Download Re-defining native woodland

Re-defining native woodland
N. BROWN
Oxford Forestry Institute, South Parks Road, Oxford, OX1 3RB, England
Summary
The term 'native' is widely used to refer to a small number of British tree species and naturally
regenerating woodland. This paper proposes that the concept is rooted in an out-dated understanding of vegetation dynamics. Consequently, its definition is arbitrary and ecologically flawed,
and should be replaced by terms that make clear the different but important aspects of biogeographical origin, local distinctiveness and conservation value.
Introduction
Growing concern in Britain over the decline of
the original woodland cover to less than 1.5 per
cent of the land surface has provoked a recent
revision of Forestry Commission policy. The
new policy seeks to promote the environmental
and social benefits of forests. One of its principal objectives has been to protect and expand
the area of native woodland. The policy of
expansion of the area of native woodland has
been put into practice in two ways. First,
changes to the Woodland Grant Scheme
(Forestry Commission, 1993) have promoted the
planting of native species. Second, the Commission has made widely available detailed information on their ecology and silviculture. As a
consequence of these measures there has been a
dramatic increase in planting of native species
for conservation and amenity purposes. While
this is to be welcomed the policy needs to be
examined critically to ensure that it is achieving
sensible results. Many foresters believe that the
promotion of native species has gone beyond
sound conservation to the point of arbitrary discrimination against exotic species and may have
<} Institute of Chartered Foresters, 1997
reinforced the unhelpful distinction between
production forests and forests for conservation
and amenity.
In this review the author examines the ways
that the term 'native' has been used by foresters
and ecologists and suggests alternatives which
may help overcome the ambiguity and apparent
arbitrariness of the term.
Conventionally, the word native implies
belonging to a natural or original state which
has not been created artificially. The word
native has been applied to both species and
woodland. When applied to a species in Britain,
it implies that it is a natural post-glacial colonist
of these islands and has not been introduced by
humans. When applied to woodland the definition varies with the region of Britain. In England and Wales the term native woodland has
been used to refer to communities of naturally
occurring species regenerating by natural
means, even where the woodland is not of natural origin. However, Stevens and Carlisle
(1959) defined the native pine woods of Scotland as the naturally regenerated remnants of
the post-glacial pine forest. They proposed that
Forestry. Vol. 70, No. 3. 1997
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woodland was only genuinely native if it was
descended from one generation to another by
natural means. Following this lead the term is
now commonly used in Scotland to refer to
woodland that is descended directly, without
human interference, from those trees which
spread over the British Isles after the last glaciation. In England and Wales the terms natural or
semi-natural are more commonly used for this
purpose.
Native species
The identification of native species is based on
the assumption that it is possible to disentangle
the effects of human interference from biogeography.
It is now frequently accepted that native tree
species are those which established themselves
in Britain after the most recent glacial period,
approximately 11 000 years ago, and before significant human civilization. It may be assumed
that only a very few tree species continued to
occupy these isles throughout the last glacial
period. As the botanical slate was all but wiped
clean the question of which of the many types
of forest that have occupied these lands in the
past is used as the base-line for assessing what
is natural can be avoided. Genera such as Tsuga
and Pterocarya, though present in Britain in previous interglacial periods can now be conveniently classified as exotics.
In Britain, climatic history has enabled us to
avoid the difficult issue of how far we should go
back into biogeographical history to find our
native woodland. However, we still face the
thorny issue of how close we dare approach the
modern day before we decide that human influence is too pervasive and woodland composition
no longer natural. Conventionally, civilized
human populations are not considered to be
natural. Therefore the advent of human civilization in Britain during the Mesolithic has often
been used to define the point at which the vegetation ceased to be natural and any further
species immigrations became ineligible for
native status. It is, however, a far from simple
biogeographic task to detect which tree species
were natural colonists of the British Isles and
which were introduced by humans. It is likely
that many species may have colonized and been
introduced to Britain on many occasions.
