Background
Species-rich
hedges have an approved Habitat Action Plan under the UK Biodiversity Action Plan,
reflecting their importance for wildlife conservation and landscape.
Hedgerows are a noted semi-natural feature of lowland agricultural landscapes
in Britain, and are important as refugia for plant diversity and support many
taxa of fauna, including farmland bird species.
The cultural importance of hedges is reflected in their protection under
the UK Hedgerow Regulations. At the two
extremes of over-management and lack of management, reduced diversity may occur
in hedgerows.
British hedges have been divided into eleven categories
according to the dominant shrub species present. The four major types are a) hawthorn dominant,
b) mixed hawthorn, c) mixed hazel dominant and d) blackthorn predominant.
There are also a number of other types, such as elm and gorse hedges, some
of which are specific to different areas, for example the beech hedges on banks
in Devon. Current management of hedges is based on trimming
with flail cutters. The traditional techniques of laying are largely abandoned
on the basis of cost, though hedge laying societies maintain the skill in many
parts of the country. Work by FWAG has
produced some useful data on the techniques that may be used for hedge management.
In particular, a reduction in frequency of trimming to biennial cutting
has benefits for wildlife. These results require further confirmation on different
hedge types in different conditions. The timing of cutting will certainly affect
fruiting in shrubs. Arable farmers try
to trim hedges after crop harvest in early autumn, when access to field margins
is easy. Grassland farmers tend to trim hedges earlier in the year.
In previous times, hedge management was a winter operation, when farm labour
was available. Advice on the management of hedges has been available for many
years. An important aim of this project was to update
this advice to include aspects of the best maintenance and to increase farmland
biodiversity. Although there is a wide variety of advisory material available
to farmers and agricultural advisors, there is relatively little quantified, scientific
information on the impacts of current hedge management on farmland wildlife.
Hedgerow
Protection
The
Hedgerow Regulations were introduced in England and Wales in 1997 in order to
protect this characteristic element of the British countryside. Landowners are no longer allowed to remove hedges, except with the
approval of the local authority. If the
hedge is of value, as defined under the regulations, for wildlife, history or
landscape, then the hedge will be protected. It is estimated that at least one fifth of the hedges in England
and Wales fall under the definitions and are protected. Currently, the regulations are being reviewed.
The following is extracted from the DEFRA website:
http://www.defra.gov.uk/wildlife-countryside/rddteam/rddthf.htm
Under
the Hedgerows Regulations 1997, it is against the law to remove most countryside
hedges without first getting the permission of your local council.
A
leaflet - The Hedgerows Regulations: Your
Question Answered - provides a brief summary of the law (available from DETR
Free Literature). More detailed guidance is in The Hedgerows Regulations 1997: A Guide to
the Law and Good Practice (£5.50 from DETR Publication Sales Centre). Neither
of these publications are currently on the DEFRA website.
The
Regulations have been reviewed by a group of experts, who reported on their ideas
for improving the system of hedgerow protection in 1998. A free summary
of their report, published by the then Department for Environment, Transport and
the Regions, has been published and is available online (or by calling 0870 1226
236). Full report, priced £10, can be obtained from Unit 21, Goldthorpe Industrial
Estate, Rotherham, S63 9BL, telephone 01709 891318.
Results
of research into the review
proposals, published in 1999, will help to inform decisions on how the current
Regulations should be revised. Hard copies of this research report cost £15 from
Unit 21, Goldthorpe Industrial Estate, Rotherham, S63 9BL, telephone 01709 891318.
Outline
and experimental design
Many
previous studies, have focussed on correlative studies between different hedgerows,
e.g. between bird species and hedgerow variables. However, this project uses an
experimental approach in order to reveal the specific role of cutting management,
and has investigated the impacts on hedge growth and fruiting, the associated
herbaceous flora and the insect fauna. Manipulated and replicated treatments of
cutting frequency (annual, biennial and triennial) and timing (September or February)
were implemented, with independent experiments on representative hedges across
southern Britain (Table 1). All sites
except one had a history of annual flail cutting.
The remaining site (Leicestershire) had a hawthorn hedge that had been
uncut for four years prior to the experiment.
