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Taxus baccata L.

2003; Wiley; Volume: 91; Issue: 3 Linguagem: Inglês

10.1046/j.1365-2745.2003.00783.x

1365-2745

Peter A. Thomas, Anthony Polwart,

Plant Pathogens and Fungal Diseases

Taxus baccata is a native evergreen non-resinous gymnosperm tree up to 20(−28) m, often with multiple trunks and spreading, rounded or pyramidal canopy. Capable of producing leafy branches from old branches and trunks, and sometimes from stools. Root system shallow with extensive horizontal roots, often above ground on calcareous substrates. Bark reddish-brown, thin, scaly. Branches long, not whorled. Twigs green and irregularly alternate; buds very small, bud scales dark-brown, rounded, imbricate and closely appressed. Leaves, spirally attached but on lateral shoots twisted more or less into two ranks, can live for up to 8 years; 1–3(−4.5) cm long and 2–3 mm wide, parallel-sided, shortly stalked, mucronate, dark glossy green above, paler and yellowish beneath with two pale stomatiferous stripes; midrib prominent on both sides, margins recurved, tapering to a petiole-like base. Normally dioecious, rarely monoecious; reproductive structures green, borne in leaf axils near the end of the previous summer's growth. Male parts in small cones (simple strobili; Mundry & Mundry 2001) 2–3 mm diameter, made up of 6–14 stalked peltate microsporophylls each with 4–9 pollen-sacs; strobili in clusters of up to 20–30 near branch ends. Female structures (ovuliferous brachyblasts sensuStützel & Röwekamp 1999) 1.5–2.0 mm long, solitary or in pairs in leaf axils on underside of shoots, not forming cones, consisting of several imbricate scales, the uppermost fertile, bearing a single ovule. Seed ovoid, smooth and shiny, brown-yellow, 6–7 × 5 mm at maturity, with a tough seed coat, partly surrounded by a fleshy red aril typically 9 × 7 mm which falls with the seed at maturity, the 'fruit' ripening in the first year. More than 70 varieties and cultivars are recognized (Vidaković 1991; Welch & Haddow 1993; Cope 1998) including the columnar Irish yew (cv Fastigiata Loudon) originating on the flank of Cuilcagh Mountain in Co. Fermanagh, N. Ireland, first found in 1780 (Elwes & Henry 1906; Nelson 1981). Taxus is a genus of around seven closely related species scattered through the northern temperate region. There is some indication from morphological and chemical characteristics that the different species are sufficiently closely related that they constitute geographical variations of T. baccata (Hess et al. 1967 cited in Voliotis 1986; Burns & Honkala 1990; Dempsey & Hook 2000). Indeed, the heartwood constituents of T. baccata, T. brevifolia, T. cuspidata and T. floridana have been found to be almost indistinguishable chemically (Erdtman & Tsuno 1969). Most species contain the alkaloid taxol and although Dempsey & Hook (2000) found wide variation in morphological characteristics and chemical features (paclitaxel content) between varieties of T. baccata they found no sufficiently distinctive interspecific differences of taxonomic value between different species. They thus question the correct identification of plant material used in many pharmacological studies. Taxus baccata has a stress-tolerant life strategy sensu Grime (1979), being slow growing, slow to reach maturity (c. 70 years), long-lived (> 1000 years), shade-tolerant but can withstand full sun; it can grow on a wide range of soils and produces strong decay-resistant wood (Lilpop 1931; Brzeziecki & Kienast 1994). It forms dense stands in oceanic climates but increasingly becomes an isolated understorey tree in more continental climates. This evergreen poisonous tree is local in Britain but may form nearly pure woodlands on chalk and limestone in England. In Britain, Taxus baccata woodlands are almost wholly confined to the chalk of the South Downs and, to a lesser extent, the North Downs in south-east England especially along the sides and bottom of dry valleys and on scarp slopes (Tittensor 1980; Rodwell 1991). There are also examples on the Magnesian Limestone of County Durham in the north-east, the Carboniferous Limestone around Morecambe Bay in the west and on the limestones of south-west Ireland, especially the Killarney woodlands (Kelly 1981). Further