I want to give a brief resumé of the reasons why the taxonomic study of Salix is so notoriously difficult. As long ago as 1737, Linné in Flora Lapponica wrote: “Hunclocum sibi postulat Salicum familia, qui si ulla in Botanicis obscura, haec sane maxima”. The phrase “crux et scandalum botanicorum” has been quoted time and again.

(1) All willows are dioecious, developing male and female flowers in different individuals.

(2) Flowers in many species are precocious, the flowers appearing before the leaves.

(3) There is a tendency for the current year's terminal and past year's lateral branches to differ in indumentum, leaf shape and leaf sequence.

The paper deals with three problem areas in the taxonomy of British willows: (1) the widespread occurrence in Perthshire and neighbouring counties of an intermediate between Salix myrsinifolia Salisb. and S. phylicifolia L., apparently of hybrid origin, though growing in an area where typical S. phylicifolia has yet to be found; (2) the complex taxonomy of the British Sallows, where two common aggregate species, Salix cinerea L. and S. caprea L., are each represented by two segregates; a third, S. aurita L., may also comprise more than one segregate; all three species interbreed, as do the segregates within each aggregate; (3) the disputed identity of the Crack Willow, Salix fragilis L., and its nomenclatural implications.

The responses of clones of a range of willow species to waterlogging and total submersion were compared in glasshouse experiments using specially constructed perspex tanks. Up to 13 weeks of waterlogging had little effect on form and growth rates of above-ground parts of two clones of Salix caprea L. and one clone of S. cinerea L. A second clone of S. cinerea responded differently to the other three clones by significantly increasing its leaf area and stem dry weight, suggesting that it is likely to be more tolerant of waterlogging.

Plant parts below the waterline in the waterlogging treatment responded very differently to the same parts on control plants. All waterlogged plants developed hypertrophied lenticels within four weeks, followed by significant increases in stem diameter. In each clone most of the pre-existing root system died by the end of the waterlogging treatment. All clones formed adventitious roots which varied considerably between the species and to a lesser extent between clones within each species. In S. cinerea many short, fine, much-branched roots grew vertically upwards until they reached the water surface. In S. cinerea clone 90 they originated from both the pre-existing woody roots and from the hypertrophied lenticels on the submerged portions of the stems. In S. cinerea clone 12 they developed only from the hypertrophied lenticels. New roots produced by waterlogged S. caprea clones were long, sparsely branched and floated on or near the surface of the water. As in S. cinerea clone 90 they originated from both the pre-existing woody roots and the submerged hypertrophied lenticels. Whereas roots of the control plants of all clones were mostly ectomycorrhizal, those of the waterlogged plants had no mycorrhizae. In a second waterlogging experiment S. × sericans, S. viminalis and S. fragilis produced adventitious roots more like those of S. caprea than S. cinerea. Plants of all clones removed from the waterlogging treatment after 16 days and kept with their soil at field capacity thereafter produced many new roots from their old root balls. After 32 days waterlogging no such recovery occurred.

Total submersion caused much more severe damage to the aerial parts of all tested species than waterlogging. After eight days submersion the shoot tips of S. fragilis were beginning to rot and by day 25 those of S. × sericans and S. viminalis were rotting too. Subsequently shoots developed from axillary buds which remained unaffected by rot throughout the experiment. In S. cinerea shoot growth ceased after eight days and terminal buds were formed which remained dormant but healthy throughout the experiment. Submersion caused progressive loss of pre-existing leaves in all species. Some newly formed leaves of each species developed epinastic deformities after > 16 days of submersion. All species except S. cinerea showed significant reductions in root dry weight after 32 days of submersion.

In a second submersion experiment responses of three clones each of S. purpurea and S. viminalis were compared. Shoot growth was reduced significantly in four of the six clones after 14 days. No further reductions occurred in the next 14 days of submersion. Root dry weight also decreased, but it took 28 days of submersion to produce the greatest effect. The faster growing clones of each species were more adversely affected by submersion in terms of both reduced shoot growth and root growth than those which grew more slowly.

Considerable intraspecific variability in responses to waterlogging and submersion have been shown in these experiments by clones not selected for their variation in flooding tolerance. This suggests that selection and breeding could lead to the development of flood-tolerant cultivars of species such as S. cinerea and S. caprea which are not noted for their ability to withstand inundation.

