Download The silviculture of conifers in Great Britain

The silviculture of conifers in Great
Britain
D. C. MALCOLM
School of Forestry, Institute of Ecology and Resource Management, University of Edinburgh, Darwin
Building, Mayfield Road, Edinburgh, EH9 3JU, Scotland
Summary
The original paper by H.M. Steven, with this title, described the silvicultural characteristics of
the main coniferous species then available in British forestry. The use of these introduced
conifers was justified by the limited site tolerances of the one native species, Scots pine, and the
need to expand forest cover on to sites with infertile soils in harsh climatic conditions.
This paper reviews the developments in silvicultural techniques and understanding that have
enabled the rapid expansion of productive forestry in the last seventy years:
• The problems associated with afforestation, requiring amelioration of soil physical and chemical conditions, have been resolved and the climatic limits to the use of individual species are
more clearly defined.
• Increased knowledge of the physiological responses of individual species to environmental factors has influenced silvicultural practice from nursery production to the regeneration of mature
stands.
• Analysis of genetic variation within introduced conifer species has refined their use and is leading to improvements in their productivity and timber quality.
• The application of ecological principles, based on the ecosystem concept, has led to an understanding of forest stand dynamics that should ensure production from exotic species on a sustainable basis.
Future requirements in the silviculture of conifers include the transfer of the results of computer
simulation modelling into practice, the development of silvicultural systems appropriate to
British conditions and greater emphasis on the quality of production.
Introduction
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commonly used conifer species throughout the
When H.M. Steven wrote the original article country. He displayed detailed knowledge of
with this title, in the first (1927) issue of this the background to and characteristics of the
journal, he was Forestry Commission Research main species and perspicacity in pointing out
Officer (South) based in Oxford. Previously he likely development in their use.
had served in Scotland and been involved with
Steven describes his article as 'a general study
early sample plot survey. He thus had first-hand of the growth of our different coniferous
experience of the range and performance of the species, their regeneration and tending'. He
O Institute of Chartered Foreneu, 1997
Forenry, Vol. 70, No. 4, 1997
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FORESTRY
divided his paper into an historical context and
sections on seven major species, a few comments on minor species and a brief conclusion.
He considered that gaining a better understanding of the soils available for afforestation and
the 'life-processes' of the tree species involved as
'the task for this century'. This would be the
foundation for the 'artificial type of forestry'
based on exotic species that he foresaw for forest expansion in Britain.
Seventy years later there has been such an
immense increase in our silvicultural knowledge
that whole books have been devoted to single
species or particular aspects. This article can
only therefore highlight some of the critical
developments since 1927, leading to our present
understanding of the site tolerances, physiology
and performance of coniferous tree species. The
coverage is not comprehensive but eclectic and
somewhat cursory. Rather than dealing with
individual species separately, the paper is structured on general topics, many of which are common to several or any species. The discussion is
further restricted to British experience although
the expansion of our knowledge has had many
international contributions.
Native woodlands
Natural coniferous forest is represented by only
one species, Scots pine (Pinus sylvestris L.) and
restricted in historical times to the Scottish
Highlands. Since 1927 the expansion of fossil
pollen studies and the availability of C 14 dating
techniques have explained much of the postglacial invasion and spread of native tree
species. Scots pine entered south England about
9500 BP spreading north over the next two millennia but apparently not reaching the Southern
Uplands of Scotland (Birks, 1989).
The subsequent disappearance of these
southerly populations remains something of a
mystery as there are sites throughout southern
Britain that might have been expected to have
retained pine against natural succession to
broadleaved species or even human depredation.
Such areas have become pine-dominated since
its seventeenth century reintroduction and active
measures have to be used to control its spread.
The Scottish populations were already pre-
sent over their historic distribution by about
8500 BP although they declined, possibly for climatic reasons, at about 4000 BP (Bennett, 1995).
This disjunct distribution between south and
north has led to speculation as to the origin of
the Scottish populations. Examination of the
population structure of the extant remnants
using biochemical genetic markers, both
monoterpenes (Forrest, 1980) and isozymes
(Kinloch et al., 1986), suggests at least two origins. The majority of remnants appear to be
related to continental populations but those in
the far north-west are distinct and may have
arisen from a postulated refugium off the west
coast (Bennett, 1995).
Treated as an exploitable resource for timber
for over two centuries and often open to grazing, many of the Scots pine remnants reached a
parlous condition by the 1950s. Although the
need to achieve regeneration was recognized,
only a few trials and experiments were installed
in those forests in public ownership before the
1960s. McVean (1963) studied the ecology of
natural regeneration in Beinn Eighe National
Nature Reserve demonstrating that even old
trees had viable seed, so that natural regeneration was possible if a satisfactory seed bed was
available. Suitable seed bed conditions were
often created a few years after the burning of
heathland vegetation and the health of seedlings
on peaty soils was improved by applying phosphate fertilizer. Numerous later studies in different areas have confirmed these results. The
problems of regeneration and resuscitation of
near-derelict native pine woods thus were
shown to be mainly to do with disturbance of
the ground vegetation developed under low density canopies and, particularly, in the control of
browsing animals. Apart from regeneration
studies most research in the native pinewoods
has concentrated on ecological questions rather
than the application of silvicultural techniques,
which indeed have been frowned upon, as
potentially interfering with the conservation of
these woods as habitat, for organisms associated
with the pine.
