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Information and Advisory Note Number 121 Back to menu
The woods of the western Highlands are among the richest
habitats for bryophytes and lichens in Europe. They are especially important for
species restricted to humid, equable (oceanic) parts of the world. These oceanic
species evidently need a humid, equable climate, and many are associated with a
long historical continuity of woodland cover or appear to have a low tolerance
of pollution. The main habitats of oceanic bryophytes are shaded rocks and
banks, and the trunks and branches of trees and shrubs. The lichens of interest
grow mainly on trees and shrubs, and favour generally better-lit conditions than
do the oceanic bryophytes, although there is considerable overlap of habitat
between both groups.
These notes give a brief description of the effects of different types of
woodland management on the bryophyte and lichen flora, and recommendations for
management which is likely to be of benefit to the bryophyte and lichen
interest. The management recommendations are necessarily generalised and it is
therefore advisable to survey the bryophyte and lichen flora of the site
concerned in order to evaluate the nature conservation interest and make a more
accurate assessment of the potential effects of woodland management.
In lightly grazed or ungrazed woodland with lush ground vegetation and some tree
or shrub regeneration the high degree of shade, shelter and humidity appears to
be more favourable to oceanic bryophytes and lichens than the less sheltered and
humid conditions in woodland
where grazing has greatly reduced the height of the ground vegetation and the
abundance of tree and shrub regeneration. In woodland with a long history of
heavy grazing the vertical zonation of bryophytes and lichens appears to be
lowered so that some species are unnaturally restricted to habitats near the
ground and might therefore be at risk from overshading by tall ground vegetation
and dense tree/shrub regeneration (especially holly) if grazing is eliminated in
future. Species particularly at risk here include relatively light-demanding
woodland plants such as the oceanic liverworts Adelanthus decipiens, Plagiochila
atlantica and Radula voluta, the oceanic moss Sematophyllum micans and various
lichens of the Lobarion community. However, this problem of potential
overshading appears to be more serious south of the Highlands, where grazing has
been particularly severe, the climate is not quite as humid, and there is more
holly (one of the most potentially overshading species in ungrazed or
lightly-grazed woodland).
The idea that grazing is important in maintaining the bryological interest of
western woods by keeping ground vegetation short originates largely from the
results of reduction or elimination of grazing in some previously long-grazed
woods in north Wales. There can be prominent carpets of common bryophytes in
short, heavily-grazed woodland ground vegetation, but bryophytes are often just
as abundant and diverse among tall, ungrazed or lightly-grazed heather Calluna
vulgaris and bilberry Vaccinium myrtillus in lightly-grazed or ungrazed
woodland. Bryophytes can be sparse among dense greater woodrush Luzula
sylvatica, although on steep rocky slopes where the woodrush is discontinuous
there can be a well-developed bryophyte layer.
Tree and shrub regeneration can be beneficial to the bryophyte and lichen flora
in providing a greater age diversity of trees and shrubs, and therefore a wider
range of habitats for epiphytes. Tall, ungrazed or minimally grazed Calluna
vulgaris and Vaccinium myrtillus provides extra habitat for epiphytic bryophytes
and lichens. The epiphytes on Calluna and Vaccinium are mainly common species
such as the liverwort Frullania tamarisci and the lichen Hypogymnia physodes,
but in some western woods the epiphytic bryophyte flora on Calluna includes
oceanic species such as the liverworts Colura calyptrifolia and Frullania
teneriffae and the moss Ulota drummondii, and the rare non-oceanic moss Ulota
coarctata.
In woodland with a high density of sheep or cattle, bryophytes and lichens might
be affected adversely by excessive trampling or by animals rubbing against trees
and rocks. A high stocking density might also cause local nutrient enrichment;
this would probably affect terrestrial bryophytes most, but runoff might also
affect bryophytes on rocks.
