Sorbus aucuparia L.

Authors


  • *

    Abbreviated references are used for standard works; see Journal of Ecology (1975), 63, 335–344. Nomenclature of vascular plants follows Flora Europaea and Stace (1997) for British species.

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).

I. Geographical and altitudinal distribution

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).

Figure 1.

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.

Figure 2.

The European distribution of Sorbus aucuparia based on information in the literature (Hultén & Fries 1986).

II. Habitat

(a) climatic and topographical limitations

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).

(b) substratum

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).

III. Communities

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 sylvaticaSorbus aucupariaDeschampsia flexuosa community typical of very acid and oligotrophic soils. Two subcommunities, characterized by either Carex pilulifera or Vaccinium myrtillus, were described. The Fagus sylvaticaSorbus aucupariaDeschampsia 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 sylvaticaSorbus aucupariaDeschampsia 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.

IV. Response to biotic factors

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).

V. Response to environment

(a) gregariousness

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).

(b) performance in different habitats

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).

(c) effect of frost, drought, etc.

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).

VI. Structure and physiology

(a) morphology

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).

(b) mycorrhiza

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.

(c) perennation: reproduction

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).

(d) chromosomes

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).

(e) physiological data

(i) Light

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).

(ii) Water relations

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%) than buds at lower altitudes (cuticle thickness 19.3 µm; water content 42%). Despite this, vital staining indicated that the buds were still viable. In a further study, buds of S. aucuparia remained viable after 20 days' desiccation in comparison with a range of other tree species including Quercus robur and Fagus sylvatica which were viable for only 5 days under the same desiccating conditions. It was concluded that the continued survival of S. aucuparia at high altitudes must be due to cytoplasmic resistance.

Transpiration declined with increasing wind speed; also the stomatal resistance declined although the stomatal response was over-shadowed by the declining leaf to air vapour pressure difference (Dixon & Grace 1984).

(iii) Temperature

The effect of temperature on growth has been little studied. In a multivariate analysis of height increment with meteorological variables, White (1974) observed that shoot elongation occurred below 5.6 °C, and the rowan was particularly responsive to the component which he labelled as ‘energy’ (i.e. temperature, day length, hours sunshine and relative humidity) and soil temperatures early in the season. Rowan seemed to benefit significantly from cold soil early in the growing season. Barclay (1979) suggested that S. aucuparia could decrease its dark respiration rate in response to increases in temperature, enabling it to conserve carbohydrates in a low energy environment. Kronenberg (1994) modelled the effect of temperature on the flowering date and concluded that it requires a cold period of 750 h below 7 °C, followed by a temperature sum of 160 day-degrees above a mean day temperature of 6 °C, with a base temperature above 6 °C.

(iv) Nutrition

Gillham (1980) suggested that rowan will grow on nutrient-poor soil, but on more fertile soils the growth rate is higher. Foliar analysis of seedling leaves indicated no significant differences between acidic (pH 3.2) or calcareous soils (pH 6.6): N 5.9 mg g−1; P 2.8 mg g−1; K 12 mg g−1. Leaf calcium contents varied depending on the location of the seedlings: those from an acidic site accumulated less calcium in the leaves (5 mg g−1) irrespective of substrate compared to those from more calcareous sites (11.5 mg g−1). Findlay (1999) also observed increased growth when rowan was grown on the more fertile substrates, although on less fertile substrates growth was improved when the substrate was not sterilized, suggesting that the microbial components of the soil may enhance nutrient uptake on marginal sites. Sperens (1997a) applied mineral fertilizers to a sample of experimental trees and recorded an increased number of flowers, fruits and seeds per tree up to 5 years after the fertilizer application compared to control trees. However the fruit–flower ratio per tree did not change, and increased fruit production was owing to increased flower production per tree and inflorescence, rather than any increase in fruit set. The addition of fertilizer increased leaf nitrogen content from 2.5 to 4 mg g−1.

