Information and Advisory Note Number 115 Back to menu
The effects of mammalian herbivores on natural regeneration of upland, native
woodland
1. Introduction
1.1 Semi-natural woodland and scrub cover about 4% of the Scottish landscape.
Grazing and browsing are major factors preventing, or restricting, the
regeneration of woody species on many of these areas on unenclosed hill land.
1.2 The two herbivore species that are considered to have the greatest impact
across Scotland are red deer and sheep, which have upland populations of about
350,000 and 2,350,000 respectively. Several other domestic and wild species such
as cattle, roe and sika deer, horses and ponies, goats, hares, rabbits and voles
can also affect rates of tree regeneration.
1.3 Under natural conditions, herbivore numbers would have fluctuated with
weather conditions and with predator numbers. Wild boar would also have
scarified the ground, creating regeneration niches. Conditions suitable for tree
regeneration were therefore likely to have occurred often enough to maintain
woodland cover across large parts of Scotland. Winter feeding of sheep and red
deer, land improvement and a lack of predators have all led to herbivore numbers
being kept continuously at artificially high levels.
2. Requirements for successful tree
regeneration and establishment
2.1 Successful natural tree regeneration
requires
• a source of viable seed or suckers;
• suitable temperatures, and adequate rainfall and light, to allow seedlings and
saplings to grow1;
• the presence of mycorrhizal fungi for some tree species;
• a suitable seed bed where roots can reach the mineral soil, nutrient levels
and soil moisture are sufficiently high and there is not too much competition
from other vegetation; and
• seed predation and browsing levels low enough to allow sufficient numbers of
saplings to reach a height where they are no longer susceptible to browsing.
Grazing and browsing directly affect the last two of these conditions.
2.2 All Scotland's dominant native tree species require relatively high light
conditions to establish (see 3.3) hence successful regeneration and
establishment happens either at the woodland edge or in gaps created by the
death of adults, or by felling. In theory, the correct combination of conditions
for regeneration and establishment need occur only once during the lifetime of
each adult tree for it to be replaced and for a woodland to survive. Normal
maximum lifespans for several native tree species in Scotland are given below.
Little is known about how the reproductive potential of different tree species
is affected by age and growing conditions. For this reason, it is hard to
predict how frequently successful regeneration and establishment events must
occur to ensure the survival of a particular woodland.
¹ There is no consistent distinction in the literature between a seedling and a
sapling. In this note a sapling is taken to be a young tree taller than about 25
cm and a seedling is one less than about 25 cm.
2.3 If grazing and browsing are very heavy, regeneration will be completely
suppressed but if less heavy, the density and species composition of surviving
saplings depends on
• the pre-browsed density and growth rate of seedlings and saplings;
• the severity of browsing on seedlings and saplings; and
• the responses of seedlings and saplings to being browsed.
An understanding of the factors affecting each of these will help in planning
appropriate grazing management for woodland regeneration.
3. The pre-browsed density and growth rate of seedlings and saplings
3.1 These are affected by the first four factors listed in section 2.1.
3.2 Availability of viable seeds depends on the abundance of seeds produced by
local seed sources, the distance to those sources, the presence of the agent of
dispersal and the viability of the seed. Abundance and viability of seed
produced by a tree can vary greatly from year to year depending particularly on
weather conditions. Both production and viability of seed often decline with
altitude. For wind-dispersed species, such as birch, pine, willow, alder and
ash, most seed falls within a distance of about twice the height of the parent
tree and there is little regeneration at more than 1 km, though small numbers of
birch seeds can travel to at least 3 km and those of pine to at least 7 km. Oak,
rowan, juniper, hazel and holly require an appropriate animal vector to be
dispersed beyond the canopy of the parent tree. In Britain, aspen reproduces
largely by suckering. The seeds of most tree species do not remain viable in the
soil for more than a year but birch seeds can survive for two to three years and
ash seeds survive for up to six years. Ash and rowan seeds take at least 18
months to germinate.
3.3 Seedling germination and survival may be affected by lack of light, not only
under an existing tree canopy but also where there is dense heather, bracken or
bramble cover. Pine, birch, alder and willow seedlings are all light demanding.
Oak seedlings can survive beneath a canopy for up to four years and ash will
also tolerate shade for the first few years but then becomes light demanding.