Recommendations
• Light grazing by deer, cattle or sheep, or (at least temporarily) no grazing,
is preferable to heavy grazing because: (a) it helps to maintain shelter and
humidity for desiccation-sensitive oceanic bryophytes and lichens, (b) it allows
the development of an epiphytic flora on dwarf shrubs, and (c) it reduces the
risk of animal damage (trampling, nutrient enrichment and rubbing epiphytes off
tree trunks) to the bryophyte and lichen flora. It is also more natural, and the
tree/shrub regeneration ensures long-term survival of the woodland.
• If some important lichens or bryophytes are found to be at risk from very
dense tree/shrub regeneration developing nearby (e.g. dense young holly growing
up around trees with a rich Lobarion lichen community in a wood where the
Lobarion is scarce), it might be justifiable to cut back some of the developing
regeneration.
Shade-demanding species such as many oceanic bryophytes have clearly been lost
as a result of felling, the main reasons evidently being the reduction in shade,
shelter and humidity, and the direct loss of habitat for epiphytes. Some
populations of shade-demanding species may have survived
episodes of small-scale selective felling during which there was always some
shade cast by trees which were left standing. Shade-demanding species on
N-facing slopes might be more tolerant of limited felling than those on S-facing
slopes. Some oceanic bryophytes such as the liverworts Plagiochila atlantica and
Adelanthus decipiens appear to need a delicate balance of at least moderate (but
not excessive) light intensity, relatively warm temperatures and high humidity;
this could be the reason why in the western Highlands they are strongly
associated with Ancient woodland.
Epiphytes of interest growing on trees selected for felling might disappear from
the site along with their tree hosts. Some populations of these species might
survive for a short time on fallen wood left on the ground, but this habitat
rapidly becomes unfavourable for such plants.
Felling usually has a negative effect by removing habitat or letting in too much
light. Less often it has a positive effect by preventing overshading. The
epiphytic flora is likely to suffer more if the age structure and/or species
composition of the tree and shrub layers are reduced. During felling operations,
falling trees could potentially damage the bryophyte and lichen flora by
scraping plants off their substrates but this is probably of less concern than
the increased light/exposure and the removal of habitat for epiphytes.
Bryophytes and lichens lost through felling might be slow to recolonize
subsequently, or may even fail to recolonize. The rate of recolonization is
likely to be influenced by the abundance of the species in the vicinity, its
ability to reproduce and the time taken for new tree and shrub growth to provide
suitable habitat. Some of the commoner oceanic bryophytes such as the moss Ulota
phyllantha can colonize 10-15 year old shoots of ash and hazel in situations
where those epiphytes are plentiful on surrounding trees and shrubs, but rich
Lobarion and other communities evidently take longer to develop. The timescale
is likely to be generally longer on slower-growing trees such as oak.
Recommendations
• Felling and thinning are best kept to a minimum.
• Large clearfells should be avoided.
• Trees with rich epiphytic bryophyte or lichen floras should not be felled.
• Felling should be avoided in the vicinity of rich oceanic bryophyte floras on
rocks, banks or trees because the increase in light and exposure is likely to
cause significant bryological losses.
• Felling, even on a small scale, should be carried out with particular care on
southerly aspects where it can lead to very dramatic changes in microclimate
which are likely to be unfavourable to drought-sensitive oceanic bryophytes.
• The species composition and age structure of the tree and shrub layers should
not be reduced in diversity by felling operations. (It does not follow that
homogeneous woodland needs diversification by selective felling; some
homogeneity can be an expected part of naturally developing woodland).
• After felling, as much dead wood should be left on the ground as possible.