Salix has many physiological features in common with other deciduous woody plants, e.g. C3 photosynthesis, occurrence of latitudinal photoperiodic ecotypes, and organic N (no ) flux to the shoot in the xylem. Special points about the physiology of Salix spp. which may have impact on their ecology and economic uses include: (i) relatively high (for woody plants) light-saturated rate of photosynthesis on a leaf area or leaf dry weight basis, (ii) sex differences in water (transpiration) costs of growth, (iii) very limited seed longevity and a wide range of temperature and light conditions permitting germination, and (iv) ready rooting and establishment of naturally or artificially detached twigs and branches. Areas in which work on Salix has been especially influential for the development of plant physiology include: (i) the analysis of phloem functioning using aphids, (ii) the role of photoinhibition under natural conditions, and (iii) the realisation that the woody habit need not constrain the achieved activity of enzymes and hence N-based metabolic rates.

Phenolic secondary compounds and trichomes are instrumental in the regulation of herbivory on Salicaceae. The roles of phenolics in willows as toxins or deterrents, as phagostimulants or ovipository signals, and as precursors in insect chemical defence are briefly reviewed. The interactions between salicaceous plants, herbivores and their predators are discussed in the context of theories on the evolution of interactions among three trophic levels.

The age structure, both on the ramet and the genet level of boreal willow populations (Salix myrsinifolia-phylicifolia) was studied in relation to fluctuations in populations of voles (Clethrionomys glareolus and Microtus agrestis). Willow ramets were collected from four different areas in the coastal part of Sweden and Finland at the Bothnian Bay. In each area we selected one mainland and one island locality. The ramets, as well as the scars left by bark-eating voles on the ramets, were aged. We found in general a high correlation between ramet ages and peaks in vole cycles (based on frequencies of scars).

From a sample of 79 willow clones where all ramets were aged, more than half of the ramets were produced the summer following a vole peak. Field surveys of seedling survival showed that successful establishment is mainly confined to disturbed ground (vole run-ways, etc.) in connection with vole peak years. It is concluded that the age structure both at the genet and the ramet level in boreal willow populations are strongly influenced by the fluctuations in vole populations. The consequences of vole herbivory are manifold, e.g. periodic damage of willows may delay senescence of individual genets and delay the rate of succession due to increased opportunities for willow recruitment.

A provisional species list for the insects galling willow in Britain is presented. Aspects of willow biology which may be important in gall-insect species diversity are briefly discussed and the process of gall formation is reviewed for one species of willow-galling sawfly. An experiment investigating willow defences against galling, by measuring phenolic levels in plant and gall tissue, is also described.

The watermark disease is caused by the bacterium Erwinia salicis. It affects six species of tree willow and of these the white willow (Salix alba) and certain of its varieties are particularly susceptible. Unusually for a bacterial disease, the pathogen only colonises the xylem tissues, which become discoloured as a result. Watermark causes severe losses in S. alba var. caerulea, the cricket bat willow, and in several Dutch clones of S. alba which have been widely planted in that country. Although numerous studies have been made of its epidemiology since the disease was first reported in 1924, the infection process remains clusive. Recent research, much of it unpublished, points to the widespread occurrence of symptomless infection, and the possible role of this in the transmission of disease through the propagating material.

The British rust diseases of willows are discussed. Only fungal characters are employed in the identification of the different species, thus avoiding the use of host plant taxa in the characterisation of the diseases. Using spore and sorus characters, 14 different species occurring in Britain could be distinguished. The density of urediniospore ornaments is analysed in particular. The rusts are described in detail, and keys for their identification are presented. A list of the rusts affecting willow hosts in Britain has been compiled.

The larger fungi associated with British willows are separated into several groups depending on their resource relationship, trophic state and whether associated with dwarf or arborescent species. Over 150 potentially mycorrhizal taxa have been recorded with willows in Britain, with nearly one-third restricted to arborescent communities. The fungi associated with creeping willow form a distinct category probably reflecting former ecological conditions. While some of the necrotrophic and mycorrhizal fungi are restricted to willows, the saprotrophs are often more widespread and exhibit less specificity.