Two hedge regeneration techniques, coppicing and laying, were included
in the treatments at this site. Subsidiary
experiments on methods of regenerating the herbaceous flora of species-poor hedge
bases were implemented on arable and grass hedge bases.
Table 1. Details of
hedge study sites established in 1996 and 1997.
| Site | Hedge type | Farm type | No. of plots | Frequency treatments |
|
|
|
|
|
|
|
Hampshire |
Mixed hawthorn |
Arable |
21 |
1/1;
1/2; 1/3; uncut |
|
Buckinghamshire |
Hawthorn/ dog rose |
Arable |
18 |
1/1;
1/2; 1/3 |
|
Norfolk |
Hawthorn/ maple |
Arable |
15 |
1/1;
1/2; uncut |
|
Leicestershire |
Hawthorn dominant |
Arable |
15 |
1/1;
layed; coppiced; uncut |
|
Somerset1 |
Blackthorn dominant |
Grassland |
15 |
1/1;
1/2; uncut |
|
Powys, Wales |
Mixed hazel |
Grassland |
15 |
1/1;
1/2; uncut |
| Exmoor | Beech dominant | Grassland | 15 | 1/1;
1/2; uncut |
1/1 = annually cut; 1/2 = biennially cut; 1/3 = triennially cut; 1abandoned in 1998.
Research findings and management implications
Hedgerow
berries
Berries
are an important winter food supply for a wide range of farmland bird and small
mammal species. Therefore increasing the abundance and availability of berries
in hedges is a desirable conservation target. Larger wintering birds will feed
on a range of berry types in hedges, but some specialisation amongst different
bird species also exists. Most hawthorn berries are generally removed by early
December; other species may be available longer.
In
woody hedge species, such as hawthorn (Crataegus monogyna), berries are produced
on second year growth. Thus hedges that
are left uncut for one year or more produce many more berries than annually-trimmed
hedges. In contrast to some current management advice, this was found to be equally
so for both September and February cutting times. Hedge cutting in September removes
formed berries directly, whereas cutting annually in late winter reduces flowering
and thus largely prevents autumn fruit-set. As there are differences in when berries
are produced depending on the time of the cut (September vs. February), hedges
cut on a biennial cycle must be cut at the same time each year.
In
general the longer the hedge is left, the more berries are produced on woody hedge
plants. However, in hedge sections left unmanaged for three years (or more), berry
production in the year after eventual cutting is particularly low. Coppicing,
and to a significantly lesser extent laying, reduces hawthorn berry production
in the immediate following years. However, these are long-term management techniques
and berry numbers would be expected to increase with time.
Bramble
(Rubus fruticosus) and other deciduous climbing
plants are less affected by cutting frequency, and there is evidence that regular
management (including annual trimming) is beneficial for fruit production. September
cutting significantly reduces availability of these berries in the autumn. Management
practice may need to reflect such responses where bramble is dominant, and/or
where the conservation of a particular bird species that feeds on these types
of fruit is targeted.
Fruiting
in woody species is highly variable between years and along individual hedges.
Summer flowering species such as bramble are much more consistent in the amount
of berries produced each year and per area of plant.
Rosa canina (dog rose) also has low variability in berry production,
but generally acts like woody shrub species in its response to frequency and timing
of cutting. Variation is likely to be a combination of genetic control and environmental
influences. Therefore local provenance hedge material may present berry producing
hedge plants best adapted to regional conditions. Mixed species hedgerows will
provide more consistent berry availability each year, but not necessarily highest
numbers of berries per length of hedge.
·
Avoid large-scale annual trimming
as this drastically reduces the availability of berries on woody hedge species.
·
February cutting is not necessarily
better for berry production than September cutting.
·
Hedges uncut for longer produce
more berries on shrubby plant species.
·
Berry production in bramble is
enhanced by regular management (at least biennial cutting)
·
Overall results suggest rotational
management is required to ensure some berry availability in hedgerows each year.
Hedgerow
flora
i) Hedge shrubs
There
is no consistent evidence that timing of cut affects hedge size.