Coppicing is generally damaging to the bryophyte and lichen flora. It evidently
causes losses of bryophytes and lichens because of (a) increased light and
exposure, and (b) a reduction in the amount of habitat (especially old bark) for
epiphytes. The degree to which the increased light and exposure affects
bryophytes and lichens is likely to vary depending on factors such as slope
aspect, rockiness, the nature of the coppicing and details of the bryophyte and
lichen flora. The effect is likely to be less severe on northerly aspects, on
rocky slopes and in places where some trees and shrubs are left uncoppiced. The
recolonization of epiphytes on coppiced trees and shrubs is evidently very
variable, depending on such factors as (a) the species of tree or shrub, (b) the
type of epiphytic flora one might expect on it given the habitat conditions at
the site (for example bryophytes are generally more abundant in shadier places,
and lichens more abundant where better lit) and (c) the nature of the coppicing
(length of coppice rotation and whether coppiced in large blocks or as scattered
individuals with other trees and shrubs left nearby). Old hazel coppice with a
dense growth of thin stems (up to about 40 years old) and no old thick stems is
generally poorer for epiphytes than hazel with old stems and little or no
evidence of coppicing. While the older hazel stems are clearly important for
many
epiphytes (e.g. Lobarion lichens) some epiphytes (e.g. Graphidion lichens and
the liverworts Frullania dilatata and Cololejeunea minutissima) are found mainly
on young or middle-aged stems. Uncoppiced hazels with both old and young stems
generally have richer epiphytic floras than coppiced hazels with relatively
even-aged stems. The greater range of stem inclination (erect to more or less
horizontal) on old uncoppiced hazels gives a greater range of microhabitats (as
regards drainage and shade) for epiphytes than on coppiced hazel on which most
stems are erect or slightly inclined. Some oceanic liverworts such as
Plagiochila exigua and small Lejeuneaceae are often most frequent on middle-aged
stems, but appear to favour hazels which also have thick old stems.
Some epiphytic bryophytes and lichens may persist for a while on cut twigs and
branches which are left on the ground, but as these twigs and branches decay
their flora will change and most oceanic bryophytes and Lobarion lichens are
likely to disappear.
Recommendations
• From a viewpoint of naturalness, coppicing should not become an extensive,
major feature of woodland. Small scale selective coppicing is preferable to
larger block coppicing.
• Trees and shrubs with rich epiphytic bryophyte or lichen floras should not be
coppiced.
• Coppicing should be avoided in the vicinity of rich oceanic bryophyte floras
on rocks, banks or trees because the increase in light and exposure is likely to
cause significant bryological losses.
• Coppicing, even on a small scale, should be carried out with particular care
on southerly aspects where it can lead to very dramatic changes in microclimate
which are likely to be unfavourable to drought-sensitive oceanic bryophytes.
• Careful selection of trees or shrubs for coppicing is probably more important
than the length of coppice rotations. Thus coppicing should be in areas where
the trees and shrubs to be coppiced, and their adjacent habitats, are of
relatively low bryological and lichenological interest. If coppicing is carried
out in more sensitive places, even a long coppice rotation cannot
be expected to maintain the lower plant interest. Selection of trees and shrubs
for coppicing should be done in such a way as to maintain the range of variation
in the species composition and age structure of the canopy and shrub layers.
• If coppicing takes place, as much dead
wood should be left on the ground as
possible.
The effects on the bryophyte and lichen flora are very varied and depend on
factors such as the conditions which existed before planting, the density and
species of trees and shrubs planted, the shadiness of the location and the
proximity to propagules of potential bryophyte and lichen colonisers. There is
likely to be an increase (often from a zero starting point in previously
unwooded situations) in the abundance of epiphytes. Details of colonizing
epiphytes are likely to depend largely on the species of tree or shrub hosts; a
good range of native tree and shrub species is likely to lead to a more diverse
epiphytic flora (although in some situations such as native pine woodland,
planting of only one or two species might better reflect the natural situation).
In woodland with a rich bryophyte or lichen flora the planting of trees should
be viewed with caution, for example if there are good populations of uncommon
light-demanding bryophytes or lichens. In new plantations of native trees and
shrubs, colonizing bryophytes and lichens will probably be mainly common species
for many years; rarer species would be expected to take a long time to appear if
at all, because they evidently have limited powers of spread.
Recommendations
• Planting can be favourable for bryophytes and lichens in areas which have
become unwooded, although natural regeneration is preferable.
Within
well-managed existing native woodland planting of native trees is usually
unnecessary unless natural regeneration is so slow that important bryophytes or
lichens are being put at risk from overexposure to sunlight and wind.
• The planted tree and shrub species should be appropriate to the site, and
should if possible be of local provenance.
