Portal de Periódicos da CAPES

Sorbus aucuparia L.

2000; Wiley; Volume: 88; Issue: 5 Linguagem: Inglês

10.1046/j.1365-2745.2000.00502.x

1365-2745

Olivier Raspé, Catherine Mary Findlay, Anne‐Laure Jacquemart,

Botany and Plant Ecology Studies

Slender tree up to 15(−20) m with narrow crown and usually erecto-patent branches; bark greyish and smooth (Hedlund 1901; Hegi Fl. 4, ed. 2; Fl. Eur. 2; Fl. Br. Isl.; Rameau et al. 1989; Guendels 1990). Twigs pubescent when young, then glabrous and greyish-brown. Buds 10–15 mm, ovoid or ovoid-conic, dark brown, somewhat pubescent (Fl. Br. Isl.; Rameau et al. 1989). Leaves 10–25 cm, pinnate, with (4–)5–7(−9) pairs of leaflets (Hedlund 1901; Hegi Fl. 4, ed. 2; Fl. Eur. 2; Fl. Br. Isl.). Leaflets 2.5–6(−9) cm, oblong, acute or subacute, more or less rounded at the often somewhat unequal base, serrate, sometimes doubly so, dark green and glabrous above; subglaucous beneath and pubescent at first, especially on the midrib, usually becoming subglabrous (Hegi Fl. 4, ed. 2; Fl. Br. Isl.). Terminal leaflet more or less equalling the lateral (never larger) (Fl. Br. Isl.). Petiole 2–4 mm (Fl. Br. Isl.). Adaxial glands present on rachis (Robertson et al. 1992). Inflorescence a compound corymb, dense, many-flowered (usually about 250 flowers), woolly pubescent in flower (Hedlund 1901; Hegi Fl. 4, ed. 2; Fl. Br. Isl.). Flowers 8–10 mm diameter (Hedlund 1901; Fl. Eur. 2), generally 5-merous. Some rare 4-merous flowers were observed (Raspé 1998). Receptacle tomentose at first, turning glabrous after blooming (Kovanda 1961). Petals 3.5 mm, white, circular to oval, with short claws (Fl. Br. Isl.; Kovanda 1961). Sepals 1.5–1.8 mm, deltate, sometimes rounded (Hegi Fl. 4, ed. 2), finely and irregularly toothed, glandular (Kovanda 1961). Stamens as long as the petals (Hegi Fl. 4, ed. 2). The mean number of the cream anthers reaches 16 (4-merous flowers) to 25, generally 20 (Raspé 1998). There are (2–)3–4(−5) styles, free or connate at the base and pubescent on the basal part (Hegi Fl. 4, ed. 2; Fl. Br. Isl.; Raspé 1998). Carpels partly free or connate; two collateral, anatropic ovules per carpel (Kovanda 1961; Sterling 1965, 1969). Ovules have two integuments and an obturator (Kovanda 1961). Carpels fused with receptacle only up to two-thirds (Kovanda 1961). The part of the hypanthium between stamen insertion and the base of the styles is nectariferous (Raspé 1998). The nectar is fructose-glucose dominant (Percival 1961). Fruit a two- to five-celled, berry-like pome, each cell containing one or two small, brown seeds (Anonymous 1963). Fruit subglobose, depressed-globose, or ovoid, with few or no stone-cells and few inconspicuous lenticels (Fl. Eur. 2; Fl. Br. Isl.; Hegi Fl.4, ed.2; Grime et al. 1988; Snow & Snow 1988; Rameau et al. 1989; Stace 1997). Pome diameter 6–9(−14) mm (Hegi Fl. 4, ed. 2; Fl. Br. Isl.) or 8–9 × 10–11 mm (Hedlund 1901; Anonymous 1963; Herrera 1987; Grime et al. 1988). For fruit and seed mass see VIII(c). A cross section of the fruit shows a star-shaped central cavity, i.e. the walls of the ovary cells are split (Kovanda 1961). Fruit colour varies from scarlet to orange in the wild (Kovanda 1961), but cv. ‘Fructu Luteo’ produces golden fruits (McAllister 1996). Sorbus aucuparia as well as S. torminalis (L.) Crantz, S. chamaemespilus (L.) Crantz and S. aria (L.) Crantz and their products form a hybrid complex (Proctor et al. 1989; Proctor & Groenhof 1992; Mikoláš 1995; see below). Five subspecies are distinguished in Europe, described below. Ssp. aucuparia: the buds, undersides of leaves and inflorescence-axis are more or less hairy. The petiole is usually more than 2.5 cm. The leaflets are firm, subobtuse or abruptly narrowed to an acute apex. Sepals are deltate and hairy. Fruit is subglobose. It occurs throughout most of the range of the species but it is rarer in the South (Fl. Eur. 2). Ssp. glabrata (Wimmer & Grab.) Cajander: is less hairy than ssp. aucuparia. The petiole is usually more than 2.5 cm. The leaflets are thin, gradually tapered to an acute apex, subglabrous or sparsely hairy on both surfaces. The inflorescence-axis is glabrous or nearly so. The sepals are rounded and hairy. Fruit is longer than wide. It occurs in northern Europe and in the mountains of central Europe (Fl. Eur. 