The renewed interest in these remnants dates
from the publication of Steven and Carlisle's
classic Native Pinewoods of Scotland (1959) in
which they gave detailed descriptions of 35 populations which they considered viable as wood-
THE SILVICULTURE OF CONIFERS IN GREAT BRITAIN
lands, and which had probably descended,
mainly by natural means, from the original
post-glacial populations. Most of these woodlands are now designated as Sites of Special
Scientific Interest thereby limiting the operations
that can be carried out to those thought not to
reduce conservation interest. A few of the larger
remnants (e.g. Glen Tanar, Deeside) have been
managed as productive forest for generations
(Ross, 1995) but for the majority the priority has
been to repair earlier exploitation damage.
Areas infilled with exotic species or non-native
origins of Scots pine are being restored
(Hamilton, 1995) by the removal of non-native
tree species, the encouragement of natural
regeneration of pine and of the often depleted
native broadleaved species. Detailed studies of
current stand structure have been published, for
example at Rannoch (Arkle and Nixon, 1996)
and Abernethy (Summers et al., 1997) with the
intention of guiding future management, however the dynamics and structure of the original
forest remains obscure, so that satisfactory longterm conservation is likely to involve silvicultural measures to maintain a balanced age-class
distribution over relatively small areas. In most
of the larger remnants, tending and regeneration
fellings could contribute to structural diversity,
produce quality material and offset maintenance
costs with no loss of conservation values.
The upsurge of concern for concepts such as
biodiversity has led to Government support for
the establishment of woodland comprising only
native species, for example the New Native
Pinewood Scheme. Strict control of origin of the
pine used is mandatory but not for associated
species, although there is a preference for supposed 'local' material. Whether this type of
afforestation of poor quality sites, often with
minimal silvicultural inputs, will lead to effective forest in the long term remains to be seen.
Forest expansion
The increase of forest cover in Britain from
about 3 per cent in 1920 to over 10 per cent now
has been achieved very largely by the afforestation of upland sites. Policy constraints, imposed
to protect agricultural use, directed forest
expansion on to generally low fertility soils at
295
higher elevations. This entailed the development
of silvicultural techniques for the establishment
of tree species beyond their previously known
limits and on site types that had not supported
forest
before,
at least for
centuries.
Silviculturalists could build on the experience
gained with a limited range of species and some
pioneering work on establishment of plantations
on difficult sites. An outstanding example was
the development of afforestation by Sir John
Stirling-Maxwell (1951) who introduced more
than 70 conifer species over a 40-year period to
Corrour, Inverness-shire where he established
them on peat soils above 400 m elevation. He
demonstrated the benefits of both a raised planting position
and
phosphate
fertilizer.
Nevertheless the scale of expansion envisaged
over the whole country required solutions for
many problems of matching species to sites and
the economic production of planting stock and
its planting.
The research approach adopted to solve the
problems by the Forestry Commission, was
based strongly on field experimentation. Over
the period from the 1920s to the mid-1950s
numerous trials of techniques for cultivation,
drainage and application of fertilizers were
established on the major soil types represented
in the uplands. The development of tracked
vehicles of sufficient power to disrupt soil layers
limiting tree root penetration, such as iron pans
or indurated layers, transformed the site preparation process while, after World War II, the
development of ploughing techniques on peaty
soils enabled the very rapid afforestation of
large areas. These developments had been foreseen and early experiments included hand-dug
mock-ploughing (Wood, 1974). The progress of
field research on afforestation has been
described comprehensively by Zehetmayr for
peatlands (1954) and upland heaths (1960).
By the 1960s most establishment problems
had been solved and later experiments have
tended to refine earlier basic findings or fill in
gaps for particular species or site types. Two
important features of the research effort were
the early adoption of the statistical techniques
being evolved for agricultural research and the
rapidity with which research findings were
transferred to field practice.
The result of the effort to develop these new
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silvicultural techniques has been the ability to
successfully establish plantations of coniferous
species on a range of sites, from the sands and
gravels of south England to the deep peat in the
north of Scotland.
Site assessment and species selection
In plantation-based forestry the satisfactory
matching of species to site is critical for the formation of healthy sustainable forest stands. In
naturally occurring forests, tree species are distributed in relation to their site tolerances (climate and soil) and their ability to tolerate
competition from other species. When used as
exotics they are generally planted in pure stands,
without competing and other associated species,
and grown for only a fraction of their true
longevity. Both factors enable exotic species to
display wider site tolerances than in their natural habitats. Nevertheless an understanding of
the ecological status of exotic species provides a
guide to their silvicultural characteristics in
plantations.
Surprisingly, little attempt was made to assess
the ecology of the conifers introduced from
North-west America in the early nineteenth century until after World War II. By then these
species were becoming dominant in silvicultural
practice in the uplands so the guidance to their
limitations provided by Wood's comprehensive
study (1955) and Day's (1957) more detailed
appreciation of Sitka spruce (Picea sitchensis
(Bong.) Carr.) were valuable. Even the commonly planted Corsican pine (Pinus nigra var.
maritima) was not studied in Corsica until 1960
(Brown, 1960).