While in a forestry context it is common to
refer to species as native to Britain, the country
does not function as an ecological unit. Species
that are technically defined as native to Britain
may be incapable of naturally colonizing many
areas. More importantly there may be considerable intraspecific local adaptation and consequently important genetic variation in local
populations of a widespread species (Soutar and
Spencer, 1991; Forrest, 1992). The use of this
term at a country scale reflects a bizarrely
nationalistic view of biogeography and has neither ecological nor practical value. There may,
however, be considerable value in being able to
be able to identify whether a particular species
is native to a given site within the country. For
example, one of the requirements for grants for
new woodland under the Woodland Grant
Scheme is that 'where nature conservation is an
important aim, this will normally mean that you
should use the species which are, or might have
been, native to the site' (Forestry Authority,
1993).
It is far from an easy task to determine
whether or not a species belongs naturally at
any particular site. The natural range of a
species can no longer be easily delineated in
Britain. One approach might be to use the original-natural distribution of species (sensu
Peterken, 1996) or its putative distribution
before humans had a significant influence. While
it has been possible to reconstruct the preMesolithic biogeography of many tree species
there is considerable uncertainty over the extent
to which the biogeography of many of those
species which did colonize Britain before the
Mesolithic is without human influence. There is
speculation that humans may have played a significant role in altering the speed or magnitude
of the post-glacial tree invasions and the subsequent dispersal across the country. For example,
Rackham (1986) has proposed that Mesolithic
communities of humans may have been responsible for the great abundance of hazel found
during this period. Indeed, the extinction of
many large herbivores, hastened by human
hunting, may have been a necessary precondition for the post-glacial re-establishment of
woodland. It is clear that while our putative
RE-DEFINING NATIVE WOODLAND
native species may have colonized one or more
parts of the British Isles naturally, their subsequent biogeography and history have been profoundly influenced by human activity. Hence,
post-glacial pre-Mesolithic vegetation distribution may not be the original-natural pattern it is
purported to be. There may not be a natural
pattern at all.
It is likely that the present-natural distribution of most species today (that which would
prevail if humans had not become a significant
influence) would be very different from a putative original-natural distribution (Peterken,
1996). The distribution of vegetation is very
sensitive to climate, especially where a species is
growing at the limit of its environmental tolerance (Cannell et al., 1989). For example, the
decline of lime (Tilia cordata) over the last 5000
years is likely to have been induced by a slight
but progressive cooling of the UK climate. The
range of any plant species is constantly shifting
and it will temporarily overlap with parts of any
number of countries as the vagaries of a
dynamic environment allow. With the exception
of the Caledonian race of Pinus sylvestris and
possibly Betula pubescens Ehrh., B. pendula
Roth., Populus tremula L., and Sorbus aucuparia L. none of our so-called native trees has
occupied Britain for longer than ten thousand
years. They are all recent invaders from Europe.
Indeed, many are pioneer species with wide ecological tolerance and remarkable geographic
ranges. The present day distribution of aspen
(Populus tremula L.) for example, stretches from
Norway to North Africa (Worrell, 1995). These
species are ecological nomads and do not have
a fixed geographical range but invade wherever
chance creates an opportunity and environmental conditions are suitable. Ingrouille (1995)
suggests that accidental events and influences
which are not necessarily predictable may have
been most influential in determining the pattern
of our original-natural vegetation. Plant species
did not re-invade Britain as predictable wave
fronts but colonized from any number of unpredictable foci where they had fortuitously established.
The modern fragmentation of British woodland cover and increase in tree planting have
obscured the present-natural tree species distributions. It is very difficult to predict what their
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natural biogeography would now be. Similarly,
it is impossible to do more than speculate as to
which of the European species of tree which
failed to colonize any part of Britain naturally
before the Mesolithic would subsequently have
arrived without human assistance. Nor can we
predict accurately what the geography of their
subsequent invasion might have been.
Native woodland
The concept of native woodland attaches great
value to naturalness. The most rigorous definition of native woodland implies that it is possible to identify components of woodland
structure and composition which have been
inherited from the original-natural forest cover.
A more modern usage of the term still places
great value on natural structure and composition and the process of natural regeneration.
The relationship between the original-natural,
or prehistoric wildwood and the fragments of
semi-natural woodland which still exist today is
a complex one. In the first half of the twentieth
century ecologists believed that, unless disrupted
and altered by humans, ecosystems would
invariably develop along well defined pathways
to a stable, self-sustaining climax community.
