Silvicultural possibilities for increasing structural diversity in British spruce forests: the case of Kielder Forest

Fores;f;ology Management ELSEVIER

Forest Ecologyand Management79 (1995) 13-28

Silvicultural possibilities for increasing structural diversity in British spruce forests: the case of Kielder Forest W.L. Mason *, C.P. Quine Forestry

Commission,

Research

Division,

Northern

Research

Station, Roslin

EH25 9SY. UK

Accepted8 May 1995

Abstract Kielder Forest comprises some 50000 ha of first- and second-rotation spruce forest created during this century on surface-water gley and peaty gley soils in northern England. The original limiting factors to tree establishment and growth, high water tables and low nutrient status, were largely eliminated by appropriate silviculture using cultivation, some drainage and remedial fertilisation. The main limiting factor to tree growth is now wind disturbance compounded by shallow rooting on the gleyed soils. To avoid the risk of windthrow, stands are left unthinned and are clear-felled at 35-40 years of age. However, the deterministic nature of the windthrow hazard classification used to predict the onset of wind damage means that the possibility of retaining stands for longer rotation$ may have been underestimated. Recent evidence suggests that, provided stands are planted using cultivation techniques that promote a stable root architecture and are respaced at an early stage to promote stem diameter growth, it should be possible to maintain some stands for at least 75-80 years to enhance structural diversity. The spread of gaps formed by windthrow in spruce forests and the development of a 72-year-old self-thinning spruce-pine mixture exemplify these possibilities. Keywords:

Afforestation;

Conifer;

Regeneration;

Wind damage;

Sustainability

1. Introduction

ished grazing land on wet soils with limited natural

tree cover. The plantations were primarily created to The most striking physical achievement of forestry policy in Great Britain in the twentieth century has

provide wood for domestic timber processing and, as is usual with such objectives, they were planted on a

been to establish extensive coniferous forests in the uplands of Scotland, Wales, northern and southwestern England. More than 1.l million ha of such forests have been planted since 1919 increasing the percentage forest cover from 3.5 to 11% (Forestry Commission, 1993). Many sites afforested were marginal for agriculture, typically being impover-

large scale with few tree species (Malcolm, 1979). The main species used has been Sitka spruce Picea sirchemis which has been found to grow well with high yields over a wide range of sites in upland Britain. Since the 196Os, forestry policy objectives have evolved to encompass a range of other benefits besides timber production. The most recent policy (Forestry Commission, 1991; Anonymous, 1994) places emphasis on sustainable management of exist-

l

Corresponding

author.

0 British Crown, 1995 SSDI 0378-1127(95)03618-O

W.L. Mason,

14

C.P. Quine/Foresr

Ecology

ing forests for multiple objectives, such as the maintenance and enhancement of biodiversity, the provision of recreational opportunities and an increase in the diversity of the forested landscapes. The provision of these benefits in the extensive, even-aged conifer forests of the British uplands presents a silvicultural challenge of at least equal magnitude to that involved in their establishment earlier this century. This paper examines a number of key factors that must be considered if this challenge is to be met, using Kielder Forest District as an example. Based upon this analysis, we attempt to draw some general conclusions for those faced with similar tasks either in Britain or elsewhere.

2. Kielder

Forest-climate

Kielder Forest District is centred on the North Tyne Valley immediately to the south of the Scottish-English border at elevations of between 150 and 450 m. It lies within the Border Carboniferous site region (Toleman, 1979) which includes parts of the forests of Newcastleton and Wauchope on the Scot-

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79 (1995)

Table 1 Percentage distribution Border Carboniferous whole (after Paterson,

13-28

of broad soil groups in forests of the site region and for upland Britain as a 1990)

Brown earths, ironpans and podzols

Surface water gleys and pew nleys

Peat ’

Border Carhoniferous

18

62

20

Upland Britain

42

38

18

a Peat is distinguished over 45 cm following

from peaty-gleys Pyatt (1982).

by having

Skeletal

2

a peat depth of

tish side of the border as well as some extensive private plantations. The total plantation area in this site region in 1993 was around 100000 ha, almost 8% of the coniferous forest area in Britain. The widespread occurrence of dense, heavy textured g&y soils in the Border forests is a major difference between this particular site region and other forest areas in upland Britain (Table 1). Further details of lithology and soils can be found in hrlclintosh (1995).

50

45

40 :rn !z TI p :: D .s

35

3 30

25

.. . . . . -

20 1920

1960

1940

Mean (Of Windspeed

1960

annual maximal (Annual gust)

maximum

YeFir

Fig. 1. Annual

maximum

windspeed

recorded

at the Eskdalemuir

Observatory

over the period

1919-1990

(from

Q&e,

1995).

W.L. Mason,

C.P. Quine/Forest

Ecology

Table 2 Mean monthly air temperature (“C) and rainfall (mm) values for the Kielder Castle meteorological station (201 m elevation) for the period 1951-1980 Air temperature Month Rainfall 1.3 1.3 3.3 5.6 8.7 11.7 13.1 12.9 11.0 8.2 4.1 2.5

January February March April May June July August September October November December

123 83 85 14 78 12 89 108 102 109 127 136

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79 (1995-J 13-28

gion. Accumulated temperatures in degree days above 56°C range from 825 to 1375 depending on elevation. The forest lies predominantly within the cool wet climatic zone (Pyatt, 1995). The growing season is around 200 days, but ground frosts can occur at any time, especially in extensive areas of grassdominated vegetation (e.g. Molinia caerulea, Deschampsiacespitosa).

Mean annual rainfall ranges from 1000 mm in the lower parts of the forest to about 1400 mm at higher elevations. This is distributed uniformly over the growing season (Table 2) with the driest periods occurring in late spring. Maximum mean monthly temperatures are about 13°C and minima are about 2°C typifying the mild, oceanic climate of the re-

Wind is a particular feature in the climate of upland Britain. Since there are no long-term wind records for the Kielder area, the best data are those from Eskdalemuir meteorological station located at 240 m elevation at a similar latitude 20 km further west. Fig. 1 shows the considerable annual variation in maximum wind speed at Eskdalemuir over a 50 year period up to 1990. A rule of thumb is that gusts in excess of 30 m s- ’ will blow over a few trees, those more than 35 m s-’ will uproot groups of trees, while those over 40 m s-’ have the potential to cause extensive damage throughout the forest (Quine, 1991; Quine et al., 1995). These data highlight the damaging gale of 1968 which caused extensive damage to forests further north in Britain (Holtam, 1971). Fig. 2 reveals the considerable vari-

80

- *.... Maximum

gust

ESKDALEMUIR LARKHILL TIREE

(m s-‘)

Fig. 2. Return period in years for a given wind speed at three meteorological stations in the British Isles: Larkhill (Oxfordshire), Eskdalemuir (Dumfries and Galloway), and Time (Western Isles of Scotland) (from Quine, 1995).

16

W.L. Mason,

C.P. Quine/Foresr

Ecology

ation in return period for damaging gusts for three different meteorological stations in Britain. The return period for a 45 m s- ’ gust at Eskdalemuir (and, by extension, at Kielder) is approximately once every 60 years. The return period will be influenced by elevation and on higher ground over 300 m at Kielder the interval might be 30 years or less.