Site quality for tree species is often assessed in
terms of productivity but it should also include
an estimate of long-term health and adaptation.
It is not good enough to demonstrate a capacity
for a species to establish in plantation conditions, when it may later be damaged by
extremes of climate or attacks of pests or
pathogens. With exotic species early plantations
tend to be established on sites of above-average
quality leading to over-estimation of their
potential. Hence the desirability of species trials
extending on to testing sites of low fertility or
harsh climate. To some extent in Britain this
was achieved by the early afforestation trials
referred to above and also by high-elevation trials installed above the recognized altitudinal
planting limits. Many of these, such as that
planted by Steven at 600 m a.s.l. in S. Laggan
Forest in 1923, demonstrated the superiority of
the then untried Sitka spruce in exposed conditions over the performance of other better
known species, particularly Norway spruce
{Picea abies (L.) Karst.).
Numerous observations of tree species performance were systematized by Anderson's
Selection of Tree Species (1950) which classified
'bare-land' vegetation types on gradients of site
fertility and wetness. The scheme had been presaged for the peat-dominated sites by the studies of Fraser (1933) but Anderson's classification
covered all sites in Britain. It provided a sound
basis for species allocation but could not properly take account of changes in site resulting
from cultivation, drainage and fertilization.
Differential climatic effects also could only be
accommodated in a general way. Nevertheless
the classification did allow a more site-based
approach to afforestation than was available
before.
Soil amelioration effects were incorporated
into site classifications during the 1960-1970
period by the development of the Site Survey
Section of the Forestry Commission. Although
existing vegetation was used as a primary site
differentiation, based on aerial photography, the
soils were mapped in detail on a 1 : 10 000 scale.
The resulting classification provided, for the
first time, a clear means of transferring experimental results to field practice. The survey
reports suggested optimal species choice,
together with recommendations for silvicultural
operations to the establishment stage as well as
an estimate of the productivity that might be
expected. The development of the complete soil
classification (Pyatt, 1982) coincided with a
period when forest expansion was strongly commercially oriented with an emphasis on maximizing overall production at least cost, leading
to concentration on few species, particularly the
well-adapted Sitka spruce. However a wider
approach was advocated by some who suggested alternative species choices where site conditions allowed (e.g. Paterson (1978) for the
former East Scotland Conservancy).
THE SILVICULTURE OF CONIFERS IN GREAT BRITAIN
Interest in the site relations of individual
species stimulated a series of autecological studies with particular attention paid to soil chemical and physical faaors. These studies depended
on the availability of multivariate statistical
analyses, such as multiple regression or principal component analysis and required appropriate computing power. The aim was to predict
productivity from combinations of site factors.
Studies were conducted on Sitka spruce
(Malcolm, 1970; Page, 1970; Blyth and
MacLeod, 1981) in various parts of the country
but also Scots pine (e.g. Cook et al., 1977) and
Corsican pine (Fourt etal., 1971). The results of
these studies proved not to be generally transferable to silvicultural practice, being often
dependent on complex sampling and laboratory
practice. However they all demonstrated that, in
healthy established stands, productivity was less
related to soil conditions than climate.
Inadequate meteorological data meant that climatic effects had to be expressed in surrogate
terms, such as elevation (e.g. Malcolm and
Studholme, 1972). Subsequently Worrell and
Malcolm (1990) were able to show that, for
Sitka spruce, the relationship between productivity and elevation varied across the uplands of
north Britain but that, at any given site, productivity (expressed as Yield Class m3 ha"1 a"1)
can largely be estimated (>75 per cent variation
accounted for) by a combination of two climatic
indices. These are accumulated temperature
(>5.6°C) and an estimate of site 'windiness'
derived from tatter flags. Other site variables
such as soil type or aspect had only slight effects
on productivity although Jarvis and Mullins
(1987) found, at lower elevations, available
moisture was important. It is probable that,
within their own site tolerances, other coniferous species will show a similar pattern of productivity-climate relations.
The reduction of tree growth rates in windy
situations, at higher elevations and closer to
coastlines, has been attributed to a complex of
mechanical and physiological responses to
wind-induced stress (Telewski, 1995). This lowered productivity is also expressed in tree form,
branchiness and stem quality, so it has negative
effects on wood quality and financial returns at
harvest. However, wind damage resulting in
windthrow of immature stands has the greater
297
influence on silvicultural practice, leading to
avoidance of thinning and fellings conducted in
anticipation of serious (>40 per cent) damage to
the stands. As much of upland forestry was
developed on unstable soils, the Windthrow
Hazard Classification (Miller, 1985) proved a
useful additional site classification for management purposes. The classification identified six
critical stand heights (based on location, elevation, local shelter and soil conditions) at which
damage would become severe. With more data
available the classification was revised (Quine
and White, 1993) and recently Quine (1995) has
emphasized the need for a classification that
takes more account of the probabilities of windspeeds exceeding damaging levels.
Soil conditions are, of course, important for
healthy established forest stands and the provision of adequate rooting space for tree stability
and lack of nutrient or moisture stress have been
the aims of site preparation for planting. Once
established satisfactorily, cycling of site nutrients through the stand becomes important
(Miller, 1981). The rate of cycling is controlled
by the decomposition of litter and mineralization of nutrients, which processes are themselves
affected by litter quality and soil microclimate,
thus integrating tree—site conditions. Long-term
species-site compatibility is therefore influenced
by humus formation allowing for some species,
natural regeneration on well-decomposed
organic soil layers (mull) and for others (e.g.
pines), demanding more drastic stand disruption
to dissipate accumulated organic matter (mor).