2). Ssp. fenenskiana Georgiev & Stoj.: has leaflets up to 9 × 1–1.8 cm. They are linear-lanceolate, thin, sparsely hairy on midrib beneath. The inflorescences are many- (up to 200-)flowered and the fruits are depressed-globose, 10–12 × 12–14 mm (Fl. Eur. 2). This subspecies occurs only in Bulgaria (Fl. Eur. 2). Ssp. praemorsa (Guss.) Nyman: the petiole is shorter than 2 cm. The leaflets are 2.5 times as long as wide, subobtuse, blunty serrate, hairy beneath. The fruits are ovoid. This subspecies occurs in southern Italy, Sicily and Corsica (Fl. Eur. 2). Ssp. sibirica (Hedl.) Krylov: is glabrous or nearly so. The petiole is usually longer than 2.5 cm. The leaflets are gradually tapered to an acute apex, glabrous or hairy only on midrib beneath. The inflorescence-axis is also glabrous. The sepals are deltate and glabrous. This subspecies occurs only in NE Russia (Fl. Eur. 2). Sorbus aucuparia is native to most of Europe (Fl. Eur. 2). It occurs in woods, scrub and mountain rocks (Fl. Br. Isl.). It could be used in forest restoration management (Emmer et al. 1998). It is also widely planted (including cultivated varieties). The distribution of S. aucuparia in the British Isles is shown in Fig. 1. It is common in the North and West, rare and perhaps not native in some lowland eastern and central English counties (Fl. Br. Isl.). It is found throughout the British Isles from sea-level to over 900 m altitude, higher than any other British tree species (Fl. Br. Isl.). It is reported in England from sea level to 550 m in West and North Yorkshire and 870 m on Helvellyn in Wales to 670 m on Cader Idris and 850 m on Snowdon, in Scotland to 840 m in Atholl and 870 m in Rannoch and in Ireland to 700 m on the Twelve Bens, Galway (Alt. Range Br. Pl.). Juveniles are particularly frequent in gritstone woodlands over 200 m and found to 400 m and adults are observed up to 900 m (Grime et al. 1988). Sorbus aucuparia occurs up to 2000 m in France (Rameau et al. 1989), 1500 m in Norway (Hemsedal) (Lid 1979), and 800 m in northern Sweden (Torne Lappmark) (Nilsson 1987). The distribution of Sorbus aucuparia in the British Isles. Each dot represents at least one record in a 10-km square of the National Grid. Native: (○) pre-1950, (●) 1950 onwards; introduced: (+) pre-1950, (×) 1950 onwards. Mapped by Mrs J.M. Croft, Biological Records Centre, Institute of Terrestrial Ecology, mainly from data collected by members of the Botanical Society of the British Isles. The species is present in most of Europe (Fig. 2), from Iceland and north (not arctic) Russia to the mountains of central Spain and Portugal, Corsica, Italy, Macedonia and the Caucasus (Fl. Eur. 2; Fl. Br. Isl.; Hegi Fl. 4, ed. 2; Atl. N. W. Eur.; Fitter 1978; Welter & Ruben Sutter 1982; Hultén & Fries 1986; Haeupler & Schönfelder 1989). It is absent from Europe only from the Azores, Balearic Islands, Crete, Faroes, Sardinia, Spitsbergen and Turkey (Fl. Eur. 2). It is common in the mountains of France, but less frequent on lower ground (Rameau et al. 1989). In Norway, it is found as far north as 71°N (B.-H. Øyen, pers. comm.). It also occurs in Morocco (high mountains) and in north Asia Minor (Fl. Br. Isl.). It is considered as an Eurasiatic Suboceanic species by Rameau et al. (1989) and Eurasian Boreo-temperate by Preston & Hill (1997). Sorbus aucuparia has been introduced in North America as a ornamental tree. The European distribution of Sorbus aucuparia based on information in the literature (Hultén & Fries 1986). The absence of Sorbus aucuparia from the higher parts of the British mountains is likely to be due to a combination of temperature, wind exposure and grazing. It grows commonly as a small tree on cliffs inaccessible to grazing livestock up to about 650 m (the probable approximate level of the natural altitudinal forest limit in Britain), but above that level is generally found only as seedlings and small and often stunted saplings. Otherwise, S. aucuparia spans virtually the entire range of temperature, rainfall and humidity within the British Isles. In Scandinavia, S. aucuparia is common up to the subalpine birch–forest zone (Nilsson 1987) and small bushes can be found high above the forest limit (Lid 1979; Kullman 1986), suggesting similar climatic limitation in Scandinavia and Britain. The distribution