Prima-Putra & Botton (1998) analysed the chemical composition of bleeding xylem sap during bud breaking. The total amino acid concentration was as high as 18 mmol L−1 in S. aucuparia, more than in the other plant families studied. Nineteen amino acids were identified: asparagine was predominant (60% by molarity) as well as aspartate, arginine, valine, leucine, isoleucine and phenylalanine. Arginine could be a potentially rich source of readily available reduced nitrogen for the early stage of leaf growth. Low amounts of nitrate (391 ± 21 µmol L−1) and ammonium (103 ± 11 µmol L−1) were detected. Organic acids were mainly malic acid (715 ± 116 µmol L−1) and citric acid (322 ± 74 µmol L−1). The main inorganic ions were potassium (4.7 mmol L−1), calcium (3.7) and magnesium (2.2) (Prima-Putra & Botton 1998).

(v) Other aspects

Horntvedt (1997) studied the accumulation of fluoride from aluminium smelter plants in Norway and observed a linear relationship in rowan between fluoride exposure and accumulation, making S. aucuparia as useful as passive fluoride collectors to monitor fluoride pollution. He calculated the fluoride accumulation coefficient (K), defined as the ratio between fluoride accumulated in leaves and fluoride exposure (airborne fluoride concentration × time of exposure), to be 1.7 m3 g−1 day−1 in S. aucuparia. This value was higher than for most other species listed by Horntvedt (1997). Vike & Håbjørg (1995) noted that leaf injury symptoms occurred at fluoride concentrations of 170 p.p.m. and also suggested that rowan was a good monitor species of fluoride emissions. Møller (1998) observed some developmental instability related to the level of radiation by caesium-137 near Chernobyl, Ukraine.

(f) biochemical data

The results of chemical analyses of the berries and their parts according to Pulliainen (1978) are given in Table 1. The flesh appeared to be rich in minerals (ash 10.2%), especially potassium (15.8 mg g−1), and sugars (17.7%). High values for crude proteins (25.5%) and crude fat (16.5%) were recorded in the seed, which also contained 5.5% sugars and relatively large amounts of calcium (2.9 mg g−1) and phosphorus (5.7 mg g-1). The highest percentage of crude fibre, 20.7%, was naturally recorded for the skin, which also contained more crude proteins and crude fat than the flesh, and was rich in potassium (Table 1). According to Herrera (1987) and Snow & Snow (1988), dry-matter content of fresh rowan berries varies between 19% and 34%, and ash content (per fresh weight) varies between 0.55% and 1.7%. Percentages, by weight, of the dry pulp are 2.3–3.1% of lipids, 3.2–6.4% of proteins, 76.1–77.0% of soluble carbohydrates and 5.3–15.9% of fibre (Pulliainen 1978; Herrera 1987; Snow & Snow 1988). Dry pulp of rowan berries contains 1.6–1.8 mg g−1 of calcium, 1.1–1.3 mg g−1 of phosphorus, 9.0–15.8 mg g−1 of potassium and 0.4–0.9 mg g−1 of magnesium (Table 1). Herrera (1987) also reported 0.2 mg g−1 of sodium, 29 mg kg−1 of iron, 1 mg kg−1 of manganese, 3 mg kg−1 of zinc and 6 mg kg−1 of copper in dry pulp. Some discrepancies appeared between the data of Pulliainen (1978) and Herrera (1987) (Table 1), especially for ash, fibre, and sugar contents. For fibre and sugars, these discrepancies might be accounted for by differences in the estimation procedure. Further studies would be needed to determine what regional and annual variations occur in the chemical composition of the fruits. The estimated energy per g of dry pulp averaged 3.48 kcal and the energy per g of whole fruit averaged 0.78 kcal (Snow & Snow 1988). The most important fatty acids of S. aucuparia seeds are linoleic, oleic and to a lesser extent palmitic acid (Johansson et al. 1997). An almost total absence of α-linolenic acid is a remarkable feature of S. aucuparia (Johansson et al. 1997). Succinic acid and methoxymalic acid were also found in the fruits (Oster et al. 1987).

Table 1.  Analytical data (% of dry matter) for S. aucuparia berries collected near Oulu, northern Finland (Pulliainen 1978) and in Andalusia, Spain (Herrera 1987)
 FinlandSpain
Entire berrySeedSkinPulpPulp
  • * 

    Non-structural carbohydrates (estimated by substracting ash, proteins, lipids and fibre from 100%).