Although the dynamics of exotic conifer species
on afforested sites in Britain is still only partially
understood, there is little reason to suppose they
will function differently here, in this context,
compared with their natural range.
Recent developments in site assessment incorporate humus condition and ground flora as a
means of ensuring site—species compatibility.
Based on European and particularly British
Columbian site classifications in natural forest,
an Ecological Site Classification (ESC) scheme is
being developed (Pyatt, 1995), that, within a climatic stratification, locates sites within a twoway gradient of soil nutrient and soil moisture
regimes. Soil nutrient condition can be assessed
by ground flora or humus type and moisture status by soil profile observation, linked with local
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FORESTRY
potential evaporation. The range of adaptability
of individual tree species can then be plotted
within the grid.
A fully developed ESC will tie site assessment
and species selection, whether for pure, production or mixed multi-benefit stands, more closely
together than hitherto. It should also permit the
rationalization of silvicultural operations on a
site basis from stand establishment and later
tending, through to regeneration.
Site amelioration
Although establishment problems, as noted
above, were overcome sufficiently to allow rapid
forest expansion in the 1960s onwards, some
aspects of site amelioration have changed in
recent years.
The introduction of ploughing, and particularly the use of tine-ploughs on heathland sites,
allowed disruption of thin iron pans or other
indurated layers and provided a weed-free
planting position on the upturned ridge. With
the availability of greater tractor draw-bar
power, larger ploughs disturbing soils to greater
depths (>60 cm) were adopted based on some
evidence of enhanced early growth of planted
trees. Such site treatments, if not well planned,
could lead to accelerated erosion, stream sedimentation and later tree instability through
alignment of roots in the direction of the
plough-ridge. Cultivation experiments, assessed
at 30 years of age, demonstrated that the initial
benefits to growth of increasing intensity of cultivation were not maintained (Wilson and Pyatt,
1984). Current practice therefore has moved to
providing a superficial disturbance of the upper
soil layers spaced at planting distance. This
avoids the deleterious aspects of continuous
plough lines and is more acceptable visually in
the landscape.
Steven (1927) described, in some detail, work
then in progress on establishing tree growth on
peaty soils. Close study of transplant root systems of spruces demonstrated the need for aerobic conditions (Steven in Zehetmayr, 1954)
which were provided by an upturned turf into
which the plants were inserted. Spruces proved
well adapted to these conditions because of their
ability to form adventitious roots that replaced
the nursery root system. Once ploughing
replaced hand-turfing the problem of later instability induced by root alignment became severe.
Attempts to reduce the risk by adopting doublemouldboard ploughs to provide turf ridges at
double plant spacing are not yet proven but
have, in any case, been superseded by a return
to creating a mound for each plant. This practice is now mainly used for establishing second
rotation stands following clearfelling, on peaty
gley and peat soils.
The importance of a radially uniform root
system for stability on soils with inadequate
rooting depth was shown by Courts (1983)
whose physiological studies confirmed both the
intolerance of Sitka spruce roots to anaerobic
conditions and the retention of dominance by
roots formed early in the tree's life. The former
habit results in the flat plate root systems, commonly found in spruce grown on soils with a
high winter water table. Increased stability can
only then be obtained by increasing the rootable
volume laterally by maintaining a lower stand
density. However low stand densities bring disadvantages for stem and wood quality.
Most site types, with the general exception of
brown earth soils, have shown responses in tree
growth to application of phosphatic fertilizers.
Numerous field trials established the appropriate rates and timing of applications which
proved to be very similar to those worked out
for conifer stands throughout the boreal and
temperate forest zones (Taylor, 1991). An earlier
reluctance to incur additional costs in establishment led to the need for large-scale remedial
aerial applications to upland spruce stands in
west and south Scotland (Dannatt et al., 1971)
and it is now accepted practice to apply 60 kgP
ha"1 at planting. It is interesting that only on the
poorest sites does it appear necessary to repeat
a phosphorus input at the start of a second rotation.
Potassium is generally deficient on soils with
organic layers (>30 cm deep) but tree growth is
normal where 100 kgK ha"1 is applied at or
shortly after planting. Although potassium is
readily lost from soils by leaching, in many cases
sufficient is obtained through atmospheric deposition and there is not yet good evidence of a
need for repeated application in young second
rotation stands.
THE SILVICULTURE OF CONIFERS IN GREAT BRITAIN
In distinction to much of the boreal and temperate zone, nitrogen availability has not generally been a problem for afforestation in Britain.
Exceptions have occurred, where Sitka spruce
has been extended on to the poorest Callunadominated heaths or more oligotrophic peats.
On these sites the mineralization of organic
nitrogen is too restricted to allow canopy development without several applications (at 3-year
intervals) of about 160 kgN ha"1 (Taylor and
Tabbush, 1990).