of rowan in Europe is thought to be limited by high summer temperatures. However, it is able to tolerate high temperatures, provided that it is not accompanied by water stress (H.A. McAllister, pers. comm.; see VI e). In France, it needs a high humidity (min. 750 mm rainfall year−1) (Rameau et al. 1989), but it grows in the birch forest at Abisko, north Sweden, where annual precipitation is said to be about 300 mm (M.C.F. Proctor, pers. comm.). According to Barclay & Crawford (1982), rowan is adapted to short growing seasons, ceasing shoot growth relatively early in the season (White 1974) allowing full hardening of buds before freezing conditions occur. Hillebrand & Rosenberg (1996) reported that, at high altitude, S. aucuparia showed less among-year radial growth oscillation than Fagus sylvatica and Picea abies, which suggests that rowan is well adapted to mountain climate. Thus, it is more likely that the distribution of S. aucuparia is limited by a combination of poor drought tolerance, adaptation to short growing seasons and cold requirement for bud burst, rather than high summer temperature per se. Sorbus aucuparia has a wide topographical range, from flat lowland sites to rocky mountain slopes and cliffs. Its sparser occurrence in the lowlands of southern and eastern England and in the midland plain of Ireland reflects the predominantly deep near-neutral, base-rich soils of these areas rather than a topographical limitation as such. No bias with reference to aspect was detected in unshaded sites, but in shaded ones juveniles are recorded more frequently on north-facing slopes (Grime et al. 1988). Sorbus aucuparia is considered as a heliophilous or semiheliophilous species (Rameau et al. 1989). Seedlings and saplings are very tolerant to shade, but light is required for flowering and fruiting (see VI e). Sorbus aucuparia is characteristic of well-drained soils; it is absent from wetlands. The juveniles are not recorded from sites with almost 100% bare soil, but otherwise the species is wide-ranging (Hegi Fl. 4, ed. 2; Grime et al. 1988). Edaphic requirements are similar to those of birch, in that it is distinctly favoured by acidic, non-waterlogged conditions, although it can persist at higher altitudes and is more shade tolerant than birch (McVean & Ratcliffe 1962). In the British Isles, it is mainly restricted to soils of pH < 5.5 but some records are up to pH 7.0 (Grime et al. 1988). Its distribution by Land Classes (Bunce & Last 1981) was assessed in the 1990 Countryside Survey. Sorbus aucuparia was most abundant on the mountain and coastal fringes of north-west Scotland, where the soils are predominantly brown rankers, brown earths, peats, peaty podzols, and peaty gleys. It was entirely absent from the alluvial clays of the Midland plains. It has also been recorded on some limestone sites (Gillham 1980), although it tends to be short-lived on these substrates. In France, Rameau et al. (1989) distinguished between low and high altitudes. At high altitude (mountain belt and higher) S. aucuparia occurs on mull with carbonates to moder (various materials). At lower altitude, it is found on acid mull to dysmoder, relatively base-poor soils, acidic pH (silts and sands, pure or stony). In northern Fennoscandia it is reported to be edaphically unspecialized (Kullman 1986). Emmer et al. (1998) found that this species could improve soil conditions and reverse borealization in central European mountains (see also Lettl & Hysek 1994; Moravčík 1994). Sorbus aucuparia does not accumulate persistent litter (Grime et al. 1988). In an experimental study, only 13% of the original dry weight remained after 5 months (Sydes & Grime 1981). Seedlings and saplings are mainly restricted to wooded sites, particularly on non-calcareous strata, but are also found in heather moorland, and in skeletal habitats including lead mines and wasteland (Grime et al. 1988). Seedlings and mature individuals are concentrated in skeletal habitats, e.g. crevices in rock outcrops and in woodland (Grime et al. 1988). In the uplands of Scotland, rowan is associated with the native Caledonian pinewoods, and occurs elsewhere occasionally in pure stands, particularly in the west Highlands, where it may have replaced oak (McVean & Ratcliffe 1962). In the following account, the