3. Evolution

of silvicultural

practice

at Kielder

The initial 1925 survey of the North Tyne Valley suggested that perhaps one-third of the land acquired was plantable (H-M. Steven, Forestry Commission internal report, 1925): in current terms, this would be equivalent to a forest area of 20000 ha instead of the current 50 000 ha. In 195 1, distinctions were drawn on the basis of vegetation, soil type and altitude with land below 250 m being classed as easier to afforest (J.W.L. Zehetmayr, Forestry Commission internal report, 195 1: see also McIntosh, 1995, table 2). Results from early experiments on peat soils in Kielder and other areas were summarised by Zehetmayr (1954). In brief, they showed: the necessity for the use of raised planting positions, produced initially by upturned turves and later by single-furrow ploughing; the superior growth of spruces over all other species on sites where Molinia caerulea was dominant or abundant; the need to use pines Pinus spp. or larches Lark spp. where Calluna uulgaris was dominant; and the need for phosphate fertilisation on the poorer soils. The normal planting spacing usedwas between 1.4 and 1.7 m giving a stocking of 3500 and 5000 trees ha- ’ Researchover the next 30 years identified a number of limiting factors to spruce growth in upland Britain especially in the nutritional field (e.g. Taylor and Tabbush, 1990; Taylor, 1991). The antagonistic effects of heather (Malcolm, 1975) could be eliminated through application of herbicides and slow initial growth rates boosted by remedial applications of fertiliser. The normal prescription was for phosphate when planting pure Sitka spruce, with applications of potassiumalso required on sites with more than 30 cm depth of peat, but nitrogen inputs were unlikely (R.M. McIntosh, Forestry Commission internal report, 1983). A further application of phosphate and potassiumwas recommendedat 6-8 years

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79 (1995’1 L--28

after which no further inputs were anticipated. Taken together, the use of ploughing to provide a raised planting position, the application of herbicides to control weeds and a regulated input of fertilisers meant that it became possible to extend the use of Sitka spruce onto most soils in Kielder. This was at the expense of both pines and Norway spruce Picea abies, since with the required inputs, the yields from Sitka spruce would be 6- 10 m3 ha ’ year- ’ more than from pine and 2--4 m3 ha -’ year ! more than with Norway spruce. As a result, the prewar policy of matching a range of conifer speciesto soil and vegetation type was replaced by one where Sitka spruce was the first choice on all sites (see also McIntosh, 1995). As the early spruce plantations reached the first and secondthinning stage at around 25-30 years of age, the first problems with windthrow occurred (Day, 1949; Petrie, 1951: Maxwell MacDonald, 1952). This became an increasingly serious issuein the Border forests due to the combination of wet, shallow rooting soils and rolling landscapewith little topographic shelter from winter gales. Surveys (e.g. Pyatt, 1966)showeda greater incidence of windthrow on gleyed soils at higher elevations. Rooting studies (Fraser and Gardiner, 19671 showed greater resistance to overturning on deeply rooting soils than on the gleys. For instance, in Newcastleton, average rooting depth of spruce on brown earths was about 90 cm compared with about 40 cm on gleyed soils (Fraser and Gardiner, 1967, table 2). There was also a tendency for wider spacing and/or thinning to result in increased root spread. Integration of soil surveys, rooting assessments and perceived windthrow risk led to the development of hazard ratings for different site types. The first classificatian (Pyatt, 1968) distinguished sites of low, moderate and high hazard on the basis of the predicted occurrence of windthrow by the time stand top height had reached 20 m. Very little windthrow was expected before rotation age (trees over 25 m tall) in the low hazard sites; it would occur only after trees attained 20 m height in the moderate category; but would he significant before 20 m height in the high hazard class. An adaptation of this system to the Border forests (Godwin, 1968) involved anticipatory clear felling of vulnerable standsof 15-20 m top height. A further development was recognition that thin-

W.L. Mason,

C.P. Quine/Forest

Ecology

ning could increase the risk of wind damage. The removal of neighbouring trees prolonged the period of sway of the remaining tree crowns since the damping effect would be reduced (Booth, 1974). Such effects were more serious on the Border gleys because shallow rooting reduced root anchorage. It became normal practice to manage spruce stands at higher elevations without thinning (Foot, 1975) since studies had shown a lower incidence of windthrow under such regimes (Booth, 1974). This policy was reinforced by the introduction of a national windthrow hazard classification (WHC) (Booth, 1977; Miller, 1985) which classed sites in terms of elevation, topographic shelter, wind zone and soil type (i.e. rooting depth) with Class 1 being most stable and Class 6 being the least. The WHC provided estimates of critical height (defined as when 3% of a stand would be windblown) and terminal height (when 40% would be windblown and clear felling would occur) for each hazard class. The impact of this classification was particularly important at Kielder since 83% of stands were classed in Hazard Classes 5 and 6 where non-thinning would be the norm. A further stimulus to non-thinning was the collapse of the domestic small roundwood market for thimrings in the late 1970’s and early 1980s just as the WHC was introduced. Subsequent revisions to the WHC system (Quine and White, 1993) have resulted in some reduction of the forest area in the higher hazard classes. However, the practical implications are largely unchanged, namely that it is technically possible to establish pure stands of Sitka spruce on nearly all sites at Kielder, but most are not thinned for risk of causing wind damage and are clear felled at 35-40 years of age which can be lo-20 years less than the optimum rotation for volume production. This is accentuated by the need to fell stands early to achieve structural diversity in the restructuring process (Hibberd, 1985; McIntosh, 1989). Because of concerns over windthrow, it has not been considered prudent to retain stands on longer rotations except in very sheltered valley bottoms. 3.1. Restocking and regeneration Sitka spruce remains the predominant species in the second rotation with a reduction in the use of

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other conifer species and an increase in the percentage of broadleaves (McIntosh, 1995). Restocking is predominantly achieved by planting which has the advantage of allowing the use of higher yielding genotypes of Sitka spruce with the potential for 15-20% improvement in yield (Rook, 1992). Replanting of the clear felled stands on gley soils has not been without difficulty. Apart from browsing pressures exerted by roe deer Capreolus capreolus, small mammals and girdling damage by the weevil Hylobius abietis, the felling of stands at a comparatively early age when there is very limited ground vegetation results in water tables rising almost to the surface since there are neither trees nor vegetation to intercept rainfall (Pyatt and Craven, 1979). The high water content of the surface layers of gleyed soils results in anaerobic conditions and low spring soil temperatures inimical to root development of newly planted trees. Satisfactory establishment in such conditions has been achieved by mounding to provide a raised, aerated and warmer rooting environment plus the use of well conditioned cold-stored plants that can be planted later in the growing season (May) when soils have started to warm up and water tables have fallen (Tabbush, 1988). Natural regeneration of Sitka spruce can occur in considerable quantity after clear felling, particularly following good seed years, and on peaty soils where vegetation recolonisation is slower. Studies in progress (C.J. Nixon, personal communication, 1994) show densities of about 15000 seedlings ha-’ on a peaty gley soil in Kielder on an area clear felled at the end of the good 1990/ 1991 seed year compared with about 1200 ha-’ on an adjoining site felled a year later after a poor seed year. Earlier studies (Mair, 1973) showed that in good seed years in Britain about 22 million seeds ha-’ were found under 40-year-old seed stands of Sitka spruce. Calculation of seed dispersal rates indicated that nearly all seed fell within four to five tree heights of the surrounding stand (Mair, 1973). P. Oliver (unpublished data, 1993) surveyed the occurrence of natural regeneration of Sitka spruce on clear felled sites in northern England and southern Scotland including Kielder. Sites with abundant regeneration on gley soils were characterised by closeness to a seed source (80 m), low vegetation dominance (15% of ground cover) with a low (14%) frequency of grass species

I8

W.L. Mason.