The nutritional problems on Calluna-dominated sites were complicated by the interaction
between spruce, and similar species, with the
Calluna itself. Restricted growth ('check') and Ndeficiency symptoms of trees in Calluna swards
can be relieved either by destruction of the
Calluna or by the tree roots developing under
other adjacent vegetation. Where Calluna swards
were an artefact of previous land use release
from 'check' was maintained but on true Calluna
heaths and on deep peats N-deficiency was
chronic. The practical solutions involved killing
Calluna by herbicide (latterly glyphosate) or suppressing it by planting mixtures of unaffected
pine (Scots pine or lodgepole pine (P. contorta
Dougl.)) and the economically desirable spruce.
Handley (1963) and Robinson (1972) showed
that the inability of spruce to access nitrogen on
these sites was due to suppression of mycorrhizal
activity in the spruce by a factor released by
either the Calluna root or its endophyte.
Mixed species stands (pine or larch with
spruce) also resolved the later N-stress for
spruce on these infertile sites. First observed in
Ireland (O'Carroll, 1978) the beneficial effects of
these mixtures were later shown to be due to
higher mineralization rates, differential rooting
depths and further mycorrhizal effects (Morgan
etal., 1991; Ryan and Alexander, 1992). These
sites are clearly marginal for spruce growth and
are better allocated to the less-exacting pine
species.
Management of stand nutrition benefited
greatly from the development of diagnoses of
nutrient stress through foliar analysis, which
integrates nutrient availability according to site
conditions. Critical levels worked out for the
main coniferous species (Binns et al., 1980)
allow the need for fertilizer applications to be
properly evaluated.
299
The demands of forest production on site
nutrient capital were first examined by Rennie
(1955) who questioned the sustainability of harvesting on low fertility sites, such as the North
Yorkshire moors. Later detailed nutrient budgets were prepared for various forest types,
which showed, for most elements, harvesting
removals were largely balanced by atmospheric
deposition and soil mineral weathering (e.g.
Miller et al., 1979). Demonstration of Nfertilizer effects on pole stage Corsican pine
{Pinus nigra var. maritima (Ait.) Melville) growing on fixed sand dunes at Culbin Forest,
enabled Miller (1981) to clarify understanding
of the role of nutrient additions to tree stands.
When stands attain full canopy, nutrient cycles,
both within the trees and between the trees and
soil litter layers, are established which reduce
the stand's demand on the soil. Fertilization is
thus most effective early in the rotation or
where leaf area has been reduced by damaging
agencies or senility. Unless over-supplied, the
added nutrient accelerates canopy closure and
increases tree growth rate, without undue loss
to drainage water or long-term effects on the
site nutrient capital.
Amelioration of site conditions has become
progressively more focused by an increasing
understanding of the morphology and physiology of root growth, tree nutrient requirements
and their utilization, together with an appreciation of the ecological relations of tree species at
different stages in their growth cycle. Problems
remaining include treatment of those sites,
where gleyed soils occur on impermeable glacial
tills, inhibiting root penetration. Sustainable
forestry in these conditions will probably
depend more on developing stand treatments
rather than physical site alterations.
Species use, provenance and improvement
Although the range of conifer species discussed
by Steven (1927) has hardly changed in forest
use in the last 70 years, their proportions have
been transformed (Table 1). The silviculture
and performance of exotic conifers in Britain
was last comprehensively summarized by
Macdonald et al. (1957). Many of their conclusions are still valid although there have been
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FORESTRY
Table 1: Changes in use of conifer species since 1920
Percentage composition of planting decades
Species
Scots pine
Corsican pine
Lodgepole pine
Sitka spruce
Norway spruce
European larch
Japanese larch
Douglas fir
Other conifers
81-90
71-80
61-70
51-60
41-50
31-40
21-30
All ages
3
6
10
68
1
—
5
4
1
5
3
15
63
3
1
6
2
1
13
4
14
42
8
2
7
4
4
22
3
8
29
10
2
13
5
3
24
3
1
37
16
4
10
2
1
26
6
1
26
19
8
8
3
1
46
6
1
11
11
8
6
7
1
18
4
10
39
9
3
8
4
2
N.B. The decade 81—90 refers to Forestry Commission only; all others include private woodlands (adapted from Malcolm,
1992).
considerable advances in understanding the ecophysiology of the major species, which have
attracted most research effort. The outstanding
example has been Sitka spruce (Henderson and
Faulkner, 1987).
The changes in the use of different species
(Table 1) have reflected the availability of sites
for afforestation. Dominating these changes has
been the expansion in the use of Sitka spruce.
The adaptation of this species to much of the
British environment, particularly in.the uplands,
its ease of establishment and its responsiveness
to cultural treatments, have ensured its place as
the mainstay of modern British forestry. High
productivity and a sought-after general purpose
timber have also encouraged investment in this
species. Steven (1927) noted its superiority to
Norway spruce (Picea abies (L.) Karst.)
although its specific requirements were not then
known. He thought it 'brings hope to our more
difficult sites' for which 'we need enrichment of
the conifer flora'. Sitka spruce has fulfilled these
expectations. Limitations to its use on low fertility sites and on sites where it suffers moisture
stress have been noted above.