list of communities is based on the National Vegetation Classification (Rodwell 1991). Sorbus aucuparia is a frequent component in Fraxinus excelsior–Sorbus aucuparia–Mercurialis perennis woodland (W9), which is characteristic of well drained, but permanently moist, brown earths derived from calcareous bedrocks and superficials in the submontane climate of north-west Britain. Tree and shrub species also include Acer pseudoplatanus, Betula pubescens, Corylus avellana, Crataegus monogyna and Ulmus glabra. The field layer is dominated by Athyrium filix-femina and Dryopteris dilatata, with Brachypodium sylvaticum, Hyacinthoides nonscripta, Mercurialis perennis, Oxalis acetosella and Primula vulgaris. Bryophytes are also abundant in this community and include Eurhynchium praelongum, E. striatum, Plagiomnium undulatum and Thuidium tamariscinum. Sorbus aucuparia is sometimes frequent in Quercus petraea–Betula pubescens–Dicranum majus woodland (W17), which typically occurs on very acid and often fragmentary soils in the cooler and wetter north-west of Britain where, along with Corylus avellana and Ilex aquifolium, it often makes up the understorey component. In some stands of Quercus spp.–Betula spp.–Deschampsia flexuosa woodland (W16) in the north-west, it contributes to the main tier of trees along with Ilex aquifolium. In the southern lowlands of Britain and upland fringes of the Pennines, this community is confined to very acid and oligotrophic soils. In Pinus sylvestris–Hylocomium splendens woodland (W18), S. aucuparia and Betula pubescens are occasionally present as scattered trees, forming thicker patches where the cover is more open. This community occurs on strongly leached soils in the cooler parts of the western submontane zone, from sea level up to 600 m. Scattered trees have also been recorded in Quercus petraea–Betula pubescens–Oxalis acetosella woodland (W11), which is typical of moist but free-draining and quite base-poor soils in the cooler and wetter north-west of Britain, gaining abundance in ungrazed stands, and attaining co-dominance with birch in the far north-west in low scrubby canopies. It sometimes contributes to the canopy of Quercus robur–Pteridium aquilinum–Rubus fruticosus woodland (W10) on base-poor soils throughout the temperate lowlands of southern Britain, or as dispersed individuals of patchy local prominence in the understorey. Sorbus aucuparia is also an occasional understorey component of the following communities: Alnus glutinosa–Fraxinus excelsior–Lysimachia nemorum woodland (W7), typical of moderately base-rich and fairly wet mineral soils which occur in the wetter parts of Britain; Fagus sylvatica–Rubus fruticosus woodland (W14) confined to brown earths of low base status with moderate to slightly impeded drainage in southern England; and Fagus sylvatica–Deschampsia flexuosa woodland (W15) which is confined to very base-poor, infertile soils in the southern lowlands of Britain. It is also sparsely distributed in Fraxinus excelsior–Acer campestre–Mercurialis perennis woodland (W8) on calcareous mull soils in the relatively warm and dry lowlands of southern Britain, and Alnus glutinosa–Carex paniculata woodland (W5) characteristic of wet to waterlogged organic, base-rich soils. In France, the species occurs in woods, mountain heaths, and borders of forests: it is present in Quercion robori-petraeae at low altitude and various communities as Fagion sylvaticae, Vaccinio-Picetea, Prunetalia spinosae, Sambuco-Salicion or Calamagrostion arundinaceae at high altitudes (Rameau et al. 1989). Schaminée et al. (1992) investigated scrub communities dominated by Sorbus species in the subalpine zone of the Monts du Forez, Massif Central. Sorbus aucuparia was the most common of the species, with the widest ecological and altitudinal amplitude, occurring up to 2000 m. Seven types of vegetation, belonging to the class Betulo-Adenostyletea, were identified. Zerbe (1993) gave a detailed account of the occurrence of S. aucuparia in southern Germany. In the upper montane belts, it can be part of the tree layer in Luzulo-Fagetum, especially in Luzulo-Fagenion, Lonicero alpigenae-Fagenion, Galio rotundifolii-Abietenion, Cephalanthero-Fagenion, Tilio platyphyllis-Acerion pseudoplatani and Galio odorati-Fagenion. The species