Betula is well represented in the Pleistocene deposits of the British Isles; B. nana is often recovered from glacial sediments and tree Betula flourished early and late in the interglacial periods. In western and northern Britain, tree Betula has remained important in the forests since the last glacial period.

The variability of the pubescent birch and the probable existence of hybrids between it and two other birches make it a difficult species to classify at an infraspecific level. To some extent, the variation is geographical and to cover its distribution, three subspecies are proposed by the authors of Flora Europaea, sspp. pubescens, carpatica and tortuosa. Two subspecies are recognized in Britain, namely ssp. pubescens and ssp. odorata, but some confusion has arisen through the undetermined relationships between sspp. odorata, carpatica and tortuosa. The rejection of ssp. odorata is proposed as its application, in this case, differs from that intended by J. M. Bechstein, the original authority. An examination of the literature indicates that ssp. tortuosa and ssp. carpatica are probably indistinguishable (a conclusion supported by multivariate analysis) and that the epithet tortuosa can be applied to both taxa. The pubescent birch in the Scottish Highlands may be referred to ssp. tortuosa and, in addition, the high frequency of small-leafed individuals in some populations in the northwest and at higher altitudes, justifies the further distinction of var. microphylla Hartman.

While B. pendula can be distingushed from B. pubescens by its diploid number of chromosomes, 28 compared with 56, separation by morphology depends upon a blend of variable characteristics, including leaf hairiness, occurrence of warts on branches, roughness of bark, pendulous habit and, most consistently, leaf shape.

Individuals of B. pendula, like those of B. pubescens, are mostly self-incompatible. The dates of flowering of individuals within a population of trees may differ markedly. However, female flowers usually mature sooner than male flowers on the same tree. The pollen of Betula spp. can be wind-dispersed considerable distances. Seed production varies greatly from year to year. Within a seedlot, the onset of germination may differ by as much as 7 days.

When side by side at latitude 56°N, trees from seed collected at southerly sites (lat. 50°N) in the natural ranges of B. pendula and B. pubescens grew taller than those from northerly (lat. 69°N) locations. However, when compared at latitude 69°N, the more northerly collections were the more vigorous. The seasonal onset of growth of different B. pubescens seedlings depended upon their origins. Seedlings from latitude 56°N commenced growth when days were 14h long; those from 63° and 70°N started when days were 16 and 20 h long respectively. Whereas northerly collections ceased growth when daytime temperatures dropped below 9°C, southerly collections continued to grow. The stem extension of B. pubescens seedlings grown from seeds collected at different altitudes (in the range of 200–1000m) was inversely related to the altitude of collection when compared at the same low altitude.

Within a population, some B. pubescens seedlings grew significantly more than others when supplied with small amounts of nutrients, the differences disappearing when amounts of nutrients were increased. In Britain, B. pendula was usually more severely attacked by the rust fungus, Melampsoridium betulinum, than B. pubescens whereas in southern Sweden, B. pubescens has been reported as the more severely attacked. At both locations, there were conspicuous within-species differences. More fruitbodies of sheathing mycorrhizal fungi were produced in association with southerly, than with northerly, collections of both B. pubescens and B. pendula—384 compared with nil fruitbodies per tree, 4 years after planting.

Betula nana L., the dwarf birch, is shrub-like in habit. Today it has a relict distribution in the north of Britain. In the Early Post Glacial period, it was widespread throughout Britain. This paper reviews the present understanding of the species in our flora and describes aspects of its morphology, palaeoecology, current distribution, habitat and hybridizaton with the arborescent birches.

The morphological variation between and within species of Betula has resulted in taxonomic confusion which chromosome studies have been aimed at reducing. The paper briefly reviews the general situation and summarizes recent work at Aberdeen.

Betula forms a polyploid series with chromosome numbers ranging from 2n = 28 to 2n = 84. The native British arborescent birches B. pendula and B. pubescens Ehrh. have respectively 2n = 28 and 2n = 56 chromosomes. Meiosis is essentially regular but many phenomena occur which may account for the production of gametes with non-haploid chromosome numbers. Variation in somatic chromosome number within the leaf tissues of both species is common and trees have been identified whose modal chromosome counts lie between 28 and 56. Some of these trees are of hybrid origin while others may aneuploid B. pubescens. The former types are sterile while the latter cross freely amongst themselves and with 2n = 28 and 2n=56 trees.