Ash (%)5.14.44.710.21.7
Proteins (%)9.325.58.76.43.2
Lipids (%)6.216.58.32.33.1
Fibre (%)8.913.320.75.315.9
Sugars (%)6.65.52.317.776.1*
Ca (mg g−1)2.02.92.31.81.6
P (mg g−1)2.15.72.21.11.3
K (mg g−1)15.89.116.315.89.0
Mg (mg g−1)1.63.61.20.90.4

The berries are rich in vitamins, especially ascorbic acid (40–60 mg per 100 g berries) (Pyysalo & Kuusi 1974). They also contain several phenolic compounds, such as trans-chlorogenic, caffeic, p-coumaric and ferulic acids, cyanidin-3-galactoside (red pigment), cyanidin-3,5-diglucoside, quercetin, isoquercetin and rutin (Pyysalo & Kuusi 1974 and references therein). Although some polyphenols are known to be bitter, the bitterness of rowan berries is not owing to the polyphenols which they contain, but rather to a precursor of sorbic acid, trans-3-d-glucopyranosyloxy-5-hexanolide (Letzig 1964; Pyysalo & Kuusi 1971, 1974; see also Eder et al. 1991).

Several biphenyl compounds are produced as phytoalexins by the sapwood and/or heartwood of S. aucuparia: aucuparin, 2′-hydroxyaucuparin, 2′-methoxyaucuparin, 4′-methoxyaucuparin and isoaucuparin (Erdtman et al. 1961, 1963; Kokubun & Harborne 1994, 1995; Kokubun et al. 1995). The two aucuparin derivatives occurring most frequently (2′-methoxyaucuparin and 4′-methoxyaucuparin) are those which are the most fungitoxic (Kokubun et al. 1995). Within the genus Sorbus, there is an atypical variation among species in the phytoalexins which they produce (either biphenyls or dibenzofurans). This variation seems congruent with and thus supports the division of the genus into several subgenera (Kokubun & Harborne 1995). In a survey of phytoalexin induction in the leaves of 130 Rosaceae species, Kokubun & Harborne (1994) observed that real phytoalexin induction (i.e. de novo synthesis) was confined to very few species, notably S. aucuparia (which produced aucuparin).

Sorbus aucuparia wood also contains constitutive antifungal phenolics. Feucht et al. (1989) reported 16 different flavan-3-ols (catechin-like substances) in the periderm and phloem of wild S. aucuparia and S. aucuparia var. edulis‘Pink Queen’ and ‘Rosina’. These were mainly catechin and epicatechin. The great variety of compounds suggests specialized defence against particular pathogens or herbivores (Feucht et al. 1989).

Malterud & Opheim (1989) isolated dihydrosapinic aldehyde from the sapwood of S. aucuparia in a yield of 2 × 10−4%. This compound is part of the dihydrocinnamic aldehyde group, which occur rarely in nature (Malterud & Opheim 1989). Arya et al. (1962) also isolated lyanoside (a lignan xyloside) from the sapwood in a yield of 1–3% dry weight.

Stams & Schipholt (1990) have analysed the content of organic nitrogen (2.4–2.7 mmol g−1), ammonium (33 µmol g−1 in the spring and 11 µmol g−1 in the autumn) and nitrate (2 µmol g−1 in the spring and 1 µmol g−1 in the autumn sample) of the leaves.

Kinnaird et al. (1979) gave the chemical composition of the bark (per cent dry weight, with 95% confidence limits): ash 4.8 ± 0.3, Ca 1.6 ± 0.2, K 0.4 ± 0.1, Na 0.02 ± 0.01, Mg 0.15 ± 0.02, P 0.07 ± 0.01, tannins 7.9 ± 1.4, pH 4.6–5.2. Lawrie et al. (1960) isolated lupeol, betulin and 23-hydroxybetulin from the bark.