The other species showing a large increase in
use (Table 1) has been lodgepole pine (P. contorta Dougl.) which Steven (1927) thought
might have a role in mixture with spruce on
heathland sites. The increase in use after the
1950s was again in response to site availability
and the inexacting nature of this species, making it popular, for a while, for peatland
afforestation. In recent years it has been less
used because of its lower productivity, the perceived poor stem form of some origins, climatic
and pest damage.
The relative decline in the use of other
species, such as Scots pine and European larch
(Larix decidua Mill.) (Table 1) reflects their
lower productivity as well as limits to their site
adaptations. On the other hand the consistency
of Douglas fir (Pseudotsuga menziesii (Mirb.)
Franco.) simply expresses the limited availability of suitable sites.
Perhaps the most important development in
the silviculture of exotic conifers has been the
recognition of geographic variation within the
natural range of species. That such variation
existed has been known for at least 200 years
and was subject to experimentation in Central
European species in the nineteenth century.
Although Steven (1927) drew attention to the
importation of undesirable (poorly adapted)
sources of Scots pine, which suffered from snow
break in Scottish conditions, little effort was
devoted to this subject until after World War II.
Early experiments were conducted with Scots
pine, demonstrating the satisfactory performance of indigenous sources (Lines and
Mitchell, 1964), however some seed from continental sources continued to be imported because
of lower costs. A similar story applies to
European larch where unsuitable alpine origins,
subject to various ills in the thicket and
polestage, were imported up to the 1940s rather
THE SILVICULTURE OF CONIFERS IN GREAT BRITAIN
than concentrating on the proven land race
established for 200 years in Britain.
Provenance testing developed strongly after
1946 and for most exotic species the more desirable origins have been identified with a few surprising results. For example, the most vigorous
(and late-flushing) origins of Norway spruce are
from the south-east of Europe (Lines, 1987).
Provenance experiments take time and are
expensive to organize in terms of seed collection, adequate replication and the area they
occupy. Too early assessment can lead to problems, such as occurred with lodgepole pine.
Origins from coastal Washington and Oregon
(ssp. contorta) showed early vigour and greater
productivity than slower growing northern or
more inland (ssp. latifolia) sources, and were
thus adopted for peatland afforestation (Lines,
1966). Unfortunately the coastal origins, which
had poor form in any case, were frequently
destroyed by wet-snowfalls in the thicket stage.
With many species the importance of obtaining origins from similar latitudes to the proposed planting sites has been demonstrated. The
adaptation to photoperiod avoids potential
damage from out-of-season frost. It was particularly fortuitous that most Sitka spruce planted
in Britain came from the Queen Charlotte
Islands, subsequently shown in provenance
studies to be the best adapted origin for much
of Britain, although slightly more southern origins (Washington) perform better in south-west
England and Wales, with less danger of damage
from out-of-season frost.
In general, provenance collections are valuable assets, beyond determining which origins
are likely to be most useful in afforestation. If
based on adequate population samples they also
provide information on the genetic structure of
the species concerned and are a resource for
more detailed physiological and genetic studies.
They may also conserve genetic material which
otherwise could be lost in the natural range.
Many of the provenance collections in
Britain, for both European and North West
American species, are part of larger studies
organized by the International Union of
Forestry Research Organizations (IUFRO), thus
providing a wider set of sites on which to test
species—environmental interactions. There are
now good collections of the range of variation
301
within the natural ranges of the exotic conifers
important to British forestry.
Normally silvicultural operations favour the
better quality and more vigorous trees in the
stand, thus leading to some improvement in
later generations. However this depends on the
hcritability of the attributes assessed. The selection of desirable phenotypes and the testing of
their progeny for superiority, for inclusion in
breeding populations, can lead to much greater
genetic gains per generation. Tree improvement
techniques have developed rapidly in the last 50
years but are expensive and demand continuity
of effort, so that the improvements obtained
have to justify the investment.
Programmes to improve Scots pine and
European larch were started in the late 1940s
although the use of these species declined thereafter. Subsequently most effort has been
directed to Sitka spruce (Rook, 1992) with
smaller programmes devoted to Douglas fir and
the production of hybrid larch {Larix X
eurolepis Henry). Rather strangely Steven (1927)
makes no mention of this hybrid which had
been known for 25 years by then.
The long delay until seed orchards are able to
produce sufficient improved seed for forest use
has encouraged the development of vegetatively
propagated clones. These clones are derived
from the controlled crossing of selected parent
trees, whose progeny have been shown to be
superior in stem form, branching habit and
vigour to unimproved material. Several cycles of
cuttings are removed from juvenile plants and
rooted in controlled environments, allowing
large-scale multiplication of a few individuals
(Mason and Gill, 1986). Mixtures of these
clones are now commonly planted in anticipation of higher yields of quality timber, more
than compensating for the greater costs of plant
production. Clonal mixtures are thought to be
sufficiently diverse to avoid biological problems
arising from lack of diversity. Vegetative propagation is most highly developed for Sitka spruce
but is also used for hybrid larch, which is difficult to guarantee from hybrid seed orchards.
The technique has, of course, been used for
mass-producing cultivars of amenity trees, such
as X Cupressocyparis leylandii (Jacks. &
Dallim.) in commercial nurseries.
Although tree improvement has concentrated
302
FORESTRY
on one species, Sitka spruce, the techniques are
readily applicable to other species. Cycles of
improvement, through recurrent selection, may
transform potential production over decades.