small, natural-seeming stands of yew are found in Ireland on limestone outcrops at Lough Derg, north Co. Tipperary, Cahir Park, south Co. Tipperary and Garryland Wood, south-east Co. Galway (McEvoy 1943). Individual yew trees are more widespread, occurring naturally in woods and on cliffs, including sea cliffs, especially on neutral to alkaline soils. They are notable on the limestone of the Peak District at the southern end of the Pennines, at Borrowdale, on Conzie Scar near Kendal (Elwes & Henry 1906) and widely distributed in the west and north of Ireland (Webb 1977). Apparently native T. baccata individuals are also known from Perth and Argyll (56° N) on to the Inner Hebrides and Orkney although whether yew is native to Scotland has been debated (Dickson 1994). The natural distribution is clouded by the extensive planting of yew trees, especially in churchyards, and it is virtually impossible to separate native and alien records; however, the occurrences mapped in Fig. 1 give the best information available. (a) The distribution of Taxus baccata in Britain. Native: (○) pre-1950, (•) 1950 onwards; introduced: (×) pre-1950, (+) 1950 onwards. Each symbol represents at least one record in a 10-km square of the National Grid. Mapped by Henry Arnold, Biological Records Centre, using Alan Morton's DMAP program, mainly from records made by members of the Botanical Society of the British Isles. Taxus baccata extends northwards to c. 63° N in Norway and Sweden (Vidaković 1991), eastwards to Estonia, Poland, to the Caspian Sea and Turkey, and southwards to Greece (including the islands of Euboea, Thasos and Samothraki; Voliotis 1986), northern Spain (Peñalba 1994), Portugal and into Algeria (Vidaković 1991). Large populations are found in the Ukraine, Poland, Hungary, Slovakia, Romania and the Caucasus Mountains (Bugala 1978). It is absent from the most continental climatic regions of Europe and also northern, south-eastern divisions of Russia, Crete, Faeroes and Iceland, and is almost extinct in the Azores. The distributions of Taxus species, including T. baccata, are given in Fig. 2. The circumpolar distribution of Taxus baccata and other Taxus species from Hultén & Fries (1986). + indicates interglacial records; • indicates isolated occurrences; hatched areas indicate regions of common or fairly common occurrence. Broken lines outline the distribution of Taxus species other than T. baccata. Reprinted with permission. In the British Isles, T. baccata grows from sea level to 425 m in England and Wales and to 470 m on Purple Mountain, Kerry (Alt. range Br. Pl.). In the north of its range, including Britain, T. baccata grows only at fairly low altitudes whereas in the south it is primarily a montane tree growing only on higher mountains in the Mediterranean region, although in the Caucasus Mountains it grows from sea-level to 2050 m. Maximum altitudes: south Slovakia, 660–1000 m; Alps, 1100–1400 m; Iran, 1400 m; Turkey, 1400–1900 m; Pyrenees, 1400–1650 m; southern Spain, 1600–1900 m; Carpathians, 1660 m; Sardinia, 1700 m; Macedonia, 1800 m; central Greece, 1950 m; northern Africa, 2000–2500 m, and Caucasus Mountains, 2050 m (Elwes & Henry 1906; Bugala 1978; Król 1978; Melzack 1979; Voliotis 1986; Hartzell 1991; Vidaković 1991; Schweingruber 1993). Taxus baccata grows best in the high humidity of mild oceanic climates. In England it thrives particularly in areas with relatively mild winters, abundant rainfall, high humidity and where mist frequency is greater than average, as in the coastlands of Hampshire and Sussex and lowland parts of the Lake District (Tittensor 1980; Melzack & Watts 1982b). Conversely, severe winter cold or strong cold drying winds in the spring appear to restrict yew growth on exposed sites in the UK (Voliotis 1986). Taxus baccata sensu stricto is classified as European Temperate by Preston & Hill (1997). In mainland Europe, the ecological barriers limiting distribution are low temperatures in the north, severe continental climate of the Baltic countries and Poland in the east, long droughts in Turkey to the south-east, and drought and high temperatures in north Africa (Kamppa 1926; Król 1978; Środoń 1978). Near these climatic extremes, T. baccata becomes more restricted to moist niches, such as near marshes and bogs on the Baltic coast (Król 1978), rock crevices, or as an understorey tree gaining protection from low humidity and frosts (Sokolowski 1921; Hoffmann 1958; Faegri 1960; Tittensor 1980; Peñalba 1994; Saniga 2000). In mountainous areas of Europe, yew tends to grow on the shaded north-western or north-eastern slopes, where under deciduous or mixed stands there is an oceanic-like climate (Markiewicz 1978; Schweingruber 1993). Taxus baccata woods on the South Downs occur on slopes of almost any aspect but primarily linked to south, east and, to a lesser extent, west slopes where there is high insolation and wind-exposure (Watt 1926; Williamson 1978). Slope is rarely limiting; stands of yew occur on moderate to very steep slopes (Rodwell 1991) and, further north, cliffs are a common habitat (Pigott 1969). On the western South Downs, 80% of yew woodlands were investigated by Tittensor (1980) and found to be between 90 and 250 m altitude, 62% on steep slopes, 57% along the main ridge and 90% on chalk. Moreover, 80% of the woods occur in the area of maximum rainfall (> 1000 mm year−1). Stands of yew are typically associated with limestone slopes carrying shallow dry rendzinas (Rodwell 1991). Soils are normally grey rendzinas of the Upton series, thin, often rich in downwashed flints and poor in earthworms. Yew grows equally well on thin, warm chalk soils, limestone pavement and fen peat (Williamson 1978). In south-west Ireland, yew grows on Carboniferous Limestone pavement with a soil pH in the hollows of 6.8 ± 0.11 (SE) (Kelly 1981). Along valley bottoms, T. baccata woodland may extend on to somewhat deeper and moister soils. Although the majority of natural stands in Europe grow on calcareous soils, yew will also grow on almost any soil, including siliceous soils derived from igneous and sedimentary rocks (Elwes & Henry 1906; Watt 1926; Webb 1977; Król 1978; Kelly 1981; Vidaković 1991). Thus in south-east England, yew grows on the chalk of the North and the South Downs, the Weald Clay and the sandstones of the Lower Greensand and the Central Weald. In the west and north of Ireland, individual trees occur on rocky terrain, siliceous as well as limestone, and in Killarney woods, individuals are found on Devonian sandstone (Kelly 1981). Voliotis (1986) states that T. baccata is favoured by slightly calcareous soil in northern Europe and siliceous soils in southern Europe. Watt (1924, 1934a,b) noted that yew is a successful pioneer on highly calcareous, exposed sites of the South Downs, but on more acidic plateau soils T. baccata is a scarce but local feature with low constancy and frequency values. Across Europe, best yew growth (in terms of rates of growth and largest dimensions) is seen on deep, moist sandy loams and well-drained clays, and worst on dry, rocky and sandy soils where it occurs as scattered and stunted individuals (Webb 1977; Bugala 1978). Yew generally is absent from wet soils, such as wet acidic peat and wet clay (Król 1978; Williamson 1978; Ellenberg 1988), although it does grow well upon calcareous fen peat, and indeed is a significant species in the wood peat accumulations of the East Anglian fens (Godw. Hist.). Even here, however, it is susceptible to poor drainage. Substratum conditions are known to be altered by T. baccata. Howard et al. (1998) compared soil physico-chemical conditions under oak (Quercus) and yew trees growing on the same soils. Under yew, the humic acids were more oxidized; ash content was less under yew (55.6%) than under oak (73.8%); a greater total C, N and Ca was found under yew (23.7%, 1.3% and 2.1%, respectively) than under oak (13.1%, 0.8% and 1.1%), attributed to the absence of large earthworms under yew. Soil pH was not, however, significantly affected by tree species. In Britain, T. baccata forms a remarkably species-poor woodland with yew as the only constant species, typically forming a closed, very dense canopy (W13 in the National Vegetation Classification; Rodwell 1991). The woodland forms 'sometimes quite extensive stretches of striking floristic poverty and uniformity' especially on the warm and sunny south-facing slopes over shallow limestone soils (Rodwell 1991). Yew thrives better than any other tree on steep calcareous slopes and so few other trees occur; when they do it is usually as scattered individuals, notably