is also very frequent (but not in the tree layer) in the following communities: Dicrano-Pinion, Piceion abietis, Aceri-Fagenion, Quercion robori-petraeae, Erico-Pinion, Prunion fruticosae and Alnion glutinosae. Oberdorfer (1978) has also recorded S. aucuparia in Epilobion angustifolii, Calamagrostion, Adenostylion alliariae and particularly in Sambuco-Salicion. Diekmann et al. (1999) conducted a twinspan cluster analysis on vegetation data from beech forest communities of nordic countries and identified a Fagus sylvatica–Sorbus aucuparia–Deschampsia flexuosa community typical of very acid and oligotrophic soils. Two subcommunities, characterized by either Carex pilulifera or Vaccinium myrtillus, were described. The Fagus sylvatica–Sorbus aucuparia–Deschampsia flexuosa community is synonymous with the Deschampsio-Fagetum described by Kielland-Lund (1981). It corresponds to the central European Luzulo-Fagetum, which also occurs on acid and nutrient-poor soils (Diekmann et al. 1999). According to Diekmann et al. (1999), the Fagus sylvatica–Sorbus aucuparia–Deschampsia flexuosa community is the most widespread beech community in northern Europe. The associated floristic diversity is low; Grime et al. (1988) recorded Luzula pilosa, Milium effusum, Oxalis acetosella, Pteridium aquilinum and Rubus fruticosus. Sorbus aucuparia is a stress-tolerant competitor (Grime et al. 1988). Nevertheless, its establishment in woodland, and presumably also in skeletal soils, appears to be adversely affected by the presence of grazing stock or game (McVean & Ratcliffe 1962; Pigott 1983; Hester et al. 1996; Linder et al. 1997). In Swedish boreal forest reserves, Linder et al. (1997) observed that S. aucuparia was the most numerous species in the seedling cohort, but was almost totally missing in the tree layer, because of high browsing pressure. After excluding sheep from hill pastures in North Wales, Hill et al. (1992) observed colonization by S. aucuparia along fences in some sites. Birds used fence posts as perches and must have introduced the seeds. Kinnaird et al. (1979) reported that, in a woodland in Aberdeenshire, as much as 99% of rowan trees was barked by beef cattle. On average 78% of the bark was removed. However, rowan is very tolerant to damage (Miller et al. 1982). The response to superficial wounding, with the formation of a wound periderm structurally similar to the intact, original one, is completed within 28 days (Woodward & Pocock 1996). The deposition of suberin was detectable relatively early in the response, i.e. 7 days after wounding. This is an important component for protecting the compromised tissues from desiccation and markedly reducing the likelihood of pathogen invasion (Woodward & Pocock 1996). Sorbus aucuparia has a maximum life span of 150 years (Grime et al. 1988). It is mostly disseminated, but can also occur in pure stands (McVean & Ratcliffe 1962; Kullman 1986). Barclay & Crawford (1984) suggested that S. aucuparia trees at higher altitudes in Scotland produced a smaller number of berries. There was a significant correlation between increasing altitude and decreasing seed weight and viability. Nevertheless, there is a gradual increase in growth rate of the seedlings (0.149 g g−1 day−1 at 567 m vs. 0.129 at 8 m) with increase in altitude of seed source (Barclay & Crawford 1984). Fruit production varies from year to year and from site to site between 50 and 3020 kg ha−1. The production is higher in open situations than in forests or borders (Kutsko et al. 1982). In the upper limits of their altitudinal distribution, rowan trees are exposed to severe winter desiccation coupled with short, cool summers in which to complete the development of over-wintering tissues. The ability of the rowan to maintain the tree growth-form well above the current altitudinal limit of most other trees suggests that it is particularly well-developed to withstand the double stress of winter desiccation and short growing season (Barclay & Crawford 1984). Dutton & Bradshaw (1982) observed a high tolerance of root desiccation: 68% of seedlings survived following 7 days' root exposure compared to only 8% survival in Betula pubescens. McEvoy & McKay (1997) recorded root frost hardiness in two-year-old trees to − 5 °C and noted that rowan displayed very