However, hedge dimensions, measured as height x width, increase with time
since last cut. Differences in growth are apparent between hedge types. In this
study, the mixed species hedge in Hampshire increased most in the first year after
cessation of cutting whereas the beech hedge in Exmoor increased more steadily
over the study period (Fig.1.). The Norfolk arable hedge showed most rapid growth,
which may reflect fertiliser inputs into the hedge base. At Leicestershire, coppiced
and laid plots reached approximately the same dimensions as annually trimmed plots
after 4 years. The generally wet spring and summer of 2000 encouraged extra prolific
hedge growth in that year, highlighting the role of climatic conditions.
Fig
1. Growth in cross-section of uncut plots
of the Exmoor hedge, 1996-2000.
There
is evidence that hedge size continues to increase after 10+ years, but that the
rate of increase declines. Other studies
have shown that hedge size is an important determinant for the number of species
of birds utilising hedgerows. It has been estimated, as a general trend, that
one additional species of bird will be encouraged for every 2m2 increase
in hedge cross-sectional area. On average from the data in this study, this would
equate to 2 species accruing per year left uncut. However, the relationship between
bird diversity and hedge size may level out in the largest hedges.
The
amount of new vegetative growth produced by hedge shrubs each year increases with
increasing cutting frequency, which may have implications for the herbivorous
insect fauna (see below).
In
mixed species hedges, there are also differences in the response to cutting between
the different hedge shrub species. Hawthorn and dog rose regrow strongly immediately
after cutting, and appear to be tolerant of regular cutting, whereas other species
such as spindle and buckthorn are positively responsive to being left uncut for
longer (2+ years). Blackthorn grows extremely vigorously each year if left uncut,
and may encroach on other hedgerow plant species and adjacent farmland. Scrambling
hedge species such as bramble and black bryony decreased in abundance with time
left uncut.
·
Decreased cutting frequency increases
the size of hedges and their suitability as a habitat for birds.---except for
linnet and yellowhammer
·
Geographical and hedge type differences
suggest that optimal cutting regime may vary. E.g. an upland beech hedge could
be left uncut for longer than a lowland hawthorn hedge.
·
Mixed species hedges in particular
should be cut biennially or less often in order to encourage non-cutting tolerant
species, such as spindle.
·
Bramble and other scrambling type
hedge plants respond well to regular management.
ii) Hedge-base flora
Hedge
base habitats generally contained very high botanical diversity, higher than in
the shrubby part of the hedge. No significant effects of hedge cutting on botanical
diversity in the hedge base are apparent. Analyses of individual herbaceous species may
reveal more subtle effects associated with the change in width of the hedge relative
to the verge, and therefore the light climate, but no consistent effects have
been established. At Leicestershire, coppicing and laying both increased the species
diversity of hedge base plants, presumably in response to ground disturbance and
increased light levels. There were no differences between these treatments and
control plots by the end of the experiment.
Analysis
of the composition in hedge-base floras between sites shows that different hedgerows
have distinct hedge-base plant communities. Differences appear to be largely determined
by adjacent land-use. Arable hedgerows are less diverse than grassland ones, although
uncropped field margins next to the hedgerow may increase botanical richness in
the hedge base.
·
Hedge base habitats can be botanically
rich.
·
Coppicing and laying can increase
plant diversity in the short-term.
·
Adjacent land-use and field operations
are important in determining plant composition of hedge bases.
Hedgerow
Invertebrates
The
high diversity and variability in invertebrate communities within hedgerows, as
well as between years, makes detecting clear management effects more difficult
than with plants. Differences of response to cutting management are apparent between
different hedges, invertebrate groups and time of year. However, some notable
patterns are discernible, with important implications.
i)
Summer distributions
Plant
diversity is an important determinant of invertebrate diversity. This is evident
at the farm scale, where the high botanical diversity in hedges equates to high
invertebrate diversity compared to other agricultural habitats. This study also
showed that plant and invertebrate diversity are positively related between different
hedge types. However, at the plot-scale within hedges this relationship is less
apparent, presumably due to mobility of many species and the distribution of individual
host plants. Importantly the hedge-base flora is found to be related to the species
composition of some insects, suggesting that
in order to enhance or maintain the biological diversity of hedgerows, both the
hedge and the hedge base need to be managed sympathetically. Restorative hedge
management that increases plant diversity, such as gapping-up with new shrub species
or hedge base restoration (see below), will thus have positive knock-on effects
for invertebrate diversity.