VII. Phenology

In the British Isles as well as in Belgium and France, buds break in early spring and S. aucuparia flowers in May and early June (Hegi Fl. 4, ed. 2; Fl. Br. Isl.; Grime et al. 1988; Snow & Snow 1988; Rameau et al. 1989). The earliest fruits ripen about mid July, and by the end of August the whole fruit-crop is ripe. The fruits tend to dry up and turn brown if not eaten, but there is a good deal of variation in this (Snow & Snow 1988). Some trees have all their remaining fruits shrivelled and bad by mid September; others stayed in good conditions until October or even early November (Snow & Snow 1988). In Germany or Scandinavia, fruits can persist in good conditions through the winter (Pulliainen 1978).

VIII. Floral and seed characters

(a) floral biology

The plant is amphimictic (Hegi Fl. 4, ed. 2; Liljefors 1953; Fl. Eur. 2; Fl. Br. Isl.). The flowers are actinomorphic, hermaphrodite and generally 5-merous. The petals are perpendicularly exposed at the beginning of the life span, but later they tend to recurve towards the pedicel (Raspé 1998). Pollen grains are oval, psilate, with a wide range of aperture types (tricolpate or tricolporate, with colpi unconstricted or constricted in various ways) but with predominantly colpi constricted by a bridge, with a maximum length of 18–27 µm (Reitsma 1966; Boyd & Dickson 1987a).

The pollen : ovule ratio is very high with a mean of 8028 ± 1978 (n = 15, Raspé 1998); according to Cruden (1977) this species can thus be considered as an outbreeder.

The stigmas are receptive before the pollen of the same flower is shed (protogyny). In dull weather the stamens converge, whereas in warm weather they diverge to expose the abundant nectar. According to Proctor & Yeo (1973) the convergence of stamens causes self-pollination. However, it is doubtful whether this phenomenon represents an adaptation to pollination in the absence of active pollinators, since Raspé (1998) has shown the species to be self-incompatible (see below). The scent is sweet and heavy (trimethylamide, Knuth Poll. 3) and perceptible at a distance of many metres (Proctor & Yeo 1973).

Percival (1965) considered the species as a short-tongued fly flower because it is shallow, with accessible nectar and massed flattish inflorescences. Indeed, Raspé (1998) reported that in the upper Ardenne (Belgium), the flowers are visited principally by Diptera (87–89%); and particularly by Syrphidae (24–40% of the Diptera) and Empididae (16–42% of the Diptera). Some Apidae (bumblebees and honeybees, 6–7%) were also observed. Faegri & van der Pijl (1979) noted Coleopterans as the principal pollinators. Proctor & Yeo (1973) recorded various insect visitors. The flowers have been noted in the English Lake District attracting exclusively large numbers of blow-flies (Calliphora spp.). However, the recorded visitors are of many species and include beetles (several families), flies (several families), bees (social and solitary) and other Hymenoptera, Lepidoptera, etc. Both nectar and pollen are taken and some of the beetles eat parts of the flowers as well (Proctor & Yeo 1973; Raspé 1998). Proctor et al. (1996) also noted Dilophus febrilis (Diptera, Bibionidae) on the flowers.

Cross-pollination significantly increased the fruit set as well as extra pollen vs. open pollinations (Table 2). Since virtually no fruit set (and very low fruit initiation) occurred after self-pollination, Raspé (1998) concluded that this species is self-incompatible. Self-incompatibility in S. aucuparia is probably of the gametophytic type, as in Pyrus serotina (Sassa et al. 1992) or Malus × domestica (Sassa et al. 1994; Broothaerts et al. 1995). Indeed, the self-incompatibility system is often conserved within a family (de Nettancourt 1977) and the Maloideae are generally considered to form a homogenous phylogenetic group. Sperens (1996) reported that S. aucuparia was only partly self-incompatible in his study sites, near Umeå (Sweden). However, he observed a fruit : flower ratio five times larger and a seed : ovule ratio two times larger in cross-pollinated inflorescences compared to self-pollinated inflorescences. This might indicate that the self-incompatibility mechanism is functional but weakened (Bixby & Levin 1996).