Rowan is tolerant of limited shade and can regenerate under a birch canopy
whereas birch itself cannot. Holly seedlings are shade-tolerant and holly can
reach 5-10 m in height underneath a woodland canopy. The high palatability of
rowan and holly probably accounts for the rarity of woodlands dominated by these
species.
3.4 Competition for nutrients and light from ground layer vegetation e.g. a
dense carpet of dog's mercury or wavy hair grass, will limit seedling survival.
Grazing helps to reduce the cover of ground layer vegetation.
3.5 Mycorrhizal fungi are found associated with almost all trees in long
standing woodlands. Mycorrhizae seem to improve the ability of trees to absorb
nutrients, especially phosphate, make them more resistant to infection and may
also improve water economy. They thus allow seedlings to grow more quickly and
seem to be essential where site conditions are particularly demanding e.g. at
the tree line. Mycorrhizal fungi may be absent from sites that have been without
tree cover for many years but they should quickly colonize any soil other than
peats or podzols.
3.6 A deep raw humus layer, or dense ground layer vegetation, can increase the
chances of seedlings drying out by preventing their roots gaining access to the
mineral soil.
3.7 Where conditions are good, and growth rates high, there is a greater chance
that saplings will get their leading shoot out of reach of large herbivores
before they are browsed. However, where soil and tree nutrient levels are high,
it is possible that there will be an increased chance of browsing (see 4.2.3).
4. The severity of browsing on seedlings and saplings
4.1 This depends on
• the species and condition of seedlings and saplings present;
• the nature of the ground layer vegetation
• the density and height of seedlings and saplings;
• the herbivore species; and
• the seasonality, intensity and spatial distribution of grazing and browsing.
4.2 Species and condition of seedlings and saplings present
4.2.1 Herbivores are selective feeders, their preferences being influenced by
several attributes of the foliage of different tree and shrub species. These are
thought to include digestibility, shoot biomass, fibre content, nutrient
concentrations, level of toxic secondary compounds and spininess.
4.2.2 In general, the relative preferences of red deer and sheep for the foliage
of different native tree species are given below.
4.2.3 All the attributes of a tree's foliage that determine its preference
ranking can vary throughout the year, with the age of the foliage and with
growing conditions. Digestibility tends to be higher, and fibre content lower,
in new, spring growth. Birch is seldom eaten by red deer when it is not in leaf
unless little other food is available. It is often heavily browsed at bud burst.
Nutrient concentrations in foliage can be higher if the tree is growing on more
fertile soils and there is evidence that deer and sheep selectively browse trees
growing on more nutrient-rich soils.
4.3 The nature of the ground layer vegetation
4.3.1 The preference shown by a herbivore for a particular tree species is
crucially dependent on what else is on offer. Pine and juniper are not favoured
in summer when other food is usually abundant because the leaves contain
terpenes, which are distasteful. Deer will, however, browse pine shoots around
the time of bud burst. Pine and juniper are eaten in winter when deciduous trees
and shrubs are bare of foliage and when other vegetation is of low digestibility
or is relatively inaccessible under snow. Leafless shoots and twigs are normally
not eaten unless there is a scarcity of other vegetation.
4.3.2 When seedlings are about the same height as the surrounding ground
vegetation, they will often be eaten along with it. Seedlings therefore have a
greater chance of survival if they are growing amongst a non-preferred
vegetation type. Thus seedlings growing amongst tall heather are less likely to
be browsed than those growing amongst short, grassy vegetation.
4.3.3 Most preferences are over-ridden by hunger. Red deer will browse pine in
summer or birch in winter if there is no alternative and, when pressed, all
herbivores may eat foliage or twigs or strip bark from species of trees which
they would otherwise avoid.
4.3.4 Heavy grazing on ground layer vegetation can reduce the availability of
good quality forage and lead to an increase in browsing rates on trees. When
grazing pressures are low, ground cover may be high and seedlings will be less
easily detected by herbivores until they emerge above the level of the
surrounding vegetation. Even then, if there is an abundant source of alternative
food, saplings will only be readily browsed by herbivores which have a high
preference for browse (goats, roe deer and, to some extent, red deer).