C.P. Quine/

Forest Ecology

(Molinia, Deschampsia spp.), whereas sites with very low regeneration (i.e. occurring on less than 20% of the site) were a greater distance from a seed source (160 m), had a higher vegetative cover (46%) with a high (90%) frequency of grasses. Abundant natural regeneration of spruce occurs in larger (over 0.1 ha) gaps in mature stands (W.L. Mason and C.P. Quine, personal observation, 19941, but these seedlings are often severely damaged by extraction machinery during clear felling, and rarely contribute to the successor stand. The lack of predictability of good seed years is difficult to integrate with management aims to produce a sustained annual production of timber from clear felled stands and to restock these coupes costeffectively. The best that can be achieved is to recognise that, in good seed years, abundant regeneration can be expected after clear felling on peaty soils with limited vegetation. Such sites can be left to regenerate naturally with infilling of any substantial gaps by planting 2-3 years later. 3.2. Maintaining

productivity

Given a policy requirement for sustainable management of existing forests in Britain, the possibility of site impoverishment and declining yields due to repeated rotations of conifers is a potential source of concern. According to Evans (1990), there have been three documented case histories of this type of decline worldwide. The case most relevant to Kielder is the Norway spruce decline reported in Germany and Switzerland in the late nineteenth and early twentieth centuries (Troup, 1952; Pritchett, 1979). Although this problem was most serious on clay soils in Lower Saxony (Smith, 1986) where mixed forests were replaced by spruce plantations, it was further aggravated by low annual rainfall (500-650 mm) which increased the amount of stress the trees experienced. Spruce forests on gleyed soils of the Kielder area occur in a more oceanic climate than those in Germany, with substantially higher rainfall and lower evaporative demand during the growing season. Limited comparisons of early growth and productivity in experiments in Kielder give no indication of any decline from the first to the second rotation on the same site (Table 3). These data are at best indicative given that establishment practice has substantially

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79 (I 995) 13-2X

Table 3 Productivity (General Yield Class; m’ ha- I year ‘> of Sitka spruce in the first and second rotation on the same site in four different experiments at Kielder -Experiment First rotation Second rotation -..Kershope 10 20 24 Kielder 100 10 20 Falstone 6 12 20 Falstone 13 14 22 Note: data for the first rotation are based upon measurements at rotation age (40-45 years), whereas those for the second rotation are taken from assessments at lo-15 years.

improved since the first rotation. In addition, the second rotation stands considered are no more than 10-l 5 years old making it risky to extrapolate at such an early age to subsequent productivity. Recently, Proe and Dutch (1994) have shown that intensive whole-tree harvesting (WTH) (i.e. removal of needles and small woody material) on a peaty gley in Kielder resulted in a loss of growth of replanted spruce compared with conventional harvesting (i.e. needles and woody material retained on site). At 10 years, the loss of growth was about 1.0 m in height with equivalent losses in volume which could amount to about 2 m3 ha-’ year-’ if this difference persisted throughout the rotation. The initial differences may be due to a lack of shelter in WTH plots during establishment, but loss of nutrient capital from the site may become more important as the stand nears canopy closure. However, these limited data support Evans’ view (1990) that cases of yield decline are rare and due more to silvicultural mismanagement (e.g. inadequate weed control, site compaction during harvesting) than to any inherent feature of repeated conifer rotations. 3.3. Summary of existing silvicultural practice In terms of stand dynamics, the Kielder spruce forests comprise a mixture of open space, stand initiation and stem exclusion stages; but the understorey re-initiation and old growth stages(Oliver and Larson, 1990) are absent. The only silvicultural system being used is patch clear felling (Matthews, 1989) with variations in coupe size ranging from 5 to 15 ha in areasof visual and public impact to 70 ha or

W.L. Mason,

C.P. Quine / Forest

Ecology

more on higher ground remote from the public view. Diversification through restructuring will result in fragmentation of the large homogeneous blocks inherited from the first rotation, therefore a smaller proportion of stands will be vulnerable to wind catastrophe at any one time. Natural regeneration of spruce will occur in profusion at periodic intervals on favourable soil types but otherwise planting will be the norm. Broadleaved regeneration, particularly of downy birch Bet& pubescens,and limited planting of other conifers and broadleaves will provide somespeciesand visual diversity. Provided harvesting methodsdo not remove too much nutrient capital from the site or cause severe compaction and damage to the soil structure, there appears to be no reason, as yet, to suggest that this short rotation silviculture cannot be maintained over a number of rotations.

4. Silviculture and diversity in a windy climate The Helsinki Guidelines on SustainableManagement of Forests in Europe. (Anonymous, 1994) strongly encourage practices facilitating multi-purpose forestry and sustainablemanagementincluding the conservation and appropriate enhancement of biodiversity. For instance, it is stated that “silvicultural practices emulating nature should be encouraged”. Similarly, Ratcliffe (1993) proposesincreasing levels of biodiversity in managed forests in Britain, citing the use of felling regimes to create disturbance patterns that mimic nature, the desirability of providing greater amounts of coarse woody debris, the need for a wider variety of tree species and the value of standsretained well beyond the age of financial rotation. It is pertinent to evaluate the silvicultural practices described in previous sections in the light of these recent initiatives. In essence, the prevailing system is one which combinesrisk avoidance in the face of climatic hazard (i.e. anticipatory clear felling) with simplified managementproducing a range of commodities (e.g. timber, visual amenity, recreation, wildlife). Compromise between conflicting demands for different commodities is achieved primarily by zonation and by adjusting the scale of harvesting disturbance. However, the forest produced by re-

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19

structuring, while spatially more diverse than in the first rotation, is largely made up of homogeneous single species stands of comparatively small trees. Under normal management,the maximum age trees will reach is 40 years with a maximum height of around 20 m except in limited sheltered sites being retained for amenity and other non-timber benefits. However, in the old-growth forests of the Pacific north-west of North America, Sitka spruce can reach over 500 years of age and heights of 70-75 m (Waring and Franklin, 1979). Promoting enhanced structural diversity through the retention of more standsbeyond financial maturity would probably be the most important single measureto improve diversity in these spruce forests (Peterken et al., 1992). However, this is generally thought impracticable because of the risk of windthrow. While the limitations causedby shallow rooting on gley soils and wind exposure are real ones, silvicultural practices may have contributed to the incidence of windthrow in the Border forests. A number of these are considered in the following sections. 4.1. Spacing

The planting density in the early development of the Border forests was between 3500 and 5000 trees ha-‘. From the 1960s prescribed stocking density was reduced to around 1800 trees ha-’ on grounds of greater cost-effectiveness (reduced establishment costs and larger mean tree size at first thinning), but was increased in the 1980s to 2500 trees ha- ’ to guard against possible loss of wood quality through larger branches, more knots and a greater juvenile core. Wider spacing will result in wider root spread since the available rooting volume is distributed over fewer trees (Fraser and Gardiner, 1967). A further important aspect is that trees will develop a lower height:diameter ratio (h/d) due to increaseddiameter growth at the wider spacing. Increased diameter growth is beneficial becausethe stems are stronger (strength is proportional to diameter cubed) and more able to withstand the overturning moment exerted by the wind (Quine et al., 1995): this assumesthere is no restriction to root spread. Table 4 shows the development of the h/d ratio for three non-thin stands of Sitka spruce planted at three different

W.L. Mason.