little seasonal variation in sensitivity to frost from the end of October to early March. Sorbus aucuparia does not appear to tolerate flooding, reduced growth being recorded (Frye & Grosse 1992). The species is also considered as extremely flammable with relatively high calorific values mainly in spring and summer (Núñez-Regueira et al. 1997). Roots are tough and fibrous. Gillham (1980) recorded a mean root length of over 50 cm in one-year-old seedlings. Sorbus aucuparia normally grows up to about 20 m in height, although individual specimens have been recorded up to 28 m in the British Isles (Anonymous 1996). Stem girth is up to 75 cm and bark thickness reaches 0.5 cm (Kinnaird et al. 1979). The tree habit is monocormic or polycormic, particularly if subjected to grazing pressure. Branch angles are acute to stem, forming a narrow crown, with a monopodial branching pattern (Linnenbrink et al. 1992). Timber is strong and fine-grained, with yellow sapwood and purple heartwood (Edlin 1978). The leaves are hypostomatous. Stomatal density was recorded by Beerling & Kelly (1997) as 132 mm−2 on the lower leaf surface, in comparison to Salisbury (1927) who observed 118 per mm−2. Morphological variation has not been widely studied in Britain. Popov (1990) investigated populations along a latitudinal gradient from Karelia to the Crimea, and observed that crown density and fruit skin colour increased along a north–south gradient. Hillebrand & Rosenberg (1996) suggested the existence of ecotypes on the basis of isoenzyme differences among three rowan populations from north-west Germany. Their conclusions seem questionable, given the very small sample analysed and the lack of reliable genetic interpretation of isozyme phenotypes. Moreover, Raspé & Jacquemart (1998) observed very low genetic differentiation among populations distributed from the Pyrenees to Finland. Some clonal selections for decorative planting have exploited morphological variations, e.g. ‘Asplenifolia’ which has deeply cut foliage; ‘Sheerwater Seedling’ which has a particularly upright habit; the weeping ‘Pendula’, and ‘Fructu Luteo’ which has amber-yellow fruits (McAllister 1986, 1996; Anonymous 1991). Sorbus aucuparia has been recorded in association with both arbuscular mycorrhizal (AM) and, less commonly, ectomycorrhizal (ECM) fungi (Harley & Harley 1987). Studies by Dominik (1957) in Poland and Trappe (1962) in the Pacific north-west of America, indicated ECM associations with the ascomycete Cenococcum geophilum Fr. Vosatka (1987) recorded AM infection levels of 13–40% in mining spoil in northern Bohemia where spores of Acaulospora spp. and Glomus spp. were isolated. Otto & Winkler (1995) noted infection levels of 30–60% in Germany. In the United States, Morrison et al. (1993) observed AM colonization levels of 10–20% in nursery plants. In the same study, inoculation with Glomus intradices Schenk & Smith under high fertility nursery conditions had no effect on growth. However, Findlay (1999) observed significant height increases (and greater cold-tolerance) following inoculation with the same fungus under experimental conditions. Phanerophyte, reproducing entirely by seeds which have a requirement for cold, moist stratification to overcome a deep physiological dormancy imposed by both the seed coat and embryo (Devillez 1979a,b,c; Gordon & Rowe 1982). Some regeneration occurs by epicormic shoots, particularly at high altitude where viable seeds are seldom produced, or in response to grazing or coppicing (Barclay & Crawford 1984; Kullman 1986). Lateral clonal spread of up to 5 m from root suckering has occasionally been recorded (Kullman 1986). In cuttings taken from 2- to 3-year-old coppice shoots, 38% rooted and rooting was improved by a basal dip in indolebutyric acid (Hansen 1990). The basic chromosome number in Sorbus aucuparia is 2n = 34 (Liljefors 1955; Fl. Eur. 3; Fl. Br. Isl.; Uotila & Pellinen 1985; Dickson et al. 1992). An isozyme study in S. aucuparia (Raspéet al. 1998) revealed the occurrence of divergent duplicated genes for most of the studied enzymes. These results support the hypothesis of an allopolyploid origin of S. aucuparia and the Maloideae, in accordance with previous studies based on a variety of characters, such as cytological and chemotaxonomical traits