4.3.5 When ground layer vegetation is tall and abundant, field vole numbers are
likely to increase and they can also damage seedlings and saplings (see 5.1.5).
4.4 The density and height of seedlings and saplings
4.4.1 For a given herbivore density, the chances of achieving successful rates of
sapling establishment are likely to increase as the initial, pre-browsed,
density of saplings increases. However the density of established saplings may
not increase in direct proportion to any increase in the initial density of
saplings. Little work has been carried out on the form of this relationship so
it is currently not possible to predict the likely effect of changing the
initial abundance of saplings on final sapling density.
4.4.2 Seedlings and saplings which project above the level of the surrounding
vegetation are more likely to be browsed until they grow taller than the mouth
height of any herbivores present. At Beinn Eighe, browsing by red deer was found
to be most severe on trees of 1-2 m in height. Browsing by sheep is likely to be
most severe on shorter saplings. Maximum
browse heights for different herbivore species are given below.
5.1 The herbivore species
5.1.1 The main impact of browsing by sheep and red deer is at the seedling and
small sapling stage, when they clip leading, and lateral, shoots. Sheep of most
breeds, and cattle, show less of a tendency to browse than do red deer. However,
the more primitive breeds, such as Hebrideans, browse more readily and are used
for scrub control on some grassland and heathland nature reserves. Sheep are
also reputed to displace red deer from favoured areas hence a reduction in sheep
numbers may lead to an increase in deer browsing on tree seedlings and saplings
in these areas. Red and sika deer can strip bark from saplings and roe deer fray
them with their antlers. Bark stripping is more likely than fraying to remove
bark in a ring around the tree and therefore to kill it. Otherwise, within
woodland, roe and sika deer can have similar impacts to those of red deer.
5.1.2 Cattle and ponies can cause damage by eating seedlings and by trampling.
Ponies have a greater tendency to browse than do cattle. They also strip bark
from trees of ail sizes.
5.1.3 Feral goat populations are restricted to a small number of sites in
Scotland but, where they occur, they can have a major effect on tree
regeneration because of their high propensity to browse. They tend to browse
taller saplings than do sheep and can affect regeneration on cliff ledges or
boulder fields that are inaccessible to other herbivore species.
5.1.4 Mountain hares browse and bark strip saplings, as do rabbits, and both can
be particularly harmful when herbs and grasses are under snow cover. Rabbit
damage can occur throughout the year but tends to
increase in winter and early spring. Bark stripping is commonest in hard winters
and late springs when snow persists.
5.1.5 Field Voles can be significant browsers of tree seedlings and, where they
occur in high numbers, will often ring-bark saplings. One study found that voles
killed 73% of seedlings regenerating on open moor by eating the main shoots.
Wood mice and squirrels eat tree seeds and bank voles may eat small tree seeds.
5.2 The seasonality, spatial distribution and intensity of grazing and browsing
5.2.1 Because of the seasonal changes in the attractiveness of different species
of seedling and sapling and in the abundance and digestibility of the ground
layer vegetation, the same density of herbivores can have a very different
effect on regeneration at different times of year. In summer, when ground layer
vegetation is usually more abundant, saplings are less likely to be browsed than
in winter, when food is often in short supply. On the other hand, deciduous
saplings are more attractive in summer, when they have leaves.
5.2.2 Herbivores do not normally distribute themselves evenly across a landscape
and young trees will suffer a heavier browsing pressure when they grow within,
or adjacent to, heavily used areas. Forage quality and abundance and, in poor
weather conditions, shelter, are the major factors affecting the distribution of
large herbivores (See SNH l&A Note 47, The grazing behaviour of large herbivores
in the uplands). Thus, areas of nutritious grassland on good soil will always be
preferred and red deer tend to spend more time in at lower altitudes in winter
than in summer. The latter can lead to lower browsing pressures, and more
successful regeneration of young trees, at higher altitudes. For rabbits, hares
and voles, cover, such as long grass or scrub, which gives protection from
predators, is also a major factor affecting their distribution in the landscape.
5.2.3 Grazing and browsing pressure
obviously have a large effect on regeneration
success but numbers of successfully
regenerating trees do not necessarily keep
increasing as herbivore numbers decline to low
levels. This is because, at low grazing
pressures, the ground layer vegetation builds
up resulting in both fewer regeneration niches
and increased competition for soil nutrients
and light.