Table 2.  Fruit initiation, fruit set, and seed set (seeds/ovules) of Sorbus aucuparia in Belgium, mean ± SE (Raspé 1998)
TreatmentsFruit initiation
(%)
Fruit set
(%)
Seed set
(%)
Open pollination28.1 ± 4.713.9 ± 3.953.2 ± 3.0
Bagged flowers00
Hand-selfed flowers8.9 ± 4.52.2 ± 1.513.0
Hand-crossed flowers84.4 ± 6.075.6 ± 6.071.0 ± 3.0
Supplemental hand-
cross-pollination
64.8 ± 6.331.7 ± 7.270.0 ± 4.0

Extra pollen significantly increased fruit initiation, fruit set and seed number per fruit when some of the flowers of a single inflorescence were additionally hand-pollinated (Sperens 1996; Raspé 1998). This suggests that pollinators do not provide enough pollen to fertilize all ovules of a single flower, and all flowers of an inflorescence. It seems, however, that pollen limitation of the total reproductive output of single trees has to be ruled out (at least for large trees) because according to Sperens (1996) resource reallocation appeared to take place between hand-pollinated flowers and nearby control (open-pollinated) flowers of the same individual.

(b) hybrids

The hybrid S. aria × aucuparia=S. × thuringiaca (Ilse) Fritsch occurs very rarely as a single tree with the parents but is sometimes planted (Hedlund 1901; Fl. Br. Isl.; Mikoláš 1995). Leaves 7–11 cm, more or less oblong, 1.6–2.3 times as long as broad, mostly with 1–3 pairs of free leaflets at the base but on some trees many of the leaves are lobed nearly to the base but without free leaflets, upper part with more or less oblong lobes, serrate, obtuse, dull green and glabrous above, greenish-grey-tomentose beneath. Veins (including free leaflets) 10–12 pairs; petiole 1.5–3 cm. Inflorescence woolly pubescent. Petals 4–6 mm. Anthers cream, more rarely pink. Styles 2–3. Fruit 8–10 mm, subglobose, scarlet, with few inconspicuous lenticels. This fertile hybrid shows marked segregation from seed but F2 plants have not been found wild in England (Fl. Br. Isl.).

According to leaf morphology, a cross between S. rupicola (AAAA, 2n = 68) and S. aucuparia (BB, 2n = 34) could have given rise to the triploid derivative, S. arranensis (AAB, 2n = 51) and this, in turn, could have produced, through the fertilization of an unreduced gamete by a backcross to S. aucuparia, the derivative S. pseudofennica (AABB) as two rare endemic Arran species (Hull & Smart 1984). It is also assumed that these two derived species have been propagated almost exclusively by apomixis from the original hybrids (Hull & Smart 1984). Other triploid derivatives, S. leyana and S. minima, were also found in Brecon and apomictic triploid hybrids may then have backcrossed with S. aucuparia to give a tetraploid group known as S. hybrida in Scandinavia (Liljefors 1955; Hull & Smart 1984). According to Bolstad & Salvesen (1999), all the variation within the S. aucuparia × S. hybrida complex in Fennoscandia should be included in one aggregate species S. meinchii, which is triploid (2n = 51).

Májovský & Bernátová (1996) described a new hybridogenous species, Sorbus pekarovae Májovskýet Bernátová, the progenitors of which are probably S. aucuparia and a species from the S. graeca aggregate. Sorbus pekarovae seems to be confined to Mt. Pekárová (Slovakia).

(c) seed production and dispersal

Seed bearing begins at about 15 years of age. The tree bears a good seed crop almost annually, with light crops in intervening years (Anonymous 1963). Wallenius (1999) reported that, in Finland, annual fruit yield correlated negatively with the previous year yield (data from 1956 to 1996).

The ripe fruit contains a variable number of rather soft, small seeds (Hegi Fl. 4, ed. 2; Iketani & Ohashi 1991). Usually 1–5 full-sized seeds are produced per fruit, but we observed up to 8 seeds in a 4-celled pome (i.e. all ovules gave rise to a seed). Fresh weight of fruit averaged 0.45–0.49 g (Herrera 1987; Snow & Snow 1988) or 0.20–0.68 g (Kutsko et al. 1982) and dry weight averaged 114 mg (Herrera 1987). Seed dry weight ranged from 1.15 mg to 4.14 mg (Barclay & Crawford 1984; given as 2.58 mg by Grime et al. 1988; 1.4 mg by Herrera 1987); seed fresh weight averaged 8.2 mg (Snow & Snow 1988). The seed ‘burden’ (seed as a percentage of the total fruit weight) is unusually low, averaging 3.4%.

