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.

Because of both the energy costs and the slowness of the reactions of the nitrogenase complex compared with those involving some form of combined nitrogen (oxidised or reduced), we argue that the evolution of nitrogen-fixing organisms required an environment which was very limited in combined nitrogen. This is thought to have occurred after phototrophy evolved, but before water was used as a hydrogen donor (and therefore oxygen was present in the atmosphere). After oxygenic photosynthesis evolved, the need for a high level of biological nitrogen-fixation remained, since abiotic inputs were insufficient to keep pace with the rapidly evolving biomass (flora and fauna). Symbiotic fixation probably first evolved in the form of casual associations between cyanobacteria and most other groups of plants. By inhabiting the sporophytic generation of evolving land plants (cycads in particular), protection against nitrogenase-inactivating oxygen and a more desiccating environment was achieved simultaneously.

We envisage nodulated plants arising by the transfer of nif genes into tumour-forming bacteria. In the case of legumes, these would be ancestors of extant agrobacteria, which gain entry into their hosts via wounds. Co-evolution of symbionts from nitrogen-fixing tumours has taken several routes, leading to extant nodules differing in mode of infection, structure and physiology. Evolution towards optimisation of oxygen usage is continuing.

Nitrogen-fixing symbiosis in animal systems is only advantageous in specialised ecological niches in which wood is the sole dietary intake. In the case of shipworms, the symbiosis has many of the advanced features associated with nitrogen fixing root nodules.

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.

A wide variety of plant-microbe nitrogen-fixing symbioses which include cyanobacteria as the nitrogenfixing partner exist. While some information has been gathered on the biochemical changes in the cyanobacterium upon entering into symbiosis, very little is known about the accompanying changes at the genetic level. Much of our present knowledge of the organisation and control of expression of nitrogenfixation (nif) genes is derived from studies of the free-living diazotroph Klebsiella pneumoniae. This organism thus provides a model system and source of experimental material for the genetic analysis of symbiotic nitrogen fixation. We describe the use of cloned K. pneumoniae genes for nitrogen fixation and its regulation in the genetic analysis' of nitrogen fixation in cyanobacteria which can enter into symbiosis with plants. These studies reveal some dissimilarities in the organisation of nif genes and raise questions as to the genetic control of nitrogen fixation in symbiosis.

The part played by lichens in the nitrogen economy of natural ecosystems is reviewed. The ability of some lichens to fix nitrogen and the consequent likelihood of an enhanced contribution is discussed, leading to the appreciation that release of nitrogen compounds from their thalli is a frequent characteristic of their growth and development, and of some nutritional benefit to associated plants.

Data on the contribution by the normal processes of growth, death and decay, to the combined nitrogen supply of those specialised habitats where lichens predominate or are frequent, are given. Such habitats are arctic and subarctic forest and tundra, and the Douglas fir forests of northwest U.S.A. The methods used for the estimation of the contribution of the lichens are outlined and details are given of recent and current studies of nitrogen movements by direct leaching during normal growth of nitrogen-fixing terricolous lichens in Finland and southwest Scotland. The latter study uses a laboratory apparatus to simulate field conditions in order to permit the use of atmospheres enriched with the heavy isotope of nitrogen, 15N.

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.

Alnus rubra growing in the field in Scotland is nodulated effectively by local strains of Frankia. Strains which have been isolated from Alnus rubra at Lennox Forest show different growth medium requirements and colony morphology compared with isolates from Alnus glutinosa and several different strains have been isolated from the same locality. Preliminary evidence suggests that some spore negative, northwest American Frankia strains may be more effective than local spore positive crushed nodule inoculum for nitrogen-fixation in Alnus rubra.

On the moderately fertile clay soil of Lennox Forest, a mixed plantation of alternate Picea silchensis and Alnus rubra showed no improvement in growth of spruce compared with pure spruce plots. However, the presence of alder increased upper soil nitrogen status by 585 kg ha ' which approximates the standard 150 kg N ha 'of fertiliser nitrogen applied in practice to nitrogen deficient stands at about 5-year intervals. Although the alder had penetrated the subsoil, there was no apparent effect on spruce rooting depth in the mixed stands in this high clay soil. In addition to nitrogen content, the total phosphorus of the upper soil horizons was improved in the mixed plots by an estimated 3–6 kg ha -1 y 1 and it is suggested that this phosphorus may be brought from the subsoil by the deeper rooting alder and deposited on the surface in its litter.