20

C.P. Quine / Forest

Ecology

Table 4 Values of top height/mean diameter ratio (h/d) for unthinned Sitka spruce growing at Yield Class 12 a and planted at three different densities Age

20 25 30 35 40 50 75

Planting

density

(trees ha- ’ )

1100

2500

5100

50 58 59 61 65 69 70

66 71 78 83 86 91 91

12 84 9x 102 104 108 101

Source: Edwards and Christie a Yield class is equivalent to stand (m’ ha- ’ year- ’ ). Yield for a Sitka spruce stand in the

(1981). the mean annual increment of the Class 12 is an average growth rate Kielder forests.

densities (Edwards and Christie, 1981). Although precise interpretation of a h/d ratio is difficult becauseof interactions with soils and wind-climate, there is general agreementthat a value larger than 90 is indicative of a potentially unstable stand since the stems may not have sufficient strength to withstand the windloading exerted on the crowns. By contrast, h/d values less than 70-80 indicate trees with a more stable configuration (Savill, 1983). The values in Table 4 indicate that standsplanted at the highest density reach potentially critical h/d values by 25-30 years after planting. By contrast, those planted at current densities only reach critical levels at 40-50 years and those at wider spacing never reach such values. A further aspect is that high h/d ratios are indicative of greater susceptibility to snow loading: wet snow combined with wind is thought to have initiated stand damage in Kielder and elsewhere (C.P. Quine, personal observation,

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planted on the upturned soil ribbon. About 70% of the spruce forests in Kielder were establishedin this way. Unfortunately, subsequentstudies (Fraser and Gardiner, 1967; Savill, 1976) have shown that the root systems developed on these upturned soil ribbons were highly asymmetrical due to preferential rooting along the long axis of the plough ribbon combined with restricted rooting at right angles to the ribbon becauseof the presenceof furrow ditches. Even where roots were able to cross ditches, the change of angle at the edge of the furrow resulted in appreciable loss of rooting strength (Coutts, 1986). Studies in Kielder showed substantialloss in rooting strength when single furrow ploughing was compared with turf planting (Booth, 1974; see Table 5). Recognition of these problems resulted in investigation of alternative ploughing or cultivation methods such as double mouldboard ploughs, rigg and furrs. and mole drainage. However, only in the last !O years has there been a concerted move away from the use of ploughing. Current recommendationsfor cultivation of both surface water gleys and peaty gleys (Quine et al., 1991; D.B. Paterson and W.L. Mason, unpublished data, 1995) are to use mounds to provide a raised planting position and to encourage all round rooting combined with drainage to remove excess surface water. As an alternative, planting can take place on the upturned plough furrow from the previous rotation provided that it occurs well away from the stump so that symmetrical root systemsdevelop. 4.3. Drainage

One function of plough furrows associatedwith single furrow ploughing was the drainage of surface

1994). 4.2. Cultivation

The desirability of providing a raised planting position to ensure rapid establishment was recognised at an early stage in the establishmentof the Kielder forests. Initially upturned turves were used, but the advent of suitable machinery resulted in the extensive use of single furrow ploughing for site preparation from the 1930sonwards, with trees being

Table 5 Resistance of 25-year-old Sitka spruce moment kg m- ’ X 1000) after planting different cultivation techniques a

to overturmng (turning on a gley soil on two

Tree stem weight

Single furrow ploughing Upturned turf

b

(kg)

50

100

150

200

250

5 5

9 12

14 19

20 27

25 35

a After Booth (1974, Fig. 1 b). b Furrows were at 1.5 m spacing

and a depth of 30-35

cm.

W.L. Mason,

C.P. Quine / Forest Ecology

water from an afforestation site. The furrows would be combined with a network of deep (60-90 cm) drainage trenches. Once the problem of shallow rooting on gleyed soils was recognised, more intensive deep drains (i.e. at closer spacings and/or deeper ploughing) were tried with the hope of removing a greater volume of water to dry the site faster and promote deeper rooting. This expectation foundered on the impermeable nature of gleyed soils. Measurements reported by Pyatt et al. (1985) from a drainage experiment at Kershope indicated extremely low hydraulic conductivities of 0.6-60 mm day- ’ , indicating that drainage at lo-40 m intervals could only be effective in lowering the water table if the presence of the trees improved the conductivity of the soil. Unfortunately, other studies carried out on gleyed soils in the Carboniferous site region suggested that the drying out of the site that occurs owing to interception by closed spruce stands (Anderson and Pyatt, 1986) does not cause shrinkage or cracking in the subsoil, but instead results in increased bulk density (Pyatt and Craven, 1979). This lack of improvement in soil hydraulic conductivity meant that drains needed to be at about 2 m spacing to reduce water tables effectively. This was clearly impractical. These studies indicated a very limited potential for using drainage to improve rooting depth on gley soils, so that rooting depth of spruce is limited to no more than 450 mm owing to a combination of soil bulk density and high winter water table. Drain spacing is determined by topography (e.g. elimination of local wet hollows) and the need to manage the flow of water coming from the plough furrows. However, a recent re-evaluation of the Kershope drainage experiment (Ray et al., 1992) suggested a difference in response to drainage intensity between the peaty and surface water gleys. There was no effect of drainage intensity on the latter soils which occurred on a 12” slope. However, on peaty gleys with slopes of less than 6” the most intensive drainage caused a 75100% increase in the hydraulic gradient (Ray et al., 1992, table 3). There was a slight response in hydraulic conductivity to more intensive drainage (D. Ray, unpublished data, 1994). Shrinkage and cracking and rooting into the peat layer also improve the hydraulic conductivity of peaty gleys. More recent work shows a positive correlation between rooting depth and the mean winter water table

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(Ray and Nicoll, 1995). Therefore, on soils with organic horizons on flatter topography, drainage can enhance greater rooting depth and resistance to windthrow, provided that the cultivation and/or drainage technique employed does not restrict root architecture. 4.4. Thinning