Sorbus aria, but also Fraxinus excelsior and widely scattered individuals of Fagus sylvatica, Acer pseudoplatanus or Quercus robur. Shrubs are rare except in gaps and edges of stands, seldom form a true shrub layer, and are usually restricted to poor specimens of Sambucus nigra and very occasional Ilex aquifolium or Crataegus monogyna. Buxus sempervirens is a rare associate. The field layer is, as would be expected, extremely sparse usually consisting of patchy herbs with extensive bare ground. The most frequently encountered species are Mercurialis perennis with very occasional Arum maculatum, Brachypodium sylvaticum, Fragaria vesca, Glechoma hederacea, Hedera helix, Rubus fruticosus agg., Urtica dioica and Viola spp. (Rodwell 1991). This is caused not just by shade but by the intense root competition in soils which are often already dry. Bryophytes are equally poorly developed in the UK, in striking contrast to the Killarney yew woods (see below). Rodwell (1991) recognizes two subcommunities. The Sorbus aria subcommunity (W13a) contains this tree as a fairly frequent associate in the canopy, at about 2–3 trees ha−1, and even more rarely, Fraxinus excelsior, Fagus sylvatica, Quercus robur or Acer pseudoplatanus. In Co. Durham, Hulme (1996) also found Corylus avellana and Ulmus glabra in all four yew woodlands visited. Sambucus nigra is the usual shrub although in a few stands in the south-east, notably Box Hill, Surrey, Buxus sempervirens can be locally important and can grow up to form a dense shrub layer or even part of the canopy. A field layer is usually absent or at most very sparse – scattered bryophytes, bare soil and litter. In the Mercurialis perennis subcommunity (W13b), yew dominates with very infrequent Sorbus aria and Fraxinus. The canopy tends to be more open, allowing a more frequent and somewhat denser growth of Sambucus beneath, sometimes with sparse Ligustrum vulgare, Euonymus europaea, Cornus sanguinea and Clematis vitalba or Tamus communis. The field layer is a little less sparse than in the Sorbus aria subcommunity (although most of the ground is still bare) with Mercurialis perennis as a patchily abundant constant component of the field layer, sometimes with Atropa belladonna, Brachypodium sylvaticum, Fragaria vesca, Inula conyza, Iris foetidissima, Rubus spp., Urtica dioica and Viola spp. Yew saplings are also a component of the Crataegus monogyna–Hedera helix scrub (W21). Young yew trees are found in the Brachypodium sylvaticum subcommunity but is a more major component of the Viburnum lantana subcommunity confined to free-draining calcareous soils, and best represented on the chalk of the south-east. Taxus baccata and Sorbus aria are the most frequent trees. Yew is associated with C. monogyna on deeper, moister soils on gentle, sheltered slopes but especially with Juniperus communis bushes on shallower, drier soils on steeper and more exposed sites where yew has a high constancy (Watt 1934a; Rodwell 1991). This reflects the need of yew for nurse plants in establishing new trees and leads to seral development. The juniper sere starts with juniper establishing (in windier areas than Crataegus) which is invaded by yew, leading to a yew scrub with associated Cornus sanguinea, Prunus spinosa, Rubus fruticosus, C. monogyna, Ilex aquifolium and Sambucus nigra (Watt 1924, 1926, 1934a). Owing to its extreme tolerance of shade and the dense shade it creates, yew eventually outcompetes other species and develops progressively into pure yew woodland, often with the dead woody remains of the former juniper scrub beneath. There can be occasional I. aquifolium and rare straggling C. monogyna and Prunus spinosa, with an occasional Sorbus aria above the yew canopy, as at the head of Kingley Vale, Sussex. The Sorbus initially survives by outgrowing the yew in earlier years to persist as emergents or invaders of gaps. But as they die they are not replaced and the canopy of yew fuses into a virtually continuous layer (Watt 1926). The association of juniper and yew is not solely found in the south-east; Larson et al. (2000) records the two species growing together on the sea cliffs of the Great Orme and the Little Orme of northern Wales in 1997. The hawthorn sere is similar, although C. monogyna is less good at providing