The natural structure and composition of this
community was believed to be determined by
the environment alone (Weaver and Clements,
1938). The climax community in which
Clementsian succession terminates was viewed
as permanent and stable since it was in total
harmony with a stable habitat. The concepts of
native and climax plant communities share considerable conceptual ground. Recent Forestry
Authority recommendations for creating new
native woodlands in Britain (Rodwell and Patterson, 1994) encourage foresters to create 'the
kind of climax vegetation that would develop .
. . if succession was allowed to take its full
course'. A similar view is reflected in Peterken's
(1996) description of potential-naturalness as
'the state which would develop if people's influence were completely removed and the resulting
succession were finished in a single instant. This
hypothetical state expresses our understanding
of the current potential of the native species on
a site under the prevailing climate.' The climax
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community is seen as natural and its structure
and composition are determined by the environment.
It is now believed that environmental determinism and the climax community are unrealistic models. Disturbance is an inherent part of all
ecosystems. Where the life-span of major organisms in a community exceeds the frequency of
disturbance then community composition will
bear little relation to the potential environmental climax of a site. This is certainly the case
for woodland where the structure and composition of vegetation reflects a continuous process
of response to and recovery from disturbance.
Since trees are typically very long lived organisms, disturbance events of a magnitude
sufficient to influence their survival and reproduction will occur with a frequency well within
their life spans. A rare weather event, such as
the hurricane which hit southern Britain in 1987,
may occur only once in many decades but may
have an effect on tree survival that has ramifications for many tree generations. Peterken and
Mountford (1996) report how the 1976 drought
has had a profound influence on the structure
and successional trajectory of Lady Park Wood
in Gwent over the last 16 years. As a consequence the composition of natural tree communities may be extremely dynamic from one
generation to the next. History is the primary
force sculpturing woodland composition.
An ecosystem is highly dynamic and cannot
be preserved in the same way that our cultural
heritage may. Chance patterns of dispersal and
survival will determine the pool of species available. Chance weather events, disease, changes in
predator pressure and human use will all alter
the way woodland regenerates. For example,
Jones (1952) attributes the even age of many
British beech woods to be the consequence of
prolific natural regeneration in very rare warm
years. The relative competitive status of different species of tree is constantly shifting as disturbance and a fluctuating forest environment
give advantage first to one species and then to
another. Present patterns are the product of
unique historic events that were neither predictable nor likely to be repeated. Harvey (1995)
concludes that its is neither desirable nor possible to manage nature in order to maintain the
status quo. Changes in species composition
should be accepted as a natural trend. The
'post-glacial pre-Mesolithic' definition of native
woodland attempts to fossilize nature. It fails to
recognize that the composition of any plant
community is continuously changing. Invasions
and extinctions are not only natural but are the
very essence of woodland ecology. Recommendations such as those of Rodwell and Patterson
(1994) that foresters should aspire to re-create
native climax woodland may simply re-inforce
an ecological myth.
Clearly the structure and composition of present-day native woodland is not a relic of a past
natural climax community. But it is also
unlikely to be a reliable indication of what wildwood might have been like today in the absence
of human influence (present-natural sensu
Peterken, 1996). Our so-called native woodland
is, by its nature, a community in transition.
Many of the 35 or so species which were naturally able to colonize this island are invasive
species. They have an ecology that is typical of
the early arrivals after disturbance. The socalled native British tree flora is an early successional one and it is highly improbable that in the
absence of human disturbance it would have
remained as a stable community up until the
present day. Late successional species would
almost certainly have followed these early
colonists had forest fragmentation not brought
their advance to a premature end. Beech (Fagus
sylvatica), hornbeam (Carpinus betulus), lime
(Tilia cordata and T. platyphyllos) and yew
(Taxus baccata) are the few truly shade tolerant,
late successional trees in the native British flora.
Beech is believed by some to have naturally
invaded parts of southern Britain and is only
regarded as native south of a line from the Wash
to South Wales (Rodwell and Patterson, 1994).
Hornbeam is a hardy species of wide ecological
range (Savill, 1991) but is deemed to be native
to the south-east of Britain alone. The putative
native ranges of these species are highly unlikely
to be their present-natural distributions. As
Sprugel (1991) has emphasized, the past provides an equivocal guide to what the present
might have been without humans.
