The natural distribution of Sitka spruce is examined in relation to the species' role in British forestry. Particular attention is therefore given to a description of habitats on the Queen Charlotte Islands, and also on the adjacent mainland of British Columbia where Sitka spruce is sympatric with white spruce (Picea glauca) and hybridisation occurs. Examples of genetic variation are given and it is suggested that a knowledge of the genecologv of the species in its natural habitat would help to elucidate its full ecological and genetic potential in Britain.

The taxonomic and phylogenetic affinities of Sitka spruce (Picea sitchensis) are reviewed in relation to the more general picture of spruce biogeography and evolution.

Because of the wide latitudinal range of Sitka spruce, trees of different origin show very large differences in growth rate and adaptability when grown on a variety of sites in Britain. The Forestry Commission has forty-eight seed origin trials, planted between 1929 and 1975, which are now beginning to provide a good overall picture of this variation. On the other hand, differences in morphology and colour are relatively minor, so that it is very difficult to identify the seed origin of a stand of trees on visual grounds alone. Southerly origins (below Latitude 47°N) grow fastest, but they suffer more severe frost damage, particularly in the nursery, and tend to have lower density timber than those from 53°N or above.

Sitka spruce is a monoecious, wind-pollinated, cross-fertilising species showing wide genetic variation which suggests heterozygosity for many alleles and natural selection against self-fertilisation. Phenotypic selection for the important trait of vigour is ineffective, so testing progenies of selected individuals and clonal testing is an essential and time-consuming part of any improvement programme. Old trees can be vegetatively propagated by grafting and very young trees by rooted cuttings.

The British tree improvement programme is based, in the short term, on the use of seed collected from superior plantation trees of desirable origins; in the mid term on seed derived from clonal orchards based on mixtures of clones previously tested for superiority in family tests; and in the longer term on highly superior seed or clones derived from a system of recurrent mating with family selection. Three populations are being developed. There is a small interspecific hybridisation programme.

Picea sitchensis has a haploid complement of twelve metacentric or submetacentric chromosomes which are similar to each other in size. Prominent secondary constrictions, which are probably the sites of nucleolus organisers, occur on five chromosomes (N.O.R.s). The DNA content of the haploid, unreplicated chromosome set is 9 × 109 nucleotide pairs and, in spite of earlier claims, probably does not vary significantly between different populations. B-chromosomes occur extensively in the southern half of the species range. They have little effect on growth rate but delay the time of ♂ and ♀ flowering. Meiosis starts and ends in spring, taking about twenty-two days to complete. The B-chromosomes do not pair at meiosis and appear to be distributed at random. There is marked accumulation of B-chromosomes when transmitted through the ♀ parent, possibly due to preferential segregation during meiosis. A balance between gain by accumulation and loss due to delay of flowering may account for the natural distribution of the B-chromosome.

Young Sitka spruce trees grow slowly, compared with poplar for instance, because of lower photosynthetic rates and slower rates of leaf area production. On the other hand, the canopies of Sitka spruce stands have structural and photosynthetic properties which make them very productive. Both the slow growth of young trees, and the high productivities of closed stands, can be attributed to the needle habit.

Experiments have shown that Sitka spruce on poor soils requires relatively frequent applications of fertiliser, phosphorus being most commonly required, other deficiencies in decreasing frequency being potassium, nitrogen and, very rarely, copper. Recent evidence suggests that growth responses to fertilisers are commonest while stands are young and the green canopy is being established. Spruce has a heavy crown and in consequence nutrient demands are high prior to canopy closure. Thereafter, the annual production of foliage by spruce is no greater than that of other species of comparable growth rate and by this stage the establishment of cycles within the tree and through the litter layer enable efficient re-use of nutrients and hence the demands placed upon the soil fall dramatically.

The general adaptations of the species are described in relation to the environment of its natural distribution and are shown to be well suited to its use as an exotic in forestry. What is known of natural stand dynamics has little relevance to the cultivation of the species in pure plantations but the interaction with climate and soils is important in determining the limits to its growth and in prediction of productivity. Atmospheric humidity, windiness and accumulated temperature appear to be the most influential climatic variables and, given adequate soil conditions, can be used to give good estimates of potential production. Sitka spruce flowers freely and regenerates readily, demonstrating its ability to survive unaided in British conditions.

Sitka spruce is the most important species grown in Scottish forests thanks to its high yield potential, site tolerance and wood properties. It is easily raised from seed or cuttings, and can be readily established and tended under a wide range of site conditions using uncomplicated silvicultural techniques. Spaced furrow ploughing and site drainage are normally required for afforestation; and cultivation may also be desirable prior to restocking. Planting at 2500 trees/ha is done in late winter and spring. Weed control may be necessary, particularly to prevent Calluna-induced growth check. On poor sites, repeated fertiliser application prior to canopy closure can be essential to ensure satisfactory growth, and use of nursing mixtures may be worthwhile. Wherever possible, pole stage crops are thinned regularly to enhance stem quality and diameter growth, but on high windthrow hazard sites the onset of windthrow can be delayed by leaving crops unthinned. After an economic rotation of forty-five to sixty years, crops are clear felled and replanted. During establishment, deer control measures are often required to prevent browsing damage. Damage by insects and fungi is seldom serious, provided that bark-feeding insects are controlled by insecticide at time of replanting, and development of fungal butt rot is minimised by urea treatment of fresh stumps.