protection for yew seedlings from grazers, even when mixed with Prunus spinosa. Moreover, on the deeper, moister soils where Juniperus is replaced, other trees, notably Fraxinus excelsior, increase in importance as invaders, leading to a more species-rich scrub. Hulme (1996) shows data which suggest a Crataegus/Fraxinus successional sequence in Co. Durham yew woods, with yew eventually killing out the scrub elements. Although yew is capable of invading grassland (Williamson 1978), either juniper or hawthorn appears essential for its regeneration (Watt 1926, 1934a; Williamson 1978). This association between yew and shrubs has been interpreted as a requirement for establishing yew to have shade and/or protection from browsing vertebrate herbivores (Watt 1926; Williamson 1978). As such, juniper may be the more favoured nurse plant because it grows on soils more suitable for the domination of yew but also because juniper provides a better defence against herbivores. Hulme (1997) and García et al. (2000) point out that scrub will attract seed-dispersing birds and so increase yew seed deposition while, conversely, attracting high seed predation from rodents under the shelter of scrub. Nevertheless, dispersal appears to direct seeds to microhabitats most suitable for seedling survival, identified by García et al. (2000) as a favourably moist and nutrient-rich microclimate (especially in the face of summer drought), protected from large herbivores. Tittensor (1980) working on the South Downs found that twice as many Recent as Ancient Woodlands contained yew, and that yew regeneration was far more common in Recent woods. Those Ancient Woodlands containing yew had few canopy yew and none with yew as a dominant species. Combining this with the estimated young age of woodland yews, she and others (e.g. Watt 1926; Williamson 1978) point out that the South Downs yew woods largely result from the abandonment of land during the last two centuries as a result of the Napoleonic wars, agricultural neglect in the 1920s and myxomatosis. Similar expansion in Irish yew woodlands has been seen following a cessation in grazing or human disturbance (Watts 1984; Mitchell 1988; O'Connell et al. 1988) although some, such as the Reenadinna Wood at Killarney, are estimated to have developed 3000–5000 years ago (Mitchell 1990b). Tittensor (1980) also suggests that because many of the woodlands straddle parish boundaries, old marker trees (which may be centuries or even millennia old) may have been the seed parents and the cause of yew woodland development which is not seen in other areas of abandonment. Yew is extremely shade tolerant; nevertheless regeneration within mature yew woodland is rare in Britain and across Europe (Król 1978). The replacement of individual trees depends entirely on the development of a shrubby sere in gaps left by dead yews but primarily around the edge of existing woodland (Watt 1926; Williamson 1978). Regeneration to adult size may take as long as 75 years and the maintenance of yew woodlands is possible only owing to extreme longevity of individual trees and yew's tolerance of shade (Watt 1926; Tittensor 1980). Newbould (1960) concludes that yew woods are single generation stands 'moving' across the landscape by edge regeneration. If shrubs fail to develop densely enough, rabbits may prevent regeneration and the woodland degenerates by a failure to recolonize gaps formed by the death of old yews, leaving individual relict trees until eventually they die leading to 'abandonment of the area' (Watt 1926). Scrub can reform and the cycle would begin again with slow yew recolonization. Newbould (1960) suggests that yew woods may be considered as a type of secondary seral scrub that will progress to a 'woodland community as yet unknown' once the life-span of its component yews is reached. Individual trees are also found much further north as a natural part of stands of Fraxinus excelsior–Acer campestre–Mercurialis perennis (W8) woodland on Carboniferous Limestone in Wales, Derbyshire, the Pennines and around Morecambe Bay (Rodwell 1991). Here the yew may form part of the canopy, a distinct lower tier to the canopy as in Matlock Dale in Derbyshire, or exist as scattered saplings or isolated specimens on cliffs as in Dove Dale Wood, Derbyshire (Pigott 