The fact that woodland may not have been
subject to direct management or exploitation
does not mean that it is free from important
human influence. Humans have had profound
RE-DEFINING NATIVE WOODLAND
indirect effects on the structure and composition
of even protected natural areas. The current
protected status of much native woodland in
Britain may itself mean that trees are no longer
exposed to the same disturbance regime that
gave rise to their present structure and composition (Hobbs and Huenneke, 1992). The natural levels of wild herbivores in British
woodland are unknown. Populations of deer
continue to increase to levels where natural
regeneration of many native species is impossible. Other herbivores which would have
inflicted different types of pressure on tree populations such as beaver (Caster fiber Linn.) and
wild boar (Sus scrofa Linn.) have gone extinct
(Kirby et al., 1994). Many important processes
of human induced environmental change now
act at a landscape scale and even protected forest areas are subject to their influence. Changes
in the composition of the atmosphere due to
pollution are already known to have had a profound effect on natural plant communities
(Woodin and Farmer, 1993). It is possible that if
climate change continues our native woodland
may become unsuited to the prevailing climatic
conditions (Billington and Pelham, 1991). The
UK Department of the Environment has concluded that the likely increase in mean annual
temperatures of 0.5°C over the next decade will
bring about noticeable changes in plant and animal distributions. There will be significant
movement of species northwards at a rate of at
least 50-80 km per decade (DoE, 1996). The
number of species in the UK is likely to increase
due to an increased rate of immigration. In the
face of these ubiquitous processes of human
induced change it is untenable to use the concept of native to imply that any present day
British woodlands have a structure or composition that is largely a function of natural
processes.
The traditional view of the climax ecosystem
is that once human disruption ceases, nature
will take over and succession will slowly return
community composition to its original-natural
state. This model has been replaced by a view
that the trajectory of succession is determined
by the starting conditions and that even within
a single environment different starting points
will lead to very different outcomes. Succession
is therefore a chaotic rather than a deterministic
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process. Past management of British woodland
has not been obliterated by the passage of time
but has played a central role in determining the
path of future natural succession. Spencer (1996)
has demonstrated that in the New Forest past
exploitation has had an impact on composition
that has persisted for many centuries due to
major changes in stand dynamics and ecology.
During the seventeenth and eighteenth centuries
the forest was heavily exploited for oak leading
to a major shift to dominance by beech. Beech
trees are now reaching the end of their natural
life span and are prone to catastrophic disruption from drought and windblow. This in turn
has profound consequences for the present day
forest disturbance regime and the nature of tree
regeneration. Forest composition has not
returned to an environmentally determined climax condition. The ramifications of past events
continue to determine the future.
Nature: a powerful myth
The concept of native species depends on the
assumption that it is possible to determine
whether or not a species belongs naturally at a
particular site. Native woodland has been
defined as a community of naturally occurring
species regenerating by natural means. Naturalness is central to the value attached to native
species and woodland. This review has sought
to demonstrate that naturalness is an abitrary
quality. Peterken (1996) suggests that we should
regard naturalness as a continuous variable such
that a forest might be classified as being more or
less natural. This review has striven to demonstrate that natural process and human influence
should not be separated. Humans have had a
diverse range of impacts on many ecosystem
processes many of which it is no longer possible
to quantify. There is a suspicion that a classification of naturalness such as that proposed by
Peterken (1996) will rank only conspicuous
effects as unnatural.
All British woodland reflects the intimate
interplay of both natural and human influences.
Not only is it naive to believe that the two
forces may be disentangled but such an
approach denies the very real biological and historical interest in understanding their relation-
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ship. While conservation biologists clearly
recognize the pervasiveness of human influence
in British woodland there is a powerful culture
of.valuing what is perceived as natural over and
above human influenced systems.