6. The responses of saplings to being
browsed
6.1 The ability of saplings to survive and
resprout after browsing varies with
• the age and species of tree or shrub;
• the proportion of the plant removed;
• the type of damage;
• the plant's growing environment; and
• the season in which the browsing occurs.
6.2 Mature trees are relatively resistant even if their lower branches are
completely removed up to the grazing height of the herbivores. Young trees are
more at risk as a greater proportion of total foliage area may be removed. Trees
up to 2 m tall are vulnerable and saplings of less than 50 cm are often killed.
6.3 Even so, birch, rowan and pine can all tolerate removal of up to half of the
current year's growth. The effect of removal of more than this differs with
species. Pine seems least tolerant of damage. Two year old pine usually dies if
eaten back into the previous season's wood whereas most birch and all rowan are
likely to survive. Other broadleaves are probably similarly resistant.
6.4 Trees which are tolerant of browsing may be kept small above ground but
develop an extensive root system, enabling them to respond rapidly if browsing
pressure is reduced or removed. In general, recovery is likely to be better on
sites with favourable growing conditions including good soil nutrient levels and
climatic conditions although, under these conditions, saplings may have to
compete with a vigorous growth of ground layer vegetation.
6.5 Browsing on seedlings and saplings in early summer, when nutrient
concentrations in the shoots and leaves are high, is likely to be more
detrimental than browsing in late summer or winter, when most of the nutrients
have been re-allocated to stems and roots. This applies to birch and may also
apply to other broadleaved species. Coniferous species store nutrients in their
leaves as well.
7. What herbivore densities will allow tree
regeneration?
7.1 Given the range of factors involved, and that large herbivores do not
usually distribute themselves evenly across the range available
to them, it is not surprising that there are no clear thresholds for the numbers
of deer or sheep at which adequate regeneration may occur. The few studies which
have been carried out in this area can, however, give us an idea of the range of
herbivore densities that might be appropriate. It should be borne in mind that,
even at low overall densities, grazing pressures in preferred areas can be very
high.
7.2 Hester, Mitchell and Kirby (1996) studied the effects of sheep grazing and
browsing on regeneration in an upland wood in Cumbria at three stocking
densities and in either summer or winter. The study looked at germination and
survival rates of seedlings and saplings of downy birch, rowan and ash.
7.3 Although germination rates were higher under heavier grazing pressures
{2.1-3.8 sheep ha¯¹), probably because of the higher number of regeneration
niches, there were still large numbers of seedlings appearing even in the most
lightly grazed plots {0.6-1.2 sheep ha¯¹).
7.4 Seedling survival was higher at lower grazing intensities, and in
summer-grazed plots. The latter was probably because summer grazing prevented
the build-up of ground vegetation that would compete with seedlings.
7.5 At high levels of summer grazing (2.1-3.4 sheep ha¯¹), almost all the
saplings were browsed whereas at low levels (0.6-0.9 sheep ha¯¹), only 25% of
the saplings were browsed. In winter, browsing rates on saplings were generally
lower, with 40% of saplings remaining undamaged even at 3.8 sheep ha¯¹. The
higher browsing rates in summer are likely to be because the saplings had leaves
at this time.
7.6 A parallel study of browsing of planted saplings also found a decreasing
impact as sheep densities decreased, apparently because the sheep browsed
saplings less frequently when more ground-layer vegetation was available.
However, planted saplings were more heavily browsed on average than naturally
regenerated saplings, which is commonly the case.
7.7 The winter browsing of naturally regenerated, leafless birch saplings at
Glen Feshie has been found to be linearly related to local densities of red deer
(Miller & Cummins 1998) with the browsing rate ranging from
about 5% of birch saplings being browsed per week at about 200 red deer km¯² (2
ha¯¹) to < 1 % of saplings browsed per week at less than about 25 red
deer km¯²
(0.25 ha¯¹). Deer numbers on low ground are often over 150 km¯² in winter.
7.8 Several attempts have been made to estimate the density of red deer that
might allow successful woodland regeneration. Fraser Darling (1947) guessed at 6
km¯², Holloway (1967) found partial regeneration at 4 km¯² and unimpeded
regeneration at 2 km² and Mitchell, Staines & Welch (1977) found regeneration
occurring at about 10 km¯². The latter was at a site with a relatively high
density of saplings of around 4,000 saplings ha¯¹. Ratcliffe (1988) found that
in forests there was a heavy impact on most broadleaved trees at red deer
densities of about 7 deer km¯².