Recommended times of first thinning for Sitka spruce(Hamilton and Christie, 1971) are between 18 and 30 years depending upon growth rate when the trees have a top height of 8.5-12.0 m. The yield models indicate the removal of 25-35% of trees in first thinning amounting to possibly 15-20% of the canopy assuminglow thinning. Since first thinning was often delayed becauseof the marginal profitability of harvesting small diameter trees, stands were opened up close to the critical height of 12- 14 m when their close initial spacing,limited root development and high h/d ratio made the trees particularly vulnerable to windthrow. This situation was aggravated by the use of line thinning to ensure machine access(Hamilton, 1976). Thus, no thinning resulted in a lower incidence of windthrow in experimental studies at Kielder than with selective or line thinning, with ratios of windthrown trees ha-’ of 1: 6.3: 23.8 respectively (Booth, 1974). An alternative strategy is to carry out early precommercial thinning before trees reach critical heights of about 12 m. If trees grow in height at between 0.5 and 1.0 m year-’ and it takes 3-5 years for canopy closure to recur after thinning, then thinning needs to be carried out when the trees are between 5 and 8 m in height. One advantageof such early intervention is that the need for later thinnings is reduced and the average tree size produced on a shortened rotation is increased (Ford, 1979). While the biological possibilities of such respacing techniques in promoting diameter growth have been known for sometime (Rollinson, 19881,their implementation on a wider scale has been limited because of the costs involved, concerns over potential losses in total yield and possible decline in timber quality. However, in terms of wind stability, standsof spruce trees that are respacedat a relatively early age when the root systemsare capable of adaptive responseto increased wind loading (Coutts, 1986) should, in

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Ecology

theory, be capable of closing canopy before reaching critical height and maintaining a favourable stand structure for a longer period assuming symmetrical root development. Such stands could develop h/d ratios close to the stand planted at 1100 stems ha-’ (Table 4) since respacing would occur at a sufficiently young age for trees to respond (Cremer et al., 1982). This strategy assumes that adaptive root growth compensates for increased wind loading on the crowns so that the risk of overturning is not increased. This assumption still requires adequate verification in the field.

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compensate for the dramatic increase in windloading that they experience. As a result, extensive windblow along the downwind edges of clear felled coupes is a common feature of spruce forests in this site region irrespective of coupe size. The only realistic way of avoiding this problem in pure spruce stands is to design the edges of felling coupes well in advance of critical height (e.g. by use of severance cuts, by increasing the spacing at the edge to increase wind permeability, by planting slower growing species in the margins) so that the trees downwind of the coupe are already acclimatised to the wind environment to which they will be exposed (Quine et al., 1995).

4.5. Coupe size 4.6. Development of windthrow There have been few studies on the effect of different size of clear felled area on stand stability and microclimate in upland spruce forests in Britain. The most important was carried out on a peaty gley soil in the Forest of Ae, Dumfriesshire where circular holes of 0.04, 0.12, 0.40 and 4.0 ha size were cut in 30-35 year old spruce stands growing on a peaty gley soil (Neustein, 1%4). This experiment was subsequently repeated in a less intensive manner using coupes of 0.12, 0.40 and 2.0 ha in Redesdale on the eastern edge of Kielder forest (Neustein, 1968). Although average windspeed was least in the smallest clearings, the incidence of windthrow along the perimeter edge in terms of number of trees blown per unit area was much higher in the 0.04 ha coupes than in the 4.0 ha size. This seemingly contradictory result can be explained by more recent work which showed that, although the average bending moment exerted on edge trees downwind of a gap only increased slowly with increasing gap size, the maximum bending moment increased very rapidly to the extent that there was no difference between the maximum force exerted at the edge of a gap two tree heights wide compared with one ten tree heights wider (Stacey et al., 1994). A further complication for the use of small coupes highlighted by these two experiments is the extreme difficulty of developing a wind stable edge due to the shallow rooting nature of the gley soils. For instance, windward roots are known to be a key component of root anchorage on gley soils (Coutts, 1986). Trees exposed by an adjoining coupe have neither the adaptive root or stem development to

gaps

Although there have been a number of surveys of the incidence of windthrow and the impact on management (e.g. Pyatt, 1968; Godwin, 19681, until recently there has been negligible investigation of the occurrence and extension of gaps in terms of their distribution in time and space. However, since 1989, two areas located either side of Kielder Water reservoir totalling 1099 ha have been monitored on an annual basis to examine the development of windthrow as part of the ongoing validation of the WHC (Quine and Reynard, 1990). The sites are removed from normal management operations (e.g. felling, thinning) for the duration of the monitoring, usually 10 years. The stands in the monitoring area are 72% spruce ranging from 7.5 to 21.5 m in top height and the elevation range is from 210 to 460 m elevation. Soil types are: brown earths, ironpans and podzols, 6.7%; peaty and surface water gleys, 62.2%; deep peats, 29.1%; skeletal soils, 2%. The area has been classified for wind hazard as follows, with 2.6% classed in Windthrow Hazard Classes 2 and 3, 16.3% in Class 4, 80.7% in Class 5 and 0.3% in Class 6. The two areas are thus broadly typical of the whole forest. Data presented in Tables 6 and 7 are derived from annual estimates of gaps taken from aerial photographs (Quine and Reynard, 1990). Figures in Table 6 show very limited change in the area windthrown over a 5 year period while those in Table 7 show most of the gaps occur at sizes of less than 30 m in diameter, i.e. at a scale of one to two tree heights. Year to year variation (Table 6) is due

W.L. Mason, Table 6 Total cumulative area windthrown monitoring areas at Kielder

from

C.P. Quine/Forest

Ecology

1989 to 1993 in the two

Year

Area windthrow (ha)

Range

1989 1990 1991 1992 1993

18.8 24.6 25.5 30.2 31.2

f 1.7 f 3.7 +2.8 + 2.3 +2.1

to the difficulty in deciding what comprises a gap or gap boundary on aerial photographs of different quality. There is no evidence of a regular progression from the initial occurrence of windthrow to terminal height as would be assumed by the current WHC. The annual rate of windthrow suggested by data in Table 7 is about 0.3% compared with a predicted 3-8% in the WHC (Quine, 199.5). 4.7. Mixtures The use of mixed species stands has been advocated in other parts of Europe as a possible solution to stability problems (e.g. Kenk, 1992). It is argued that different species can exploit different rooting zones in the soil and that, where a deciduous species is present in mixture, the leafless crowns allow greater filtering of wind through the upper canopy. Thus, in theory, the admixture of spruce with a deeper rooting, possibly deciduous species should result in the development of a more stable, stratified stand structure.