Cronon (1995) claims that attempts to
exclude human influence from the concept of
nature is a product of the very social history
that it seeks to deny. He suggests that a consumer society which is divorced from the country as a place for production has created a myth
that true nature can only occur when humans
are not there to despoil it. In this case that myth
is the Clementsian climax forest. This vision of
nature appeals because it provides a standard
against which the failings of a modern society
can be judged and is a central tenet of the environmental movement. A romantic urban-based
wilderness fantasy has no place for human
beings to make a living. It is dangerous because
it leaves no place for discovering what a sustainable human place in nature might be. Furthermore it sets an arbitrarily high standard
against which cultural and productive landscapes are regarded as inferior. As both Peterken
(1996) and Rackham (1986) have pointed out
much of the value for nature conservation of
ancient woodland may lie in a detailed historical understanding of human impacts. Much of
our woodland may be locally distinctive in both
structure and composition as a consequence of
human use. The grove of wind-torn sycamores
sheltering a Pennine farmhouse are a perfect
example. These cultural landscapes may well be
unique to Britain but their importance is
reduced by a forest policy in which native
woodlands are prioritized for conservation and
amenity values.
A need for a new definition
The concept of native woodland as it is
presently used in British forestry confuses the
separate issues of local adaptation, local distinctiveness and species biogeography. The recent
development of classifications of native woodlands (Dunlop, 1994; Forestry Commission,
1994) which measure nativeness on the basis of
whether woods are natural or planted and composed of appropriate species of local genotype
reflects the fact that the term is insufficiently
precise to distinguish important woodland
types.
The term native is misleading as it creates
arbitrary and ecologically unsound distinctions.
The biogeography and cultural history of a
species should be of academic and silvicultural
interest not a reason for discrimination. We
should adopt a more sophisticated approach to
our use and conservation of our tree flora, promoting and conserving woodland for more
rationally defined values. For example, there is
obvious conservation value in protecting the
ancient Caledonian pine forests. However, that
value is not a consequence of the fact that the
species qualifies for the definition 'native' but
because of their' awesome beauty, their habitat
value, and the unique genetic structure of the
Scottish population.
Ecotype
The ecological term 'ecotype' is used to describe
a population adapted to local environmental circumstances. It may be composed of trees that
are genetically distinct or that have been managed in a way that has created a unique or
important structure. Thus it is entirely possible
for an introduced species to develop a local ecotype. While it is likely that much of the beech
woodland in the Chilterns has been planted, the
local ecotype has a distinctive white timber and
is integral to the landscape.
Locally distinctive
There is clearly considerable value in being able
to identify woodland which is locally distinctive.
Such woodland is composed of trees that are an
intimate part of the local landscape, regardless
of whether or not they are native. Sycamore
trees for example, are an integral part of a bleak
Pennine farm landscape, as are Scots pines in
Breckland. The campaign group Common
Ground has promoted the role of trees, particularly apple trees, in bestowing important local
character, patterns of planting may give distinctive character, and create an essential component of the cultural landscape.
RE-DEFINING NATIVE WOODLAND
Natural colonist
There is both conservation value and scientific
interest in distinguishing the natural colonists in
any ecosystem from introduced species. The
issue of scale is important in describing the colonization process. Many tree species have been
introduced to this country but have subsequently colonized large areas naturally. On the
other hand all of the post-glacial, pre-Mesolithic
natural colonists of the British Isles have been
introduced to a significant part of their present
day range.
Conclusions
There would seem to be no rational, objective
criterion by which native woodland in Britain
can be distinguished from non-native woodland.
British woodland comprises a continuum of naturalness, from the ancient descendants of the
prehistoric wildwood to modern, intensive plantations. It is composed of species that range
from those recently introduced from other parts
of the world to those that have occupied parts
of this island since prehistoric times. The
decision about where, along these continua the
line is drawn between native and non-native is
arbitrary and ecologically unsound. It has been
open to a wide variety of interpretations for different purposes. The use of the term native has
confused the important issues of biogeographic
origin, local distinctiveness and conservation
value. It is time that a more precise forestry
made clear the important difference. British
forestry needs to shake off a concept that is jingoistic and rooted in an outdated understanding
of vegetation dynamics.
Acknowledgements
I am grateful to Keith Kirby, Peter Savill, Steve Jennings, Mary Stockdale and Dan Bebber for comments. This paper is partially based on ideas
presented at Institute of Chartered Foresters Discussion Meeting on Native and Non-native in British
Forestry, Warwick, 31 March-2 April 1995.