Although
increases in invertebrate diversity with decreasing management intensity were
not found in the study, annual cutting in a previously uncut hedge site (at Leicestershire)
reduced the number of invertebrate taxa in the first year. Furthermore, the abundance
of individual invertebrate groups was widely, but not uniformly, negatively affected
by regular cutting. This was particularly noticeable for flying insects, such
as Diptera (flies) and Hymenoptera (parasitic wasps), in early summer, which may
reflect both a reduced flowering of woody hedge shrubs and/or diminished wind
shelter in annually trimmed hedge sections
Timing
of hedge cutting is also important. Cutting in February reduces the numbers of
insect larvae found in early summer compared to September trimming; notably the
larvae of Lepidoptera (moths and butterflies) and to some extent Diptera (true
flies) (Fig. 2). Late winter cutting is likely to entail
the removal of insect eggs laid during the autumn (after a September cut), and
will have important consequences for nesting birds that preferentially forage
for such insect larvae in hedges at this time of year.
Fig.
2. Mean number of hedge invertebrates
sampled per plot (Exmoor site, May 1998).
In
mid-summer there is contrasting evidence that annual cutting increases the abundance
of some invertebrate types, notably herbivores closely associated with the hedge
vegetation, such as Coleoptera (beetles), Heteroptera (plant bugs) and Collembola
(springtails). This may be a response to the stimulated new hedge growth following
cutting providing a more suitable habitat and food source for such invertebrate
species groups. In some cases, annually cutting in February appears to be particularly
beneficial for invertebrates.
Hedge
laying is particularly beneficial for invertebrate communities, maintaining high
taxonomic diversity as well as high total invertebrate abundance. This hedgerow
rejuvenation technique may be represent a combination of stimulated new shrub
growth and good physical structure. In contrast, coppicing, with a drastic reduction
in vegetation, appears to be detrimental to hedgerow invertebrates, at least initially.
·
Encourage plant diversity in the
hedge and hedge base, either through direct management (e.g. gapping up) or indirectly
through sensitive field management.
·
Avoid wholesale annual hedge cutting
as this is detrimental to a range of invertebrate groups.
·
Late winter cutting can reduce
insect larval populations in spring. Do not undertake all cutting management at
this time.
·
Active hedge management can increase
populations of herbivorous insects; adopt an appropriate rotational management
regime.
·
Based on the Leicestershire site,
hedge laying can maintain high diversity and abundance of associated invertebrates.
·
Hedgerow features need to be managed
as an integrated unit.
[
If cut every other Sept, not many berries. North/ South hedges?]
ii) Overwintering invertebrates
Apart
from some species that overwinter as eggs, most invertebrates occur in the base
of hedges during winter. These include many groups found in the shrub layer later
in the year, as well as those that migrate to adjacent farmland in spring. Hedge
cutting has little discernible effect on overwintering invertebrates, although
at Leicestershire overall abundance was reduced by coppicing in the short term.
Vegetation biomass is positively correlated the numbers of invertebrates overwintering
in hedge bases, therefore direct (e.g. autumn cutting of hedge base vegetation)
and indirect (e.g. herbicide drift) management will have detrimental effects on
the quality of this habitat for overwintering invertebrates. There is evidence
that some overwintering invertebrate groups are spatially limited within individual
fields, therefore simultaneous hedge base management at the field scale is not
appropriate.
·
Avoid routine cutting of hedge-base
vegetation, or cut in spring where applicable.
·
Always leave some areas of hedge
base unmanaged per field as a refuge for recolonisation of hedge shrubs and adjacent
habitats by invertebrates. Similarly, avoid widespread hedge coppicing in any
one year.