Table 7 Number of windthrown gaps of different size in 1993 in the windthrow monitoring areas at Bellingbum and Lewisbum, Kielder Gap size class (ha)

Bellingbum

Lewisbum

< 0.01 0.01-0.03 0.03-0.07 0.07-0.15 0.15-0.31 0.31-0.63 0.63-l .25 1.25-2.50

18 26 35 24 16 13 6 2

15 22 19 3 1 2 1

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23

There is, at present, only limited evidence to suggest that this hypothesis may be worth consideration on the Carboniferous gley soils. For instance, in an experiment in Gisbum Forest in northwest England, a greater percentage of fine roots of Scats pine Pinus sylvestris were found at a depth of 5-25 cm on a surface water gley soil whereas Norway spruce fine roots were primarily confined to the litter layer and the first O-5 cm (Brown, 1992). However,* this rooting difference did not confer greater wind stability since the experiment was clear felled at 34 years of age after windthrow that began in a Norway spruce/Scats pine plot (R.E.J. Howes, personal communication, 1994). The susceptibility to windthrow may have been induced by the fact that the experiment was planted on shallow (20-25 cm> single furrow ploughing at 1.5 m spacing at right angles to the prevailing wind. Other experimental trials of mixed species stands in the Kielder area have shown little benefit in terms of improving the wind stability of spruce (W.L. Mason, unpublished data, 1994). Often the admixed species has proved too vigorous (e.g. larch species) and has threatened to suppress the spruce or has itself proved vulnerable to windthrow. Alternatively, the non-spruce component has proved difficult to establish and has either died out in the first years after planting or has gradually been suppressed by the spruce during the development of the stand. However, an example of the latter type of ‘sacrificial’ mixture gives some support to the view that it may be possible to grow spruce trees up to at least 30 m tall over a much wider area of Kielder than is currently considered possible. This stand of 1.6 ha was planted in 1916 (before the Forestry Commission acquired land in Kielder) in a 50:50 row mixture of Scats pine and Sitka spruce, in alternating rows of each species at 1.2 m spacing. The planting took place on upturned turves, and the site is a peaty-gley soil at 230 m elevation with an estimated WHC of 4, indicating critical height for the onset of windblow as 19 m assuming a non-thinning regime. A single low thinning occurred in 1960. Estimated general yield class (Hamilton and Christie, 1971) of the Sitka spruce is between 16 and 18 m3 ha-’ year- ’ . Investigation of this stand began in the early 1980s once it became evident that its susceptibility

24

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Ecology

to windthrow did not conform to existing models. Rooting studies showed no evidence of deeper rooting than on other peaty-gley soils in the Kielder area (C.P. Quine, unpublished data, 1994). Interception loss was substantially higher (49%) than in younger spruce stands (29%) (Anderson and Pyatt, 1986) but apparently this has not affected the soil. Mensurational data (Table 8) show a two storied structure due to the overtopping and suppression of the pine by the spruce. Thus, although the two species were planted approximately in equal ratio, the spruce now account for 67% of surviving trees, 85% of basal area and 80% of standing volume. Recent measurements suggest that this sacrificial mixture may be reaching its biological limit at between 30 and 35 m in height for Sitka spruce. Thus in 1987, 3% of the stand was windthrown whereas by 1994 this had reached 19%. The number of trees blown in the last 7 years is equivalent to 11.6 trees ha-l year ’ , but this elimination of one or two trees in each gale may be part of an inevitable process of initial stand break-up on shallow rooting gleys postulated by Day (1963) and the structure may yet stabilise. The basal area in this unique spruce/pine mixture is almost three times higher than values recorded by Jonsson and Dynesius (1993) for old growth spruce forests in northern Sweden. The enhanced stability of this stand is probably due to the lack of any impediment to the development of a symmetrical root architecture and to the gradual stratification of the canopy being equivalent to a response to thinning, but without the loss of any damping effect because of physical removal of sup-

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pressed trees. A similar effect could have been produced in a pure spruce stand by chemical thinning of unwanted stems (Ogilvie and Taylor, 1984) since the poisoned trees take a number of years to die and so continue to exert a limited damping effect while surviving crowns expand to fill the space available. While a similar process of differentiation should occur between trees in a pure non-thin spruce stand, there may be limited site-induced variation on the relatively uniform soils of the Kielder area. with the consequence that non-thin stands can become tall.. drawn and eventually unstable as density dependant competition develops (Oliver and Larson, 1990, p. 220).

5. Discussion The requirements of the Helsinki Guidelines imply progress beyond the current strategy of seeking a compromise between conflicting demands placed upon the forest to a synthesis of these demands in an ecosystem approach to forest management. This would value the forest as a living system in its own right as promoted in the New Perspectives Initiative in the United States (Kessler et al., 1992). Unfortunately, while the philosophical ideal of an ecosystem management approach to the Kielder spruce forests may be alluring, there are a formidable number of practical topics which require research and resolution. Among these are: what are the natural forest systems that should be mimicked; what is the scale, frequency and intensity of disturbance to

Table 8 Details of a self-thinning Sitka spmce/Scots pine mixture (Birkley Wood) at 73 years of age contrasted with predictions from Forestry Commission yield tables (Edwards and Christie, 1981). The stand had 829 surviving trees of which 554 were Sitka spruce. Top height of spruce is 3 1.7 m and of pine 20.0 m .Mixture Yield tables (Yield Class 16, 1.4 m spacing) Spruce Pine Live stems (ha- ’ ) Dead stems (ha- ’ ) Basal area (m* ha- ’ ) Standing volume (m” ha- ’ ) Mean tree volume (m3) NA, not applicable.

346 41 54.8 600 2.22

172 166 9.6 150 NA

No thin

Intermediate

1247

286

86.0 943 0.76

41.5 575 2.01

thin

--

W.L. Mason,

C.P. Quine / Forest

Ecology

be expected in spruce forests where wind is the major disturbance agent; what species are appropriate to promote or to introduce to increase the diversity of the forest; and to what age can stands be maintained so that vegetation re-initiation or even old-growth stages can develop? Despite having a major and several minor species in common, there are few clear parallels with the coniferous forests of the Pacific North-West, primarily because of the much shorter return period of potentially damaging storms and the difference of the soils in the Kielder area. Examination of Fig. 2 suggests that catastrophic wind damage (gusts over 40 m s - ’ > may be expected to have a return period of between 1 in 15 years up to 1 in 50 years depending upon stand location in the forest (i.e. hilltop, valley bottom). This suggests closer similarities with tropical forests subject to periodic hurricanes (Attiwill, 1994) than with temperate rain forests of the Pacific north-west of North America where there is a pattern of small-scale gap development interrupted at intervals of perhaps 150-200 years or more by catastrophic fires or windstorms (Hansen et al., 1991; Agee, 1993). There are similar difficulties in making analogies with boreal forests in Europe where fire is the primary disturbance agent recurring at intervals of perhaps 80- 100 years (Zackrisson, 1977) although in recent history the return period has been almost doubled by human intervention. However, there is a similarity between the role of clear felling and of fire management, in that controlled burning can reduce the intensity of major fires in the same way that clear felling on short rotations seeks to minimise the area of forest at risk from a catastrophic gale (Waring and Schlesinger, 1985). However, it is not strictly correct to consider clear felling, whether on a scale of 5 or 70 ha, as analogous to natural disturbance, since, in normal practice dead or dying trees will be harvested along with the remainder of the stand. Increasing the amount of coarse woody debris left on a site is a way of increasing structural diversity in the subsequent rotation (Hansen et al., 1991). Unfortunately, in the wind dominated forests of Kielder, it is not easy to ensure that either dead trees or live trees can be retained standing on clear felled areas. Such trees will be prone to windthrow in the same way as the edge trees round the small coupes tested in the 1960s