Currently the Sitka spruce breeding population
comprises about 200 selected trees and is divided
into sub-populations for use in different geographic areas and the aim is to include wood
density in the selection criteria.
Silvicultural operations
Plant production has undergone great changes
in the last 70 years. The numerous small nurseries located close to planting areas have been
replaced by a few large-scale nurseries on
selected sites with facilities for irrigation and
cold storage of dormant plants.
These developments were based on detailed
research into the nutritional requirement, weed
control and pest and pathogen control in nursery soils (Benzian, 1965) as well as later studies
on the physiology of the young trees. Tests of
the viability of bare-rooted planting stock
(McKay, 1992) have increased the efficiency of
plant handling and transport to planting sites.
Opinion remains divided on the relative merits of bare-rooted transplants and containergrown trees for the planting of most conifer
species. However planting difficulties with
Corsican pine have led to general use of
seedlings in containers for this species and now
these are often mechanically planted. In general,
planting stock sizes, now subject to British standards (Aldhous and Mason, 1994), are still chosen with regard to the site and anticipated weed
competition.
The improvements in the uniformity and general quality of planting stock, its handling and
transport, have led to an increased efficiency in
nursery practice and a reduction in real terms in
its cost.
The advent of chemical herbicides in the
1950s greatly reduced problems with weed control in the forest thereby reducing a labour
intensive aspect of stand establishment. The
range of herbicides encompasses special treatments for winter application, grass, herbaceous
and woody weeds (Willoughby and Dewar,
1995). Current emphasis is on herbicides that
have a wide-spectrum effect and which are
rapidly broken down on soil contact with little
influence on other parts of the ecosystem.
A crucial decision in planted stands is the
choice of initial spacing. Close spacing, at about
1.3 m between trees, which was common when
Steven wrote, was adopted to ensure rapid
canopy closure, control of weed growth and to
tolerate losses. Steven (1927) advocated wider
spacing (>2 m) for the shade intolerant larches
and fast growing Douglas fir. Clearly there is a
balance to be found between increased cost of
close spacing and excessive branch development, loss of early production and weed competition at wide spacing. In general a spacing of
2 m is now adopted and, for Sitka spruce,
appears to be a maximum to avoid unacceptably
low density in the wood produced (Brazier and
Mobbs, 1993).
Manipulation of stand density, through thinning, is the only way of influencing stand development after canopy closure. The effects of
inter- and intra-specific competition between
trees have been the subject of much ecological
and silvicultural research so that thinning,
which was a largely intuitive practice in 1927,
now rests on a secure scientific basis. The adoption of standard thinning grades and intensities
allowed comparisons to be made of the performance of species on different sites leading to the
production of yield models (Hamilton and
Christie, 1971; Edwards, 1981). These models
now underpin much of forest management but
also define the limits of interference available to
the silviculturalist. The limits range from density-dependent mortality (self-thinning) to loss
of overall stand production at higher thinning
intensities. Although originally determined
empirically from numerous sample plots these
results have been substantiated in properly replicated experiments (Hamilton, 1976 and 1981).
The growth, and therefore yield, of forest
stands, subject to silvicultural interventions of
various kinds, has become a major research area
in response to the need for ecosystem understanding, where timber production may only be
one of many objectives. Because of the time
scales and number of variables involved, these
studies now rely heavily on the computer simulation of either individual tree or stand
responses to environmental variables, or pertur-
THE SILVICULTURE OF CONIFERS IN GREAT BRITAIN
bations due to nutritional inputs, acid deposition and, recently, enhanced carbon dioxide in
the atmosphere. Mechanistic models based on
measured tree growth, are being replaced by socalled process models which utilize the physiological relationships between tree functions,
such as photosynthesis, and environmental variables, like radiation input. While greatly
increasing knowledge of the dynamics of stand
development, the verification of these models
can only be achieved through their application
in silvicultural practice, which requires a degree
of generalization not yet available.
Silvicultural treatments aim to bring forest
stands to maturity, when harvesting of the trees
creates the conditions for regenerating the
stand. Because most conifer forest is recent,
there has been little advance in this area of silvicultural practice in the last 70 years. Steven
(1927) drew attention to ad hoc examples of natural regenerated stands and since then there
have been a few attempts to initiate the use of
silvicultural regeneration systems other than
clearfelling. Planned natural regeneration of
Scots pine stands has been managed in some private estates such as Glen Tanar, Deeside (Low,
1988) but elsewhere regeneration has depended
mainly on planting or fortuitous seeding.
Since the 1950s it has become clear that Sitka
spruce in the uplands will regenerate freely, particularly on soils with organic surface layers.
Seed years at about 4-year intervals initiate a
seedling population which, if not released by
windblow or removal of the overstorey, rapidly
dies off (McNeill and Thompson, 1982).
However, clearfelled sites may also be seeded
from surrounding stands and dense thickets of
spruce can result (Clarke, 1992). Treatment of
these regenerated stands by respacing at different stages is still under trial.
Some regeneration of other commonly used
conifers is observable and therefore could be
planned for, although opinion of its desirability
remains divided. Natural regeneration of welladapted species and provenance is acceptable
but if improved stock or alternative species are
required planting is necessary.