7.9 At Creag Meagaidh, following culling of red deer, regeneration of birch from
previously suppressed saplings appears to be taking place at densities of 8 red
deer km¯². At Abernethy Forest densities of up to 12 km¯² in 1988/9 were reduced
by culling to about 5 km¯² by 1993. Roe were also culled but the proportion of
the population this represented is not known. A 20% increase in the number of
established seedlings and saplings was recorded over this time, with rowan
accounting for 68% of new saplings. Most of the rest were pine (the first
species to respond), with some birch and willow.
7.10 Densities of herbivores low enough to
permit regeneration may still be high enough to
affect the composition of ground layer
vegetation. At about 5 red deer km¯², Ratcliffe
(1988) suggested that plant diversity in forests
would be reduced.
8. Planning regeneration
8.1 Thus, some general conclusions on the impact of grazing on tree regeneration
in the uplands are as follows.
• There will normally be insufficient seeds of wind-dispersed species to
initiate much regeneration at distances of more than about 1 km from existing
woods. Seed densities fall off rapidly with distance from the parent tree.
• Ground conditions may be unsuitable for regeneration, especially if there is a
deep raw litter layer or dense vegetation.
• Grazing and trampling may assist initial establishment of seedlings by
suppressing
competing vegetation, creating regeneration niches and limiting field vole
abundance.
• Grazing and browsing impact on regenerating trees is likely to be concentrated
in the most heavily used areas such as those providing high quality grassland or
winter shelter.
• Seedlings are less likely to be browsed where the surrounding vegetation
partly conceals them but if the vegetation is dense there will be more
competition with the seedlings for resources.
• The greatest grazing and browsing pressure will fall on the most preferred
species present.
• There may be less browsing impact on saplings where sufficient, preferred
foods are available elsewhere.
• Stopping grazing (e.g. by fencing) may lead to increases in grass growth,
field vole numbers and seedling destruction by voles.
• Some trees are relatively tolerant and will recover if pressure reduces or
ceases.
• Tolerance and recovery will be better on sites where the growing conditions
are more favourable but herbivores may selectively browse those on more
nutrient-rich sites.
• Deer are more likely to browse saplings than are sheep or cattle.
8.2 This suggests that total exclusion of grazing may be a mistake where there
is a lack of germination and /or low densities of seedlings. Where adequate
numbers of seedlings or saplings already exist, very low grazing pressures may
be the best means of allowing these young trees to establish. Herbivores are an
important component of natural ecosystems, so the preferred policy is to achieve
regeneration without the need for their total removal.
8.3 Options include:
• Where there are sufficient numbers of suppressed seedlings or saplings already
present, the elimination, or drastic reduction, of grazing and browsing pressure
will allow the young trees to 'get away'. This regime should be continued until
the saplings are tall enough to withstand browsing.
• Where there are insufficient numbers of seedlings or saplings present, the
instigation of a low and/or periodic grazing regime may be appropriate. This
could be brought about by a reduction in overall stock, or wild herbivore,
densities or by periodic, or seasonal, exclusion of stock by
fencing. Animal numbers might have to be progressively reduced until the optimum
balance between the positive and negative effects of herbivores is reached. The
regime should be continued until the required density of saplings becomes tall
enough to withstand browsing. This is likely to be about the lifetime of a
conventional fence. It should, however, be borne in mind that, if fences are
used, they can have a number of important side effects (see the SNH I & A Note
59 Fences and upland conservation management).
• Replacement of red deer with sheep may lead to lower browsing rates on
saplings.
• If ground layer vegetation is dense, mechanical scarification may be needed to
increase the number of regeneration niches. Pigs and cattle can also be used for
this purpose.
• Felling, or coppicing, of selected trees will increase the amount of light
available and may increase the density of seedlings and/or suckers of
light-demanding species.
• If the supply of seed is limiting, it may be necessary to increase the density
of trees nearby either by encouraging the spread of woodland from existing sites
or by planting trees to provide a seed source. This is a long term strategy and,
if regeneration is wanted in the short term, direct planting will be necessary.