The respective ecological amplitudes of the two British tree species of Betula are known only imperfectly. B. pendula is better represented on drier, lighter soils and is more of a southern, eastern and lowland species than B. pubescens, which belongs rather to the wetter, colder conditions of north, west and upland Britain. The contribution of the two species to different types of woodland community is briefly reviewed in this paper, and consideration is given to the status of ‘pure’ birchwoods. In some cases, the latter may have been derived from other woodland types; in others, by invasion of heath or grassland. However, some relict birchwoods above the altitudinal limits of dominance by other trees, and in the extreme north and west of Scotland, may be ‘natural’ or ‘climax’ woods.

Birches show a number of characteristics of ‘r-selected’ plants and hence tend to be trees of secondary or seral woodlands. Factors determining the capacity of birch species to colonize bare ground or vegetation such as heath and grassland, are reviewed, with special reference to a study of invasion of Calluna heath at Dinnet, Aberdeenshire. The importance of the development of gaps in the Calluna canopy, or alternatively of fire, in providing suitable sites for establishment, is emphasized.

Changes in the ground vegetation following the establishment of birch stands are outlined, as well as the subsequent development of seral birchwoods. Attention is drawn to the fact that many upland birchwoods are becoming moribund, and to the effects of grazing and other influences in preventing regeneration. The possibility of reversing this trend is discussed.

The relationship between stand structure of Calluna vulgaris and the ability of Betula spp. to establish was examined at Dinnet Moor, Aberdeenshire. As a result of management by burning, a series of even-aged stands of Calluna is available including recently-burnt examples and stands with Calluna in the pioneer, building, mature and degenerate phases. Uneven-aged stands were included for comparison. The density of birch was found to be high very shortly after burning, much less in pioneer phase stands and minimal in the building and mature phases, indicating high mortality as the stand ages. Some indication of slightly higher densities in degenerate stands was obtained, suggesting some recruitment at this stage. Comparisons of the age of individual birches (seedlings or saplings) with that of the adjacent Calluna showed that, in general, birch is established very shortly after Calluna begins to regenerate, the surviving birch individuals being consistently one or two years younger than the Calluna. Again, results indicated little or no recruitment after the Calluna stands enter the building phase. Experimental introduction of birch seedlings to stands of various age since burning showed that survival and performance was best in the pioneer stands, and poorest in building stands.

It is concluded that birch is virtually excluded by dense Calluna stands in the building and mature phases, and that carefully controlled management involving regular burning may therefore prevent the entry of birch. However, occasional fires followed by slow or patchy Calluna regeneration may provide ideal conditions for birch establishment.

Birch has a long-established reputation as a soil-improver, a characteristic often presumed to result from some aspect of the nutrient cycle peculiar to this species. However, models for the cycles of nitrogen, phosphorus, potassium, calcium and magnesium by age in slow-growing birch based on data from the literature for biomass development, nutrient levels and rates of decomposition suggest that nutrient cycling in birchwoods is comparable to that in forests of other species with similar rates and patterns of growth.

Aspects of whole-plant physiology of birch are reviewed, and previously unpublished research is presented. Seasonal cycles of shoot growth and bud dormancy are discussed, with environmental and hormonal factors affecting the rate and duration of shoot extension and leaf production. Also covered are variability in branching patterns, aspects of cambial activity, onset of the reproductive phase, factors affecting flowering and sex ratio, and seed viability and dormancy. Finally, the regenerative capacity of birch is described, and its future roles are mentioned.

The general environmental factors controlling the epiphytic vegetation on birch are discussed, as is the physio-chemical nature of birch bark as a substratum for epiphytes (especially lichens). In terms of species diversity, birch is found to be a poor host (phorophyte) for epiphytes and various reasons (e.g. low bark pH) for this are suggested. The lichen communities commonly encountered on different parts of a birch tree in different situations in the British Isles are summarized. Annotated appendices provide lists of the lichens (235 species) and bryophytes (58 species) recorded on Betula in the British Isles.

The meagre history of the study of the fungi of birchwoods in Britain is offered as an introduction to more recent studies in the British Isles and in continental Europe. Four categories of fungi (parasites, suspected mycorrhizals and lignicolous and humicolous saprophytes) are recognized and discussed. Examples of the most frequently found species-associations are given in relation to environmental factors.