1969). Although Rodwell (1991) considers these northern examples as part of the mixed deciduous woodland of W8, Hall et al. (2001) report that it is now common practice that they be included within the yew woodlands proper (W13). Yew establishment is hindered and saplings often die or show poor growth with few leaves when beneath the shade of beech. In this way, beech may prevent the succession to yew. Yew does better on drier, more exposed conditions and this may explain why yew dominates here but beech is the dominant on the deeper soils of Fagus–Mercurialis woodland (W12). The circumstances under which yew develops from being an occasional component of woodland to becoming the dominant species remain poorly understood. Yew is an occasional and sometimes prominent associate in various kinds of deciduous woodlands, most notably those in the Fagion alliance, especially the three kinds of beech woodland, Fagus sylvatica–Mercurialis perennis (W12), Fagus sylvatica–Rubus fruticosus (W14) and Fagus sylvatica–Deschampsia flexuosa (W15) woodlands. Here it forms part of a subcanopy of shade-tolerant trees being especially prominent in areas such as the New Forest and the chalk of south-east England. In Fagus–Mercurialis woodland, the T. baccata subcommunity (W12c) is found predominantly on south-facing steep calcareous slopes of the North Downs and some gaps in the Chilterns, with high exposure to wind and sun. Here T. baccata is a constant and it can be abundant as a lower tier of saplings beneath the Fagus or sometimes even break through into the canopy. It seems likely that in these woodlands, as in W13, T. baccata establishes in Juniperus-dominated scrub and the invasion of F. sylvatica is hampered by the shallow, impoverished soils of these steep, sun-baked slopes. Sometimes T. baccata occurs with Sorbus aria, although Fraxinus excelsior or Buxus sempervirens can be scarce companions but with little Crataegus monogyna or Corylus avellana. The field layer is usually a sparse mixture of Mercurialis perennis and Rubus fruticosus with a number of other species at low frequency. Thus there is a transition between W12c and T. baccata woodland proper (W13). On the low base-status moist brown earths of the Fagus–Rubus woodland (W14), such as occasionally in the New Forest and sometimes in Surrey and Sussex, much more rarely in the Chilterns, and on the very base-poor, infertile soils of the Fagus-Deschampsia woodlands (W15) in the southern lowlands, the under-tier of Ilex aquifolium can be sporadically accompanied by scattered individuals of T. baccata, which may reach high enough to break almost through the canopy. These fall within Corine Code 9120/Palaearctic Habitat Classification 41.12 – Atlantic acidophilous beech forests (Quercinion robori-petraeae or Ilici Fagenion) (Interpretation Manual of European Union Habitats 1999). In a similar way, T. baccata can also be found in Quercus–Pteridium–Rubus woodlands (W10) on base-poor brown earths as a patchy lower tier beneath the oak, as in some New Forest stands. As noted above, T. baccata woodlands are found in close association with grasslands and scrub which reflect grazing relaxation over the south-eastern chalk. Thus, yew stands can be found in mosaics and zonations with a complete sequence of vegetation types from close-cropped turf of Festuca ovina–Avenula pratensis (CG2), other swards dominated by coarse grasses to various kinds of Crataegus monogyna–Hedera helix scrub (W21). Taxus baccata woodland (W13) is synonymous with the yew-woods of Tansley & Rankin (1911), Watt (1926), Tansley, Br. Isl. and Ratcliffe (1977) and in Europe is classified as Corine Code 91J0 and Palaearctic Habitat Code 42.A71 (Interpretation Manual of European Union Habitats 1999). In south-west Ireland, the Killarney woodlands contain extensive stands of yew dominating on bare Carboniferous Limestone pavement areas, and classified as facies of Taxus baccata of the Association Corylo × Fraxinetum (Order Fagetalia, Class Querco-Fagetea) (Ir. Pfl.) and also belong within Corine Code 8240 and Palaearctic Habitat Classification 62.3 (Interpretation Manual of European Union Habitats 1999). On more broken rocky terrain the shorter yew trees on the limestone outcrops are intermingled with taller Quercus (largely Q. petraea) in