Although improved growth of spruce in mixture with Alnus rubra is only likely where pure spruce stands are stressed for nutrients, the potential benefits of symbiotic nitrogen-fixation in silvicultural practice make it desirable to investigate other species and provenances of Alnus suited to British conditions, to achieve maximum symbiotic fixation of nitrogen by selection and inoculation with superior strains of Frankia and to include such plants in trials of mixtures on sites where nitrogen-availability may be critical.

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.

Interactions amongst plants and different endophytes are prevalent in soils deficient in both nitrogen and phosphorus. Several systems are now recognised, combining infections by both fungi and prokaryotes. Symbiotic associations are ancient and reflect the requirements for the maximum uptake of nitrogen and phosphorus in plant nutrition.

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.

Sequential observations were made of the effects on root development of inoculating Betula pendula seedlings during propagation with sheathing mycorrhizal fungi, Amanita muscaria, Hebeloma sacchariolens or Paxillus involutus. After propagation, the seedlings were grown for two seasons in four different soils (two mineral soils and two peats) which were not sterilised.

While all inoculated seedlings had similar numbers of mycorrhizas at planting, numbers subsequently increased most rapidly with P. involutus and least rapidly with A. muscaria. From the end of the first season onwards, the uninoculated controls had as many mycorrhizas as seedlings inoculated with either A. muscaria or H. sacchariolens. These numbers were, however, usually exceeded on plants inoculated with P. involutus.

The mycorrhizas developing on the controls, and on seedlings inoculated with A. muscaria, were attributable after planting into the unsterile soils, to naturally occurring soil-borne inocula of species of Cenococcum, Hebeloma, Inocybe, Laccaria, Paxillus and Thelephora etc. Those developing on seedlings inoculated with P. involutus were all attributable to this inoculant fungus in the first year but, in the second year, the proportions decreased to 19—14%. With H. sacchariolens, at least 90% of the mycorrhizas in the two mineral soils and one of the peats (a sedge peat) were attributable in years 1 and 2 to the inoculant fungus. In the other peat (Sphagnum peat), the proportion of mycorrhizas attributable to H. sacchariolens decreased to 25% by the end of the first year and totally disappeared in the second.

The continued ability of H. sacchariolensto form mycorrhizas in a range of unsterile soils, to the virtual exclusion of mycorrhizas attributable to other fungi, is thought to be characteristic of fungi that occur early in mycorrhizal successions. In contrast,A. muscaria(a late-stage fungus), was unable to form mycorrhizas after inoculated seedlings were transferred from controlled axenic conditions to unsterilised field soils.

Two arbiters of root growth were measured—numbers of roots and their dry weights. Whereas inoculations with H. sacchariolens significantly decreased numbers in the latter part of the first season, they either had no effects on dry weights as in the mineral soils or significantly increased them as in the peats. In the second season, there was further evidence, sometimes contradictory, showing that inoculations can appreciably alter the structure of developing root systems.

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.

Some aspects of the role of the ericoid mycorrhizal symbiosis in the ecology and physiology of ericaceous plants are described. Mycorrhizal infection leads to enhancement of plant nitrogen content and an experimental analysis of the basis of this effect is reported. In addition to improving the efficiency of ammonium absorption at low concentrations, the mycorrhizal endophyte utilises amino acids, peptides and proteins as nitrogen substrates for growth. These are the predominant nitrogen sources in organic heathland soil. It is suggested that the success of ericaceous plants in such soils may arise through the capacity of the mycorrhizal fungus to provide its host with access to this nutrient resource. A model is described in which absorption of ammonium and amino nitrogen leads to soil acidification, increased acid protease activity and improved vigour of the ericaceous plants.

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.