Eighty-four potentially mycorrhizal macrofungi have been recorded with Sitka spruce in Scotland. They derive primarily from the native flora of birch and pine and many are fungi with a wide host range. Specific mycorrhizal associates do not occur. In its native range hypogeous fungi are important but these are largely absent from Scotland. The regularly encountered mycorrhizal flora is less abundant and less diverse than that of Norway spruce or European larch and reflects the importation of Sitka spruce by seed and the ecological conditions to which it gives rise. The saprotrophic macrofungi are species which are widespread and common in a range of vegetation types.

Relatively little research has been done on mycorrhizas of Sitka spruce, although greenhouse studies have confirmed that its growth can be improved by the introduction of mycorrhizal fungi. Work in nurseries is difficult under British conditions and it probably will be necessary to disinfest soil in seedbeds before mycorrhizal treatment can be applied. Results from forest trials show that inoculation with selected mycorrhizal fungi can give significant early growth effects, though how long these will persist is unknown. In such work, careful attention must be given to selection of controls. Features covered in this review include a discussion of fungi mycorrhizal with Sitka spruce, identification of mycorrhizas, nutrient uptake, and growth enhancement of seedlings and transplants.

The growth and development of Sitka spruce roots is reviewed against the practical background of tree stability. Two features of the roots which make Sitka spruce liable to windthrow on upland sites are the shallow development in unfavourable soil conditions, and restricted lateral spread and anchorage caused by some forms of cultivation. Responses to different aspects of the soil environment are discussed, with emphasis on mineral nutrition, drying of the soil, waterlogging and temperature. The development of a small number of major woody roots from an assortment of primary laterals on the transplant occurs at an early age and is determined both by the relative size and vigour of the primary roots and by their local environment. The roots, which respond to increases in nitrogen and phosphorus supply with enhanced growth, are sensitive to drying of the soil and are particularly so to waterlogging, but they show no unusual response to soil temperature. Adventitious roots can play a role in the survival and development of the root system.

An account of the pathology of Sitka spruce in northern Britain is given based on enquiries dealt with by the Pathology Branch of the Forestry Commission Research Division in the last fifteen years. Sitka spruce occupies the major proportion of our forest estate but has given rise to only 15% of enquiries concerning tree damage. The most frequently diagnosed injuries have been those due to frost and to root diseases caused by Heterobasidion annosum and Armillaria spp. Two other root pathogens, Rhizina undulata and Phaeolus schweinitzii, and two needle pathogens, Lophodermiun piceae and Rhizosphaera kalkhoffii, have also been recorded in Sitka spruce plantations as have several wound-infecting stem decay fungi. Only five other fungal diseases, those caused by Botrytis cinerea, Phytophthora citricola, Helicobasidium purpureum, Sirococcus strobilinus and Pythium spp. have been recorded, all in nurseries.

The status of insects on Sitka spruce in Britain is examined in relation to the attributes of this tree as a widely planted exotic. Both time in Britain and particularly the area planted influence insect species diversity. The abundance and status of selected insects of pest importance are examined. Future trends receive attention, particularly in relation to potential importation of pest insects and this is examined for a number of species from Europe and North America.

Sitka spruce comprises the largest single forest type in Britain and provides a vast habitat for wildlife. However, many improvements are still possible. No species of bird or mammal occurs equally throughout a conifer rotation, their distribution being affected by both the temporal and spatial dynamics of plantations. We suggest a number of practical improvements, including the modifications to felling-coup size and the incorporation of other tree and shrub species, to increase the diversity and abundance of wildlife in Sitka spruce forests without seriously affecting the forest enterprise.

Sitka spruce grown from seed has a juvenile phase of about twenty years before coning commences; mature trees and grafted scions cone intermittently and produce good cone crops every three–five years. These characteristics have lengthened the breeding programme and reduced seed production in orchards. Production of male and female cones can be enhanced by treatments such as heat and drought and by application of a mixture of gibberellins A4 and A7 (GA4/7) to mature grafts. To enhance flowering consistently in container grown grafts the GA4/7 must be applied together with a cultural treatment, but with larger grafts in the field GA4/7 alone is often effective. Attempts to induce cone production in juvenile trees, however, have been less successful. Female cones initiated on field grown trees in response to inductive treatments yield seed of a similar quantity and quality to that from cones on untreated trees. The physiological mechanisms of coning, and cone induction techniques, are discussed.

A combination of two vegetative techniques is seen as a possibility for large-scale production of juvenile, rooted Sitka spruce cuttings of improved genotype. Tissue culture techniques, under development, would be used to produce large numbers of stock plants for stem cuttings production. Cuttings techniques, currently under commercial trial, would be used to produce the rooted plants for forest establishment.

The wood properties of Sitka spruce (Picea sitchensis) which influence its use in the production of sawn timber, veneer, particleboard, fibreboard, pulp and paper are discussed. Although the low strength properties of Sitka spruce restrict its use as constructional timber, it is highly suitable for the manufacture of particleboard and fibreboard and is an excellent pulping species.

Man's use of the timber of Sitka spruce grown in North America and in the United Kingdom is reviewed. The contrast in character and properties of the timber from the two sources is considered and an explanation for the differences given in terms of the growth. The importance of Sitka spruce to the British forest and timber industries is examined and consideration given to how Sitka spruce should be managed to ensure the production of timber which will meet the requirements of the British timber user.