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25

(Neustein, 1964). The most satisfactory way of achieving the desired structural diversity will be to identify potential stands for long-term retention at an early stage and promote a more wind-firm structure through early respacement. Examination of natural regeneration on favourable sites shows abundant regeneration of Sitka spruce and infrequent occurrence of Norway spruce, larch, Scats and lodgepole pine (W.L. Mason, personal observation, 1994). There is also profuse regeneration of downy birch and rowan Sorbus aucuparia where browsing pressurefrom deer is eliminated by fencing. While mixed speciesstandsmay develop in the stand establishmentphase,it seemsunlikely that these will persist beyond early canopy closure when Sitka spruce is likely to dominate most standsowing to a combination of more vigorous height growth, stronger branching and much greater abundance.The mixture of speciesshowssomeaffinity with Scandinavian boreal forests with a relatively shadetolerant spruce, a light demandingconifer (pine, larch) and a fast growing pioneer broadleaved species(birch). By contrast, shade-tolerant speciescharacteristic of the coastal British Columbian forests such as western hemlock Tsuga heterophylla, westernred cedar Thuja plicata and Pacific silver fir Abies amabilis are very scarce in these British spruce forests so that their regeneration potential is minimised. Given a rotation of 40 years and a forest structure characterised by open space, stand initiation and stem exclusion phases,the most valuable means of increasing diversity appearsto be to managea proportion of the forest on a longer rotation. At present, only Birkley Wood (Table 8) and some other isolated examples (D.C. Jardine, personal communication, 1994) suggest that there may be potential to maintain some standsto more than 30 m in height and for upto 80 years. A key research requirement is to identify further stands and sites that possessthe potential attributes for long-term stability, e.g. symmetrical rooting, adequate growing space for a favourable h/d ratio, wind-firm edge and to maintain these without intervention but with regular monitoring to see their biological potential. Such standsmight become rich in standing and fallen dead wood and should provide valuable insights into the long-term structural development of these new conifer forests (see also Pe-

26

W.L. Mason.

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Ecology

terken, 1987). Given the likelihood of more mixed species stands developing as a result of natural regeneration (e.g. spruce-birch), some long-term experimentation is desirable to consider the dynamics of such mixtures including the possible impact upon stability and product outturn. Given present knowledge, the introduction of alternative silvicultural systems (Matthews, 1989) to patch clear cutting into these spruce forests would be a considerable risk. Such systems are generally based upon central European experience in a climate with a lower frequency (one in 100 or more years) of catastrophic disturbance (Larkhill curve in Fig. 2) than that at Kielder, and without the aggravating problems of shallow rooting on gleyed soils. There needs to be a better understanding of the interaction between stand structure and wind flow before such systems could be used on any scale. The nature of the wind climate may also change over time which may have a major impact on forest stability because of the non-linear nature of the relationship between windspeed and recurrence (Quine, 1995). The present practice of patch clear felling at Kielder is unlikely to mimic the pattern of natural disturbance. The limited data (Table 7) suggest a natural pattern of small scale gap formation every 3-5 years and catastrophic damage on a landscape level perhaps once every 30-40 years. The current restructuring processis fragmenting the uniform forest structure so as to limit the potential impact of the catastrophic gale that will inevitably occur. The only way of promoting diversity within coupes is by retaining areasas leave strips or potential dead wood, provided these have been identified well in advance. If there is a lesson to be drawn, it is that the possibilities of promoting diversity in a forest depend upon an appreciation of the interaction between forest, soils and climate which sets the frame for the realisation of any managementobjective.

6. Conclusion There are a number of specific and general conclusions that can be drawn from this paper. At the specific level, it seemsprobable that a combination of particular cultivation, spacing and thinning practices on these gleyed soils resulted in the creation of

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forests that were particularly predisposedto the risk of windthrow. This problem was exacerbated by the occurrence of damaging gales in the late 1960s and early 1970s (Fig. I). The rarity of these events was not fully appreciated at the time. A better appreciation of the importance of root architecture, stem sturdiness and canopy interaction after a critical height should result in secondrotation spruce stands that are more wind-stable, assumingthe frequency of damaging gales does not increase. Reaction to the threat of windthrow has resulted in attempts to predict it in a deterministic way so as to avoid risk. This has underestimatedthe possibility that stable, comparatively long retention standsmay occur due to the interaction of particular silvicultural managementand the non-recurrence of damaging winds. Planning needsto cater more explicitly for the probability of wind damagein spaceand time rather than assuming that it will occur in a certain stand at a preordained moment. Finally, if there is some potential, for retaining standsfor a longer period on these intractable gley soils where wind disturbance dominates siiviculture, then the potential for increasing structural diversity in other spruceforest site types in Britain is appreciably greater, given the higher incidence of deeper rooting soils and lessexposed sitescompared with the shallow rooting Border gleys (Paterson, 1990). The first general precept is that the successful creation of new forests, whether of native or introduced species,dependsupon an understandingof the limiting factors that will affect the trees throughout their lifespan. Limiting factors at one stageof development may not be those that apply at a later one. The secondis the need for the prevailing disturbance agent to be identified and studied, particularly in relation to the presence or absenceof silvicultural intervention. Thirdly, disturbance can occur at a scalemuch larger than conventional experimentation which typically focuses on the stand or its components. The need to integrate information at different spatial scalesis essential if meaningful information is to be provided to the forest manager so that a sustainableflow of wood products and non-market values can be maintained. Fourthly, and perhaps most important for those involved with plantation forests which are required to meet new objectives, there is an overriding imperative to learn from and

W.L. Mason,

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Ecology

about the forests as they exist on the ground, and not to attempt to mimic forests in other soils, regions or climates.

Acknowledgements

We are grateful to many of our colleagues for their ideas, observations and comments. In particular, Chris Nixon, Janet Dutch, Graham Pyatt and Duncan Ray have provided valuable criticism. Douglas Malcolm, Steve Petty and an anonymous referee provided valuable comment on the draft.

References Agee, J.K., 1993. Fire Ecology of Pacific North-West Forests. Island Press, Washington, 493 pp. Anderson, A.R. and Pyatt, D.G., 1986. The interception of precipitation by pole-stage Sitka spruce and lodgepole pine and mature Sitka spruce at Kielder Forest, Northumberland. Forestry, 59: 29-38. Anonymous, 1994. Sustainable Forestry: The UK Programme. HMSO, London, 32 pp. Attiwill, P.M., 1994. The disturbance of forest ecosystems: The ecological basis for conservative management. For. Ecol. Manage., 63: 247-300. Booth, T.C., 1974. Management of high-risk forests in Great Britain. Ir. For., 31: 145-153. Booth, T.C., 1977. Windthrow Hazard Classification. Res. Inf. Note 22/77/SILN, Forestry Commission, Edinburgh, 4 pp. Brown, A.H.F., 1992. Functioning of mixed-species stands at Gisbum, N.W. England. In: M.G.R. Cannell, D.C. Malcolm and P.A. Robertson (Editors), The Ecology of Mixed Species Stands of Trees. Blackwell, Oxford, pp. 125-150. Coutts, M.P., 1986. Components of tree stability in Sitka spruce on peaty gley soil. Forestry, 59: 173-197. Cremer, K.W., Borough, C.J., McKinnell, F.H. and Carter, P.R., 1982. Effect of stocking and thinning on wind damage in plantations. N.Z. J. For. Sci., 12: 244-268. Day, W.R., 1949. The soil conditions which determine windthrow in forests. Forestry, 23: 90-95. Day, W.R., 1963. The development of Sitka spruce on shallow peat. Scot. For., 17: 219-236. Edwards, P.N. and Christie, J.M., 1981. Yield Models for Forest Management. Forestry Commission Booklet 48, HMSO, London, 32 pp. Evans, J., 1990. Long-term productivity of forest plantationsstatus in 1990. Proc. XIV IUFRO World Congress, 5-11 August 1990, Montreal, Canada. IUFRO Secretariat, Vienna, pp. 165-180.