As an increasing proportion of plantations
reach maturity there has been a developing
interest in adopting silvicultural systems other
than clearfelling (Matthews, 1989). Systems that
303
rely on retention of part of the previous stand
for regeneration are unsuitable for exposed sites
and, in some cases, shade intolerant species
but opportunities to diversify silvicultural
approaches do exist (Paterson, 1990).
Systems based on small-scale fellings (<0.25
ha) have been instituted in a number of places
(Hart, 1995) where landscape, amenity or
wildlife considerations are important. These are
essentially transformations from regular plantation structures to irregular, usually mixed
species, stands (Mclver et al., 1992) and thus
involve planting as well as accepting natural
regeneration. The main silvicultural advantages
are the retention of a forest microclimate and
the possibility of mixed litter layers, particularly
where broadleaved species are included. The
dispersed size classes give rise to more costly
operations but these may well be offset by the
benefits obtained.
Experimental comparisons of different silvicultural systems are not possible because of
scale and time problems so that development of
new systems, based on exotic species and
afforested sites, must arise from trials. These trials have inevitably to adapt to changing external circumstances (e.g. machinery development
and market availability). Nevertheless sufficient
is known of the characteristics (seed production,
shade and site tolerance) of the major conifers
to be able to adapt systems developed elsewhere
to British conditions. A sine qua non for success
is continuity in management objects and flexibility in scheduling production.
Conclusion
In trying to distil the important developments in
the silviculture of conifers since 1927, this paper
has had to ignore whole rafts of information
which have silvicultural implications. In particular the interaction of tree species with other
organisms has been neglected. Fortunately pests
and pathogens have not been, so far, dominant
in silvicultural developments. There are some
notable exceptions, such as the ubiquitous
Hylobius abietis affecting replanting sites, the
potentially serious outbreak of Dendroctonus
micans on spruce and the damaging moths
(Bupalus piniaria, Panolis flammea) on extensive
304
FORESTRY
pine plantations. Among disease organisms the
more important have remained Heterobasidion
annosum and Armillaria spp. In these, and
many other less important cases, sufficiently
effective control measures have been devised to
avoid altering silvicultural practices.
Steven (1927) considered the red squirrel a
damaging pest and does not mention any deer
species at all. The squirrel now is sufficiently
rare to require conservation and deer populations have exploded. Deer are a major constraint on expanding the use of several conifers
(e.g. Abies spp.) desirable for diversifying existing forests and increasing the silvicultural
options, that more shade tolerant species (e.g.
Tsuga heterophylla (Raf.) Sarg.) would make
possible. Red deer numbers have to be about 2
animals km"2 (Ratcliffe, 1987) for susceptible
conifers to avoid serious damage.
Another major constraint on silvicultural practice is the threat of windblow in stands formed
on shallowly rootable soil types, at higher elevations in the uplands. In the most difficult situations these stands (mainly Sitka spruce) reach a
critical height (13 m) before they attain maximum mean annual increment (Quine and White,
1993). Canopy disturbance exacerbates windblow so these stands are normally left unthinned
to gain, perhaps, a further 3 m in height before
clearfelling. Whether stability can be improved
by early respacing, chemical thinning or severance fellings to give wind-firm margins remains
to be seen. However, without improved rooting
conditions it is difficult to see how such stands
can be diversified in species or structure, so their
silviculture is likely to remain rather simple.
The tasks set out by Steven (1927), to learn
more of the soils and tree species for afforestation have been more than fulfilled. The successful expansion of forest on to all the available site
types is sufficient confirmation of his expectations. The response of different species to site
conditions and the importance of within-species
variation, are now well-known for the major
conifers, leading to greater assurance in their use.
Silvicultural techniques from the nursery to stand
maturity have been the subject of detailed
research although the results are not always
applied for cost reasons.
The next stage is to develop appropriate silvicultural systems for ensuring a transition from
simple plantation forestry to properly structured
and ecologically sustainable forests. Such systems
can only be introduced on a trial basis, making
use of the known characteristics of the major
species. There is likely to be more planned use of
natural regeneration, although this requires more
flexible management planning so operations can
be tied to seed year frequency. To meet the current demand for structural and species diversity
many forests dominated by single species will
need to have other species introduced. To some
extent that is being achieved now by the requirement that all plantings should contain a proportion of broadleaved species but there is
considerable scope for developing stands of
mixed species of conifer.
An important area of development is the transfer of the results being obtained through computer modelling of forest stands into silvicultural
practice. The ability to short-circuit long periods
of field trial through stand modelling should
allow more options to be considered in the handling of stands, to meet increasingly complex
management objectives. At present it appears
that the modelling capabilities are outstripping
the application in practice. This area is likely to
become more important in view of potential climate change over the next 50 years. The putative
changes in environmental quality may occur well
within a normal rotation length for most conifers
but will not be necessarily deleterious (CCIRG,
1996) for all species or sites.
Finally, the emphasis within silviculture needs
to be shifted further to the production of high
quality products. In 1927 the objective was to
increase the total forest resource and later to
maximize the quantity of timber produced and
the return on capital invested. With an established forest resource, there is now a need to
improve the quality of forest stands to meet the
multiple objectives of present management policy. The main product of British forests, coniferous timber, has to be of high quality if it is to
compete effectively for a share of the more
rewarding markets.
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