Excursion flora of the British Isles, 3rd edn. Cambridge: Cambridge University Press. Countryside Commission (1984). Agricultural Landscapes: a second look. CCP 168, 20 pp. Cheltenham. Edmond, J. B., Senn, T. L., Andrews, F. S. and Halfacre, R. G. (1975). Fundamentals of Horticulture. 4th ed. 560 pp. New York: McGraw-Hill Book Co. Farming and Wildlife Trust (1983). A hedgerow code of practice. 6 pp. Sandy Bedfordshire. Genstat 5 Committee (1988). Genstat 5 Reference Manual. Oxford Science Publications. 749 pp. Oxford: Clarendon Press. Hackett, W. P. (1985). Juvenility, maturation and rejuvenation in woody plants. Horticulture Review 7, 109–155. Klecka, W. R. (1980). Discriminant Analysis. Sage University Paper no. 19. London: Sage Publications. Kramer, P. J. and Kozlowski, T. T. (1979). Physiology of woody plants. 811 pp. London: Academic Press. Maclean, M. (1992). New hedges for the countryside. 276 pp. Ipswich Farming Press. Maggs, D. H. (1965). Dormant and summer pruning compared by pruning young apple trees once on a succession of dates. Journal of Horticultural Science 40, 249–265. Malden, W. H. (1899). Hedges and hedge-making. Journal of the Royal Agricultural Society 10, 87–117. Mika, A. (1986). Physiological responses of fruit trees to pruning. Horticultural Review 8, 337–378. Mitchell, P. L. (1989). Repollarding large neglected pollards; a review of current practice and results. Arboricultural Journal 13, 125–142. Rackham, O. (1980). Ancient woodland: its history, vegetation and uses in England. 402 pp. London: Edward Arnold. SAS Institute Inc. (1985). Users’ guide: Statistics. Version 5. Cary N. C. SAS Institute Inc., pp. 155–169. Semple, D. A., Dyson, J. and Godwin, R. J. (1994). Effect of mechanised cutting on the short term regrowth of hawthorn hedgerows. In Field margins: integrating agriculture and conservation. (N. Boatman, ed.) pp. 235–240. BCPC Monograph no. 58. Farnham: British Crop Protection Council. Whelon, D. (1994). The Hedgerow Incentive Scheme. In Hedgerow Management and Nature Conservation. (T. A. Watt and G. P. Buckley, eds) pp. 137–145. Wye: Wye College Press.