9. Further reading
1. Beaumont, D., Dugan, D., Evans, G. & Taylor, S. (1995) Deer management and
tree regeneration in the RSPB reserve at Abernethy Forest In: Aldhous, J.R.
(Ed). Our Pinewood Heritage. Forestry Commission, RSPB, SNH.
2. Fraser Darling, F.F. (1947) Natural history in the Highlands and Islands.
Collins, London.
3. Harmer, R. (1995) Natural regeneration of
broadleaved trees in Britain: III. Germination
and establishment. Forestry, 68, 1-9.
4. Hester, A.J. & Miller, G.R. (1995) Scrub and Woodland Regeneration: Prospects
for the Future. In: Thompson, D.B.A., Hester, A.J. & Usher M.B. (Eds). Heaths
and Moorland: Cultural Landscapes. HMSO, Edinburgh.
5. Hester, A.J., Mitchell, F.J.G. & Kirby, K.J. (1996) Effects of season and
intensity of sheep grazing on tree regeneration in a British upland woodland.
Forest Ecology and Management, 88,99-106.
6. Hodge, S. & Pepper, H. (1998) The prevention of mammal damage to trees in
woodland. Forestry Commission Practice Note No. 3.
7. Holloway, C.W. (1967) The effects of red deer and other animals on the
natural regeneration of Scots pine. Ph.D. thesis. University of Aberdeen.
8. Kinnaird, J.W. (1974) Effect of site conditions on the regeneration of birch
(Betula pendula Roth and B. pubescens Ehrh.). Journal of Ecology, 62, 467-472.
9. Miller, G.R. & Cummins, R.P. (1998) Browsing by red deer on naturally
regenerated birch and juniper saplings on wintering ground at Glen Feshie.
Scottish Forestry, 52,138-145.
10. Miller, G.R., Cummins, R.P. & Hester, A.J. (1998) Red deer and woodland
regeneration in the Cairngorms. Scottish Forestry, 52, 14-20.
11. Miller, G.R., Kinnaird, J.W. & Cummins, R.P (1982) Liability of saplings to
browsing on a red deer range in the Scottish Highlands. Journal of Applied
Ecology, 19, 941-951.
12. Mitchell, B., Staines, B.W. & Welch, D. (1977) Ecology of red deer.
Institute of Terrestrial Ecology, Cambridge.
13. Pepper, H. (1998) The prevention of rabbit damage to trees in woodland.
Forestry Commission Practice Note No. 2.
14. Ratciiffe, P.R. (1988) The management of red deer populations resident in
upland forests. In: D.C. Jardine (Ed). Wildlife Management in Forests.
Proceedings of a symposium held by the Institute of Foresters.
15. Staines, B.W., Balharry, R. & Welch, D. (1995) Moorland management and
impacts of red deer. In: Thompson, D.B.A., Hester, A.J. and Usher M.B. (Eds).
Heaths and Moorland: Cultural Landscapes. HMSO, Edinburgh
10. Authors
John Andrews
Andrews Ward Associates
17 West Perry
HUNTINGDON
Cambs
PE18 0BX
Gordon Miller
Gilbank
School Hill
BANCHORY
Aberdeenshire
AB31 5TQ
Helen Armstrong
Woodland Ecology Branch
Forest Research
Northern Research Station
ROSLIN
Midlothian
EH25 9SY
11. Acknowledgements
Comments were kindly provided by Alison Hester (Macaulay Land Use Research
Institute), Richard Thompson, Brenda Mayle, Jonathan Humphrey and Robin Gill
(Forest Research), Kate Holl and Roland Stiven (SNH), Steve Palmer (Institute of
Terrestrial Ecology), Lars Edenius (Swedish University of Agricultural
Sciences), Pete Reynolds (Capreolus consultants), Fiona Stewart and Dick
Balharry.
12. Contacts for advice and information
Kate Holl
Land Use Group
Scottish Natural Heritage
2, Anderson Place
EDINBURGH
EH6 5NP
Tel: 0131447 4784
Duncan Stone
Land Use Group
Scottish Natural Heritage
27 Ardconnel Terrace
INVERNESS
IV2 3AE
Tel: 01463 712221
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