• Any existing standing or fallen dead wood
on the site is best left in place.
Rhododendron
The overshading effect of rhododendron combined with its dense, acidic leaf
litter causes severe impoverishment of the bryophyte and lichen flora. Usually,
all that is left on the ground and rocks is a sparse cover of a limited range of
common mosses. Rhododendron is poor for epiphytes, although the Sub-Atlantic
liverwort Metzgeria temperata can grow on it in some quantity. Where
rhododendron grows more thinly in bryophyte-rich woodland a limited number of
calcifuge, shade-tolerant bryophytes (including the oceanic liverworts Metzgeria
temperata, Lejeunea ulicina and Lepidozia cupressina) and ferns (including the
filmy fern Hymenophyllum tunbrigense) appear able to show a slight increase in
abundance before finally dying out when the rhododendron becomes too dense. The
rate of recovery of the bryophyte and lichen flora after rhododendron clearance
is evidently slow. If some of the most shade-tolerant oceanic bryophytes had
increased in abundance in response to the shade cast by rhododendron, their
populations may suffer a decline (a return to the "normal" situation) after the
rhododendron is removed. Any damage done to bryophytes and lichens on trees and
rocks during the process of rhododendron clearance is likely to be of relatively
little significance because in these places the bryophyte and lichen flora has
probably already become severely impoverished by the rhododendron.
Recommendations
• Clearly, rhododendron is best removed from native deciduous woodland because
of its disastrous overshading and acidifying effects not only on the bryophyte
and lichen flora but on the total woodland ecosystem.
• Woodland with dense rhododendron will have already lost most of its nature
conservation interest, so it is important to keep intact woodland free from
rhododendron invasion.
• In dense stands of rhododendron the difference between manual and mechanical
cutting is probably insignificant in relation to the already impoverished nature
of the woodland. With mechanical clearance
there is a risk of some damage to native trees (e.g. some lower branches can get
broken off). It is desirable to attempt to minimise such damage. It might also
be desirable to remove at least some of the finer cut rhododendron wood which
carpets the ground after mechanical clearance, otherwise the dense layer of dead
wood might inhibit the establishment and growth of ground vegetation. With
manual cutting, subsequent spraying of chemicals onto the stumps should be done
carefully so that it does not affect rocks and the bases of native trees and
shrubs.
Other non-native trees and shrubs
The bryophyte and lichen flora on the ground and rocks becomes impoverished in
the deep shade and acidic leaf litter beneath dense non-native conifers. Species
that can tolerate these conditions are mainly common mosses. Where conifers have
been planted within native deciduous woodland the epiphytic flora on native
trees and shrubs tends to become more mossy, and there can be an increased
abundance of certain common bryophytes. Most lichens are evidently intolerant of
the increased shade beneath dense conifers. Among the bryophytes which can
remain or perhaps even increase slightly in abundance in the vicinity of planted
conifers are the calcifuge oceanic bryophytes Plagiochila punctata, Leptoscyphus
cuneifolius, Metzgeria temperata (all on trees), Scapania gracilis, Plagiochila
spinulosa and Anastrepta orcadensis (all on rocks and banks). Beech and sycamore
have a less severe effect than alien conifers on the bryophyte and lichen flora
but their shade and leaf litter may cause some losses of species on rocks.
However, these trees can support some bryophytes and lichens of interest.
Sycamore is one of the favoured tree hosts for lichens of the Lobarion
community, and can provide important habitats for these lichens in woodland in
which most trees are acid-barked species such as birch (on which the Lobarion is
usually absent or only poorly developed). Very locally, beech can support a rich
Lobarion lichen flora, as can some other non-native trees such as lime and horse
chestnut. In general, none of these tree species are notable for oceanic
bryophytes.
Recommendations
• Planting non-native trees within native woodland is generally more detrimental
than beneficial to the bryophyte and lichen flora.
• Retention of some existing mature non-native trees is often desirable - for
example, old sycamore, beech, lime and horse chestnut with rich Lobarion lichen
floras.