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Foot, D.L., 1975. Forest management in Craik: a review of current thinking with special reference to windblow. Scot. For., 29: 129-134. Ford, E.D., 1979. An ecological basis for predicting the growth and stability of plantation forests. In: E.D. Ford, DC. Malcolm and J. Atterson (Editors), The Ecology of Even Aged Plantations. Institute of Terrestrial Ecology, Cambridge, pp. 147-174. Forestry Commission, 1991. Forestry Policy for Great Britain. Forestry Commission, Edinburgh, 14 pp. Forestry Commission, 1993. Forestry Facts and Figures 1992-93. Forestry Commission, Edinburgh, 10 pp. Fraser, A.I. and Gardiner, J.B.H., 1967. Rooting and Stability in Sitka Spruce. Forestry Commission Bulletin 40, HMSO, London, 28 pp. Godwin, G.E., 1968. The influence of wind on forest management and planning. Forestry, 41 (SuppI.): 60-66. Hamilton, G.J., 1976. Aspects of Thinning. Forestry Commission Bulletin 55, HMSO, London, 138 pp. Hamilton, G.J. and Christie, J.M., 1971. Forest Management Tables. Forestry Commission Booklet 34, HMSO, London, 201 pp. Hansen, A.T., Spiers, T.A., Swanson, F.J. and Ohmann, J.L., 1991. Conserving biodiversity in managed forests: Lessons from natural forests. Bioscience, 41: 382-392. Hibberd, B.G., 1985. Restructuring plantations in Kielder Forest District. Forestry, 58: 119-129. Holtam, B.W., 1971. Windblow of Scottish Forests in January 1968. Forestry Commission Bulletin 45, HMSO, London, 53 PP. Jonsson, G. and Dynesius, M., 1993. Uprooting in boreal spruce forests: Long-term variation in disturbance rate. Can. J. For. Res., 23: 2383-2388. Kenk, G.K., 1992. Silviculture of mixed-species stands in Germany. In: M.G.R. Cannell, D.C. Malcolm and P.A. Robertson (Editors), The Ecology of Mixed Species Stands of Trees. Blackwell, Oxford, pp. 53-63. Kessler, W.B., Salwasser, H., Cartwright, C.W., Jr. and Caplan, J.A., 1992. New perspectives for sustainable natural resources management. Ecol. Appl., 2: 221-225. Mair, A.R., 1973. Dissemination of tree seed; Sitka spruce, westem hemlock and Douglas fir. Scot. For., 27: 308-314. Malcolm, D.C., 1975. The influence of heather on silvicultural practice-an appraisal. Scot. For., 29: 14-24. Malcolm, D.C., 1979. The future development of even-aged plantations: silvicultural implications. In: E.D. Ford, D.C. Malcolm and J. Atterson (Editors), The Ecology of Even-Aged Plantations. Institute of Terrestrial Ecology, Cambridge, pp. 481504. Matthews, J.D., 1989. Silvicultural Systems. Oxford University Press, Oxford, 284 pp. Maxwell MacDonald, J., 1952. Wind damage in middle-aged crops of Sitka spruce and its prevention. Scot. For., 6: 82-85. McIntosh, R.M., 1985. The history and multi-purpose management of Kielder Forest. For. Ecol. Manage., 79: 1- 11. McIntosh, R.M., 1989. Forest design: Kielder Forest restructuring. Timber Grower, 19-20.

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Quine, C.P. and White, I.M.S.. 1993. Revised windiness scores for the windthrow hazard classification: the revised scoring method. Res. Inf. Note 230. Forestry Commission, Edinbur& 5 PP. Quine, C.P., Bumand, A.C., Coutts. M.P. and Reynard. B.R.. 1991. Effects of mounds and stumps on the root architecture of Sitka spruce on a peaty gley restocking site. Forestry, 64: 385401. Quine, C.P., Coutts, M.P., Gardiner, B.A. and Pyatt, D.G., 1995. Forests and Wind: Management to Minimise Damage. Forestry Commission Bulletin 114, HMSO, London. Ratcliffe. P.R., 1993. Biodiversity in Britain’s Forests. Forestry Commission, Edinburgh, 27 pp. Ray, D. and Nicoll, B., 1995. Effects of soil water on root development and stability of Sitka spruce. Proc. of Plant Biomechanics Congress, Montpellier, France, September 1994. Ray. D., White, I.M.S. and Pyatt, D.G.. 1992. The effect of ditches. slope and peat thickness on the water regime of a forested gley soil. Soil Use Manage., 8: 105 I1 I Rollinson. T.J.D.. 1988. Respacing Sitka spruce. Forestry, 61: I-22. Rook, D.A., 1992. Super Sitka for the 1990’s. Forestry Commirsion Bulletin 103, HMSO, Landon, 75 pp. Savill, P.S., 1976. The effect of drainage and ploughing of surface water gleys on rooting and windthrow of Sitka spruce in Northern Ireland. Forestry, 49: 133-141. Savill, P.S., 1983. Silviculture in windy climates. For. Absb.. 44 473-488. Smith, D.M., 1986. The Practice of Silviculture, 8th edn. John Wiley. New York, 480 pp. Stacey, G.R., Belcher, R.E., Wood, C.J. and Gardiner, B.A., 1994. Windflows and forces in a model spruce forest. BoundaryLayer Meteorol., 69: 3 1 l-334. Tabbush. P.M., 1988. Principles of Upland Restocking. Forestry Commission Bulletin 76, HMSO, London, 22 pp. Taylor, C.M.A., 1991. Forest Fertilisation in Great Britain. Forestry Commission Bulletin 95, HMSO, London, 45 pp. Taylor, C.M.A. and Tabbush, P.M., 1990. Nitrogen Defmiency in Sitka spruce Plantations. Bull. 89, HMSO, London, 20 pp. Toleman, R.D.L., 1979. Ecology of even-aged plantations-site classification. In: E.D. Ford, D.C. Malcolm and J. Atterson (Editors), The Ecology of Even-Aged Plantations. Institute of Terrestrial Ecology, Cambridge, pp. 23-37. Troup, R.S., 1952. Silvicultural Systems, 2nd edn. Oxford University Press, Oxford, 216 pp. Waring, R.H. and Franklin, J.F., 1979. Evergreen coniferous forests of the Pacific Northwest. Science, 204: 1380-1386. Waring, R.H. and Schlesinger, W.H., 1985. Forest Ecosystems: Concepts and Management. Academic Press, London, 340 pp. Zackrisson, O., 1977. Influence of forest fires on the North Swedish boreal forest. Oikos, 29: 22-37. Zehetmayr, J.W.L., 1954. Experiments in Tree Planting on Peat. Forestry Commission Bulletin 22, HMSO, London, 110 pp.