Birds are the main seed dispersers (see IX), but mammals play also a role (Snow & Snow 1988). A mean diameter of about 9 mm allows the fruits to be swallowed whole by all except the smallest frugivores. Warblers may be able to swallow the smaller fruits whole but cannot swallow full-sized ones (Snow & Snow 1988). Where suitable habitats are adjacent to mountain streams, the seeds are also dispersed by water (Disp. Pl.).

No persistent seed bank has been detected within the Sheffield region (Grime et al. 1988) but Hill (1979) suggested that seeds have considerable longevity in the soil. This observation, if confirmed, would make S. aucuparia the only tree in the British flora characterized by a long-persistent seed bank.

(d) viability of seed: germination

Sorbus aucuparia seeds are characterized by a deep and absolute dormancy, which has two components, an embryo dormancy and a dormancy imposed by the seed coat (Flemion 1931; Devillez 1979a,b,c; Devillez et al. 1980). Indeed, no germination of freshly collected seeds or excised embryos was observed after 1 month of incubation at 20 °C (Devillez 1979a,c; Barclay & Crawford 1984; Grime et al. 1988; Table 3). Dormancy breakdown involves complex processes concerning water intake and circulation, growth regulators and trophic relations (Devillez et al. 1980). The embryo dormancy could be related to an insufficient digestion of the perisperm by the cotyledons, and varies from year to year (Anonymous 1963; Devillez 1979c).

Table 3.  Germination after various treatments of Sorbus aucuparia seeds collected in Belgium (Devillez 1979a,b,c; Devillez et al. 1980), each treatment with 4 × 50 seeds
TreatmentsGermination (%)
  • * 

    Germination capacity, i.e. percentages of seeds/embryos that germinate, relative to the sum of germinated seeds/embryos and ungerminated but viable seeds/embryos.

Freshly collected (1 month incubation at 20 °C)0
Cold stratification (6 months at 2 °C) and response to light
 Light89.5
 Dark64.0
Warm stratification (10∼30 °C daily) + 6 months' stratification at 2 °C
 16 days100*
 45 days100*
 60 days83.3*
 90 days68.3*
Excision of embryos—cold stratification (2 °C)
 2 months31.1*
 3 months92.0*

Stratification at 2 °C can break both embryo and seed coat dormancy. According to Devillez (1979a), a 6-month cold stratification is necessary to obtain the highest germination percentages. Light seems to increase germination during the cold stratification (Devillez 1979a; Table 3). All germinations could be obtained during the cold stratification, although incubation at 20 °C sped up germination and growth of those seeds of which dormancy had been broken by low temperatures. The first seedlings appeared after 50 days of cold stratification, germination taking place between 50 and 190 days, which seems to indicate a great variation of cold requirements, and the intensity of dormancy, among individuals (Devillez 1979a). Barclay & Crawford (1984) reported that seedling emergence during an 18-week stratification period occurred only (and less than 10% germination) in the three low altitude samples (means of 8, 102 and 402 m). They also mentioned that the altitude of seed source affected the length of cold stratification required to break dormancy, seeds from lower altitudes needing longer treatments. Cold stratification of seeds within fruits induces a secondary dormancy which makes the treatment ineffective (Devillez 1979a); cold stratification (2 °C) seems the only way to break simultaneously the dormancy imposed by the seed coat and the embryo dormancy (Devillez 1979c). Partially after-ripened seeds exposed to higher temperatures also go into secondary dormancy.

Warm stratification (10, 20, 30 °C, or 12 h-12 h cycles at 30–10 °C) can also break the embryo dormancy (Devillez 1979c). Warm stratification might even be more effective than cold stratification: germination capacity of excised embryos was 10% after 2 days' warm stratification and more than 45% after 30 days at 30 °C (Devillez 1979c). However, it remains unclear whether seed coat dormancy can be broken by warm stratification. The embryo dormancy may thus be broken both by warm or cold stratification, and the highest germination rates (100%) may be obtained by a warm stratification of 16–45 days (at 30 °C or 30–10 °C, 12 h-12 h), followed by cold stratification (2 °C) for 6 months (Anonymous 1963; Devillez 1979c). Germination rate and speed decreased when a warm stratification longer than 45 days was applied (Devillez 1979c; Table 3).