The production of fruitbodies of mycorrhizal fungi associated with Betula spp. in an experimental site was observed to occur in a pattern ordered in time and space. Certain species of Inocybe, Hebeloma and Laccaria and Thelephora terrestris appeared in the first five years after planting: they were followed by species of Leccinum, Cortinarius and Russula. Usually more fungi were associated with B. pendula than with B. pubescens. More toadstools of Laccaria tortilis developed in association with one clone of B. pubescens than with another which favoured instead the development of Inocybe petiginosa.

The idea of a fungal succession is supported by below-ground observation of mycorrhizas. Early- and late-stage fungi formed mycorrhizas on seedlings growing in sterile conditions whether inoculated with mycelia or spores. However, only early-stage fungi formed mycorrhizas in unsterile conditions.

The concept of succession is based on observations made when treeless sites were planted with Betula spp. However, experimental planting of seedlings into soil with intact roots of older trees with mycorrhizas of Lactarius pubescens resulted in mycorrhizas on the seedlings: but no mycorrhizas formed if the roots of the older tree had been severed. This suggests that the pattern or sequence of mycorrhizal fungi associated with seedlings regenerating within natural stands is likely to differ from that associated with Betula spp. colonizing previously treeless sites.

Fungi reported as parasites of Betula in Britain are reviewed. Pathogens colonizing the major host organs: roots, stems, branches twigs and bark, foliage, flowers fruit and seed, together with those attacking seedlings, are assessed in turn. Other species of fungi not yet known from Britain, but having potential importance as disease agents are also discussed. The major stem decay pathogens and the occurrence of leaf rust in Britain and its significance as a major nursery disease are examined in detail. The status of the fungal parasites on birches in woodland and plantations, in ornamental plantings and in the nursery is considered.

The entomology of birch in Britain (and to a lesser extent in Fennoscandia) is reviewed in broad ecological terms. In particular, the size of the associated fauna, the kinds of geographical distribution patterns seen for birch insects in Britain, birch defoliators, and the conservation needs of insects associated with birch in Scotland are considered. A closing section briefly outlines additional literature on the ecology of birch insects.

Seven virus-like agents have been recognized naturally infecting birches in Europe or North America. Of these, only two have been studied in detail. Apple mosaic virus (AMV) infects Betula papyrifera and B. alleghaniensis in North America, and is associated with golden-white ring/line patterns in leaves. There is no evidence that AMV spreads between birches and no natural means of dissemination is known. In Europe, B. verrucosa and B. pubescens are naturally infected with cherry leaf roll virus (CLRV) which causes foliar mottle, pale yellow spots and bright green vein banding. In the United Kingdom, the prevalence of CLRV in unmanaged birch populations is less than in street trees. The virus is spread via birch seed and pollen which it infects and also contaminates. Sparse data suggest that the following viruses infect birch but their effects are unknown: yellow birch latent, tobacco rattle, tobacco necrosis, prunus necrotic ringspot and prune dwarf. There is a similar paucity of information concerning the cause of wood abnormalities that enhance the veneer value of affected timber: agents transmitted in soil or by grafting have been associated.

The authenticity of birch collections is always in doubt unless one has wild source material; even the collections of Forrest, Wilson, Rock et at. may be suspect in that labels and records have been lost or confused. Collections are best maintained from vegetative sources and clonal selection is discussed. Site tolerances, planting problems and after-care are given detailed coverage. Species, hybrids and cultivars in general cultivation are listed and a summary of representation in British arboreta is included.

Man has found a diversity of uses for birch at least since Mesolithic times. In the mediaeval period, it was used for all kinds of domestic and agricultural purposes and it is remarkable how early its beneficial effects in silviculture were appreciated. Similarly, its amenity value has been appreciated for centuries in the rural scene and today it is employed in urban improvement too. Its importance for fuel and charcoal goes back into the distant past. Its bark is unique, with a range of end products from trinket boxes in northern Europe to large canoes in Canada. The trade in birch products is very large in eastern North America, while in Finland the plywood industry is also important. Birch will continue to play a significant role in British Forestry, both as a nurse species and as the main amenity species for poor and exposed sites.