Regenerating
herbaceous hedge bases
In
degraded arable hedge bases, sowing seed mixtures to re-establish a diverse perennial
flora can be successful. In grassland, resowing hedge-bases with seed
mixtures is less successful and natural regeneration techniques are probably more
appropriate. Reducing fertiliser contamination
of the hedge base can also maintain plant diversity. Where annual weeds, such as cleavers (Galium aparine) and barren brome (Bromus sterilis), dominate the herbaceous
flora, selective herbicides can effectively reduce their abundance. Negative relationships
between abundance of weed species and associated insect diversity suggest that
control of such species in hedge bases may have an ecological as well as an agronomic
basis.
As
agricultural practices are a crucial determinant of the field boundary ecology,
even where a diverse habitat is successfully restored sensitive applications of
field operations will be necessary to prevent continued degradation of such habitats
in agricultural landscapes.
·
Restoring botanical diversity
in degraded arable hedge-bases is best achieved through resowing with wild flower
seed mixtures and use of selective herbicides.
·
In grassland sites, encouraging
natural regeneration is more effective than resowing.
·
Control of annual weeds in hedge
bases is important to maintain the wildlife value of hedges.
Economic
considerations
Although
there is evidence of yield reductions at the edges of fields, assessment of yields
of adjacent crops did not show significant effects of hedge management within
the scale and timing of this study. Hedge
management represents a cost for farmers. Averaged over the whole cutting cycle,
measures of cutting time showed that total time taken was 40-50% lower in hedges
left uncut for two or three years compared to annual cutting. Thus there are cost
advantages in relaxing hedge cutting from annual trimming. Not cutting the hedge-base
vegetation each year will also have an associated management cost saving
Overall summary of findings and recommendations
·
Hedge trimming
using flail cutters should be relaxed to biennial and triennial cycles where safety
and sightlines are not compromised
·
Mixed species
hedges should be encouraged, either through new planting, gapping up or appropriate
field edge management.
·
A mixture of
cutting regimes should be planned on each farm, including at least some September
cutting to allow insect larvae to survive in spring.
·
Where possible, allow some sections
of hedge (e.g. hedge nodes) to be left uncut for longer time periods (10+ years)
to allow extra berry production and to support specialist species dependent on
these features (e.g. greater horseshoe bat).
·
Hedge laying should be encouraged
as a regeneration technique that enhances farm wildlife. In contrast, coppicing
has short-term detrimental effects on wildlife.
·
Hedge bases are an important habitat
for plant and invertebrate diversity, and consequently other wildlife. There are
important ecological interactions between hedge and hedge base, such that an integrated
approach to hedgerow management should be adopted.
·
Cutting of hedge base vegetation,
particularly in autumn, should be avoided where possible. Rotational cutting can
be adopted where necessary, ensuring undisturbed habitat refuges.
·
Degraded hedge bases may be enhanced
by restorative techniques such as resowing. However, minimising ongoing damage
through sensitive field operations remains a priority.
·
Reduced cutting regimes are likely
to result in financial savings.
Farm-scale
hedge management
The
application of beneficial hedge management at the farm-scale has not been directly
investigated, although a number of useful indications can be highlighted. In particular,
the diversity of responses to management between wildlife groups, especially within
invertebrate communities, suggests a rotational approach to farm hedge management.
Each farm will need a tailored management plan based on the type and location
of hedgerow, the specific wildlife objectives alongside practical and financial
considerations. In general, bi or triennial cutting regimes, with allowance for
some sections of hedge left uncut for longer periods, should be achievable on
most farms.
The
importance of second year growth of shrubby hedge plants for berry production
and egg laying insects from this study, suggest two other approaches to the application
of mixed cutting management regimes (given below). These may be practically applied
to hedges where some cutting management is required each year. Furthermore, the
high degree of small-scale spatial variation in invertebrate distributions within
hedgerows indicates that such management procedures will allow refuges from unfavourable
management events.
i)
Incremental cutting. This allows some increase in the hedge shrubs
each year, thus preserving some second year woody growth. Hedges would retain
a ‘tidy’ appearance and would only need to be severely cut infrequently.
ii)
Differential top and sides cutting. If cut in alternate years,
this would leave part of the hedge uncut each year, allowing a diversity in management
condition across the farm.
Further
studies of these techniques under different conditions are indicated.
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