and fruit and leaf morphology (Raspéet al. 1998; references therein). Meiosis is normal (Liljefors 1955). Sorbus aucuparia is adapted to completing its growth cycle within the short growing seasons which occur at high altitude and latitude sites. Håbjørg (1978) did not observe any effect of photoperiod on shoot elongation, although this may have resulted from equipment failure. In the same experiment, other tree species (e.g. Betula verrucosa), growing at similar latitudes to rowan, had higher shoot elongation under longer daylength—an adaptation to short growing seasons. Heide (1993) observed no response to long days in thermal time to bud burst. White (1974) measured growth at 560 m altitude in the Pennines, where shoot elongation ceased in mid-August, allowing sufficient shoot hardening before winter. Rowan is generally considered to be relatively shade tolerant, particularly during the seedling stage (Hegi Fl. 4, ed. 2; McVean & Ratcliffe 1962). Pigott (1983) planted seedlings of S. aucuparia, Betula pendula and Quercus petraea in pots beneath the dense canopy of Acer pseudoplatanus. Betula failed to survive; only Sorbus demonstrated increased dry mass. Lunde-Hoie & Anderson (1993) observed that, in Norway, S. aucuparia tends to regenerate in established vegetation close to the mother tree. Similarly, Vanha-Majamaa et al. (1996) compared rowan regeneration in clear-cut and boreal forest areas in southern Finland. Greatest regeneration was recorded in the forest areas, particularly in the shade of dead trees, although it was also suggested that the dead trees may have provided birds with places to sit and defecate the seeds. In the established phases, rowan trees also tolerate partial shade, particularly at low altitudes, although flowering tends to be reduced (Schaminée et al. 1992). In wooded habitats, S. aucuparia occurs more frequently in woodland than in scrub, suggesting that establishment by seed and the subsequent development of the tree usually take place under shaded conditions (Grime et al. 1988). The rate of photosynthesis was estimated as 9.4 ± 2.5 µmol CO2 m−2 s−1 in May and 4.2 ± 1.2 µmol CO2 m−2 s−1 in September (Jahnke et al. 1998). Translocation of photoassimilates (sorbitol and sucrose) was studied on 2-year-old trees (Jahnke et al. 1998). Within 2 h after pulse feeding with CO2 even the upper leaves were involved. Early in the season (April), within 2 h after pulse labelling, 20% of the fixed radiolabel was exported from the leaves. The rates of photoassimilates translocation monitored at the rachis were as high as 50–130 cm h−1 (Jahnke et al. 1998). Linnenbrink et al. (1992) recorded the bulk water relations of a range of hedgerow shrubs in northern Germany, and classified rowan as a euryhydric species. It showed the greatest diurnal amplitude of water potential (1.9 MPa) and the lowest leaf water potential (− 3.0 MPa). Leaf water content depended on the position of the leaf within the canopy, although it did not fluctuate as widely throughout the growing season in S. aucuparia (80–120% dry weight) as in Sambucus nigra (200% dry weight), which they suggested was owing to a greater allocation of solutes to the leaves in S. aucuparia. Although both species were able to tolerate leaf water saturation deficits of more than 40%, damage occurred much sooner in S. aucuparia (4–5 hours' desiccation in early summer) than in Sambucus nigra (14 h). Vogt & Lösch (1999) reported that despite stem water potentials as low as −4.0 MPa, S. aucuparia showed high values of leaf conductance, indicating that the stomata were kept open. Vogt & Lösch (1999) claimed that the ability of S. aucuparia to maintain high leaf conductance under water stress conditions is an adaptation to drought. However, Linnenbrink et al. (1992) argued that since plasmatic drought tolerance of leaves was not exceptionally high, water shortage may sometimes reduce competitive vigour, and S. aucuparia is not well adapted to habitats where water stress can occur. The survival of S. aucuparia at high altitudes has been in part attributed to the ability of the winter buds to tolerate winter desiccation (Barclay & Crawford 1982). At higher altitudes, the bud scale cuticles were thinner (13.0 µm), less mature and had the greatest decrease in relative water content (19