the intervening soil-filled hollows (Kelly 1981). Little grows under the yew canopy although Corylus avellana or Ilex aquifolium become frequent locally and Corylus can replace yew as the dominant. The field layer is sparse, the most constant species being Brachypodium sylvaticum, Fragaria vesca, Oxalis acetosella, Potentilla sterilis and Sanicula europaea. Unlike the yew woodlands of southern England, here there is a very dense moss cover, primarily Thamnium alopecurum with Eurhynchium striatum and Thuidium tamariscinum (Kelly 1981). There is no direct equivalent of these species-poor mature stands in continental Europe. Delelis-Dusollier & Géhu (1972) and others put samples of fairly species-rich yew scrub from south-east England into a Roso-Sorbetum ariae which they took as the British equivalent to the Taxo-Prunetum mahalebis, a community found on chalk cliffs along the Seine. British yew woodland is best considered as part of the Fagion alliance in the Querco-Fagetea (see Kelly 1981; above), alongside the Fagus–Mercurialis woodland and the European communities such as the Taxo-Fagetum (e.g. Król 1978; Rodwell 1991; Saniga 2000). From Portugal to the Crimea in the east, constrained by the need for a humid climate, T. baccata is usually a minor component of climax deciduous, coniferous or mixed woods (Tittensor 1980). For example, in northern Spain yew grows only as isolated individuals within the beech forest (Peñalba 1994); in central and eastern Europe yew is an understorey component of beech, or mixed forests of beech and conifers (e.g. Sokolowski 1921; Hoffmann 1958; Saniga 2000). Yew occurs in all ecological regions of Cephalanthero-Taxetum on shell-limestone or chalk (Król 1978). These communities are intermediate between Melico-Fagetum and Seslerio-Taxetum with the occurrence of Fraxinus excelsior and Tilia cordata. The community Seslerio-Taxetum is used to distinguish stands with yew from those with beech (Seslerio-Fagetum) – Moor 1952 cited in Król (1978). In the mountainous regions of Czechoslovakia, yew grows in fir and beech forests, and at lower elevations it is part of the oak and hornbeam forests (Futak et al. 1966). In Poland, yews grow amongst beeches on podsolic soils (Melico-Fagetum) and in a calcium-rich ash and alder carr (Circaeo-Alnetum) (Bojarska & Plitcha 1968, cited in Król 1978). Also in Poland, yew is found in Querco-Carpinetum medio-europaeum stands and with alder on fen soils (Carici elongatae-Alnetum). Outside of Britain, T. baccata appears to form single-species stands only in the oceanic climates of the Crimea and Caucasus Mountains on a small scale (Pridnya 1984). Król (1978) records that, in the Caucasus, stands of pure yew or where yew dominates can be up to 10 ha with trees up to 32.5 m in height. These grade into forests formed by Fagus sylvatica, Carpinus spp., Fraxinus excelsior, Acer platanoides, A. campestre, A. pictum, Ulmus elliptica and Fagus orientalis mixed with Abies nordmaniana and Picea orientalis, with an understorey of Buxus sempervirens (Bugala 1978; Pridnya 1984). Purer yew woodlands are apparently being invaded by beech and ash. The islands of Corsica and Sardinia also have very local woodlands dominated by T. baccata, often with Ilex aquifolium and Buxus sempervirens (in Corsica), Sorbus aria and Mercurialis perennis (classified as Corine Code 9580 and Palaearctic Habitat Classification 42.A72 and 42.A73 in the Interpretation Manual of European Union Habitats 1999). These are thought to represent a senescent phase of the original beech or beech-fir wood. Similar T. baccata relicts, sometimes of small isolated formations, exist in the north and centre of Portugal. Despite its poisonous properties, yew is very susceptible to browsing and bark stripping by rabbits, hares, deer and domestic animals such as sheep and sometimes even cattle (Elwes & Henry 1906; Watt 1926; Kelly 1975, 1981; Mitchell 1988, 1990a; Haeggström 1990; Sarmaja-Korjonen et al. 1991). Indeed, Kelly (1975) describes T. baccata as one of the most grazing-sensitive trees in the Irish Killarney woodlands. Despite this, yew is tolerant of repeated pruning (as demonstrated by its use in topiary) and is able to continue growth under severe browsing pressure (Tittensor 1980). Tabbush & White (1996) suggest that the swelli