References
Billington, H.L. and Pelham, J. 1991 Genetic variation in the date of budburst in Scottish birch pop-
197
ulations: implications for climate change. Fund.
Eco. 5(3), 403-409.
Cannell, M.G.R., Grace, J. and Booth, A. 1989 Possible impacts of climatic warming on trees and
forests in the United Kingdom: a review. Forestry
62(4), 337-364.
Cronon, W. 1995 The trouble with wilderness; or,
getting back to the wrong nature. In Cronon, W.
(ed.) Uncommon Ground: Towards Re-itwenting
Nature. W.W. Norton, New York.
Department of the Environment 1996 Review of the
Potential Effects of Climate Change in the United
Kingdom. HMSO, London.
Dunlop, B.M.S. 1994 The native Woodlands of Strathspey. Scottish Natural Heritage Research, Survey
and Monitoring Report No 33, Scottish Natural
Heritage, Publications Section, Battleby, Redgorton, Perth, PHI 3EW.
Forestry Authority 1993 Woodland Grants Scheme:
grants for planting trees and looking after woodland. Forestry Authority, Forest Research Station,
Alice Holt Lodge, Wrecclesham, Farnham, Surrey.
Forestry Commission 1994 First Report of the Advisory Panel on Native Woodlands in the Highlands.
Forestry Authority, Glasgow.
Forrest, G.I. 1992 Conservation of the native
pinewoods of Scotland: the role of gene pool analysis. Scott. For. 46(3), 194-201.
Harvey, H.J. 1995 The National Trust and nature
conservation: prospects for the future. Biol.J. Linn.
Soc. 56(Suppl.), 231-248.
Hobbs, R.J. and Huenneke, L.F. 1992 Disturbance,
diversity and invasion: implications for conservation. Conserv. Biol. 6(3), 324—337.
Ingrouille, M. 1995 Historical Ecology of the British
Flora. Chapman and Hall, London.
Jones, E.W. 1952 Natural regeneration of beech
abroad and in England. Q. / . For. 46, 75-82.
Kirby, K.J., Mitchell, F.J. and Hester, A.J. 1994 A
role for large herbivores (deer and domestic stock)
in nature conservation management in British seminatural woods. Aboric. J. 18(4), 381-399.
Peterken, G.F. 1996 Natural Woodland: ecology and
conservation in northern temperate regions. Cambridge University Press, Cambridge.
Peterken, G.F. and Mounrford, E.P. 1996 Effects of
drought on beech in Lady Park Wood, an unmanaged mixed deciduous woodland. Forestry 69(2),
125-136.
Rackham, O. 1986 The History of the Countryside. J.
M. Dent, London.
Rodwell, J.R. and Patterson, G.S. 1994 Creating new
native woodlands. Forestry Commission Bulletin
No 112. HMSO, London.
Savill, P.S. 1991 The Silviculture of Trees Used in
British Forestry. CAB International, Wallingford.
198
FORESTRY
Soutar, R.G. and Spencer, J.W. 1991 The conservative of genetic variation in Britain's native trees.
Forestry 64(1), 1-12.
Spencer, J. 1996 Forest exploitation in the New Forest; the biological consequences of a thousand
years of forest use and ahuse in lowland England.
Paper presented at a conference on Sustaining
Ecosystems and People in Temperature and Boreal
Forests. Victoria, British Columbia, September
1996.
Sprugel, D.G. 1991 Disturbance, equilibrium and
environmental variability: what is 'natural' vegetation in a changing environment? Biol. Conserv.
58(1), 1-18.
Stevens, H.M. and Carlisle, A. 1959 The Native
Pinewoods of Scotland. Oliver & Boyd, Edinburgh.
Weaver, J.E. and Clements, F.E. 1938 Plant Ecology.
McGraw-Hill, New York.
Woodin, S.J. and Farmer, A.M. 1993 Impacts of sulphur and nitrogen deposition on sites and species
of nature conservation importance in Great Britain.
Biol. Conserv. 63, 23-30.
Worrell, R. 1995 European aspen {Populus tremula
L.): a review with particular reference to Scotland.
I. Distribution, ecology and genetic variation.
Forestry 68(2), 93-105.
Received 20 August 1996