Palynological and macrofossil evidence of the former presence and abundance of Salix spp. in the British Isles is discussed. The occurrence of Salix remains in archaeological excavations is reviewed. It is concluded that, whereas Salix spp. of shrub and dwarf-shrub habits were abundant during the glacial and late-glacial periods, tall-shrub and tree species have been of only local occurrence during the post-glacial. The wood of these species has been used opportunistically by humans since at least the Neolithic period.

Current evidence for early fossil Salicaceae is summarised. Most data come from Europe and North America where revisionary studies are in progress in several laboratories. The earliest records of the modern genera are from North America with Populus section Abaso in the Late Palaeocene and Salix subgenus Salix in the early Eocene. This evidence is based mainly on leaves but the presence of Populus is confirmed by a leafy shoot with attached fruiting raceme. The two genera first occur later in Europe with Populus in the uppermost Eocene and Salix in the Middle Oligocene. Members of both genera in both continents apparently occupied riparian habitats early in their history.

Some main uses of the versatile genus Salix are described. The basket willow industry, once nationwide but now concentrated in Somerset, is covered in some detail, noting the site requirements and attributes of the three main basket willow species, Salix triandra L., S. viminalis L. and S. purpurea L. The management of the crops and methods of processing to give the ‘white’ or ‘buff’ coloured rods preferred by basket makers are described, as are the ability and versatility of basketry to produce containers uniquely suited to meet specific needs.

Other uses of the shrub willows (sub-genus Vetrix) are outlined, including the stabilisation of slopes and other aspects of bioengineering, amelioration of difficult environments and large-scale amenity urban and motorway plantings.

Attention is drawn to opportunities to develop the tree willows (sub-genus Salix) to meet projected timber shortages, to the use of willows for windbreaks and shelter, and to the culture and use of that very British tree – the Cricket Bat Willow (S. alba var. caerulea (Sm.) Sm.).

The ornamental use of Salix in Britain must be measured against any detrimental impact introduced plants may have on the natural or artificial environment. Willows in amenity planting have the capacity to be both rewarding and at the same time damaging. Anyone using them should be aware of the constraints that may be imposed by the plants and by any potential site. Existing published plant lists are long and inaccurate, often failing to give proper advice on ornamental use. Commercial plant supply may be wanting. Forty species and cultivars are recommended here for a range of situations, but some require further taxonomic research and others need wider introduction to the trade.

Work on short-rotation coppice willow as an alternative and renewable energy source began in Northern Ireland in the mid-1970s, prompted by the massive rise in oil prices during that period. Although in the short run oil prices have dropped in real terms, interest in short-rotation coppice willow has ben sustained because of the potential role it has in the development of agriculture, particularly in marginal areas. This is particularly relevant in the current situation of over production of a wide range of agricultural commodities within the European Community and the moves to reduce Government support in the form of farm and export subsidies.

Although Salix cultivars have yielded in excess of 30 tonnes dry matter (DM) ha−1 annually under experimental conditions, it is considered that 10–12 tonnes DM ha−1 is a sustainable commercial yield.

Melampsora spp. rust has emerged as one of the most important factors limiting the development of short-rotation coppice as a commercial crop. For economic and environmental reasons, the application of fungicide for rust control is not a possibility. Consequently, other disease control strategies have to be established. The main focus of this work is in the selection, for suitability for coppice application, of the widening range of genetic material becoming available from breeding programmes in Canada, Sweden and Finland with a view to their incorporation into mixed stands.

End product utilisation is considered a priority area for investigation if short-rotation coppice is to make a contribution to land use and the development of agriculture in marginal areas. Currently two potential end uses are being investigated: firstly fractionation – to produce cellulose for paper manufacture, hemi-cellulose for the production of molasses and lignin for further processing into other industrial chemicals, and secondly the simultaneous generation of heat and power using gasification – ‘combined heat and power’.

Coppiced willow supports a much higher density of breeding birds than unmanaged more mature willow. This appears to be because it retains richer vegetation cover, especially in the herb layer, and so provides more breeding and feeding sites for both marsh and woodland birds.

The ecological role of native willows is described in terms of the diverse structure of the species involved, the wide range of plant communities they form and the large numbers of invertebrates associated with them. The conservation importance of the genus Salix is discussed along with comments on the necessary management to retain willow habitats.