• It is desirable to remove dense stands of non-native conifers or beech if they
make up a significant proportion of the wood.
The most obvious effect on bryophytes and lichens is the removal of habitat for
species such as the liverworts Nowellia curvifolia, Riccardia palmata, Scapania
umbrosa, Cephalozia catenulata, Odontoschisma denudatum and Calypogeia suecica.
Removal of dead wood might therefore reduce the diversity of the lower plant
flora, especially where habitat diversity is limited (e.g. few or no rocks) so
that the flora on logs make a relatively important contribution to the total
bryophyte and lichen flora of the site. Removal of dead wood from the ground
might lower humidity levels in nearby rock habitats; this could adversely affect
some saxicolous oceanic bryophytes by making them more prone to desiccation.
Removal of dead wood might increase the area available to herbivores, resulting
in a decrease in the diversity of ground vegetation.
Recommendation
• It is best to leave standing and fallen dead
wood in place.
Abundant bracken does not seem to have a significant effect on the bryophyte and
lichen floras of west Highland woods. The shade cast by bracken mostly affects
the woodland floor which is not rich in bryophytes and lichens, but tall bracken
can cast shade on tree bases and low rocks. Because the effect of shade is
greatest in summer when the climate is driest, some shading of rocks and tree
bases by bracken might be of benefit to oceanic bryophytes.
Recommendations
• If spraying with herbicides such as Asulam
or Roundup, this should be done with care,
on still days and avoiding spraying on rocks
and tree bases with important bryophyte
and lichen floras.
• Introduction of cattle might help to control
bracken.
A general downward shift of vegetation zones on rocks and banks alongside rivers
and streams might mean that some habitats previously dominated by riparian
bryophytes and lichens become colonised by common non-riparian species. This
could lead to some losses of the riparian oceanic mosses Hyocomium armoricum,
Isothecium holtii, Fissidens polyphyllus and Rhynchostegium lusitanicum and the
Sub-Atlantic moss Fontinalis squamosa. On rock faces c.0-3m above water level a
reduction in the effects of spray and splash, leading to lowered levels of
atmospheric humidity, might cause losses of the hygrophilous oceanic liverworts
Lejeunea patens, L lamacerina, Aphanolejeunea microscopica, Drepanolejeunea
hamatifolia, Harpalejeunea ovata, Colura calyptrifolia, Radula aquilegia, R.
voluta, R. carringtonii, Jubula hutchinsiae, Acrobolbus wilsonii, Plagiochila
exigua, Lophocolea fragrans and Dumortiera hirsuta.
Recommendation
• It is important that hydro-electric power
schemes are not sited on streams with rich
oceanic bryophyte floras, otherwise the
general reduction in humidity could cause
serious bryological losses.
Averis, A.B.G. (in press). The effects of woodland management on bryophytes and
lichens in the western Highlands. Scottish Natural Heritage Research, Survey and
Monitoring Report No 171.
Hodgetts, N.G. (1996). The conservation of lower plants in woodland. Joint
Nature Conservation Committee, Peterborough.
Ratcliffe, D.A. (1968). An Ecological Account of Atlantic Bryophytes in the
British Isles. New Phytol. 67, 365-439.
Rodwell, J.S. (Ed.) (1991a). British Plant Communities. Volume 1 - Woodlands and
Scrub. Cambridge University Press, Cambridge.
A B G Averis
2 Traprain Cottages
HADDINGTON East Lothian
EH41 4PY
12.1 Stephen D Ward
12.2 Species Group
12.3 Advisory Services
12.4 Scottish Natural Heritage
12.5 2 Anderson Place
12.6 EDINBURGH
12.7 EH6 5NP
12.8 Telephone: 0131-446 2431
Kate Holl
Land and Water Use and Management Unit
Advisory Services
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP
Telephone: 0131-446 2426
N G Hodgetts
Joint Nature Conservation Committee
Monkstone House
City Road
PETERBOROUGH
PE1 1JY
Tel: 01733 562626
Summary of main effects of woodland management on bryophytes and lichens in west
Highland woods