The removal of the seed coat reduces the length of low temperature treatment necessary for seed germination (Table 3) although excised embryos still require a short period of treatment (Flemion 1931; Devillez et al. 1980; O. Raspé, unpublished).

Seeds stored dry for 6 months require only 60–80 days to break dormancy (Anonymous 1963). Dry storage (20 °C) has a small positive effect: it slightly increases the germination of isolated embryos but intensifies the dormancy imposed by the seed coat (Devillez 1979b). Cleaned seeds or intact berries have been stored at various temperatures from − 8.3 °C to 21.1 °C for 2 years without significant loss in viability. Higher temperatures were injurious (Anonymous 1963). Relative humidities much below or above 25% are unfavourable to long storage at the higher temperatures; ordinary room conditions are usually satisfactory. Storage in sealed containers or under vacuum showed no advantages. Seed may be stored over winter in outdoor stratification pits.

Kinetin (10 mg L−1), gibberellic acid (100 mg L−1) and thiourea (7.5 g L−1) stimulate the germination of isolated embryos, but are ineffective when perisperm is intact, even if the testa has been removed (Frankland 1961; Nikolaeva et al. 1987). Oster et al. (1987) studied the inhibitors of germination in the fruits and seeds. Three different inhibitors were found: parasorbic acid, abscisic acid and isopropylmalic acid. The most important was parasorbic acid (4–7 and 0.08–0.12 mg g−1 fresh weight in fruits and seeds, respectively).

(e) seedling morphology

Seedling development is shown in Fig. 3. Germination is epigeal. In the field, it is largely complete in the second spring after sowing (Anonymous 1963). The hypocotyl is 2–4 cm long, glabrous and woody (Muller 1978; O. Raspé, unpublished). The two cotyledons have a glabrous petiole of 1–2 mm; they are elliptically oblong, their base is cuneate, their length reaches 5.5–8.5 mm, they are herbaceous, glabrous and with a rounded tip. The epicotyl is 2–7 mm, with straight unicellular hairs, slightly woody. The leaves are alternate, with a petiole 2–5 mm, more or less hairy, with linear gland-tipped stipules, ovate, pinnatipartite-pinnate; their base is more or less truncate, and they are 1–1.5 cm long, herbaceous, glabrous or with few unicellular hairs. The leaf segments are serrate to lobed; the tip of the lobes is acute (Muller 1978).

Figure 3.

Stages in the germination of Sorbus aucuparia: (a) 2 days, (b) 8 days, (c) 3 weeks, and (d) 7 weeks.

(f) effective reproduction

Sorbus aucuparia regenerates entirely by seeds, which germinate in the first or second springs (Grime et al. 1988). In vitro propagation can be achieved by means of softwood cuttings and organ cultures on MS medium containing cytokinin and auxin (Chalupa 1987, 1988; Hansen 1990). Jörgensen & Binding (1984) developed a method for callus regeneration from protoplasts (isolated from shoot tips).

IX. Herbivory and disease

(a) animal feeders or parasites

(i) Mammalia and Aves

Seedlings are frequently nipped by deer in USA (Anonymous 1963). Moose selected S. aucuparia in winter in northern Sweden (Lundberg et al. 1990; Andren & Angelstam 1993; Shipley et al. 1998). In south-eastern Norway, moose diet also contained rowan in summer (Hjeljord & Histøl 1999). Red deer chose S. aucuparia in summer in the Scottish Highlands (Miller et al. 1982). In the French Jura, Jacob (1988) has reported that hazel grouse (Bonasa bonasia) diet was dominated by S. aucuparia buds from January to April.

In northern Spain, pine martens (Martes martes) eat fruits as much as 9.4% of their diet; these fruits are of S. aucuparia, Rubus spp. and Vaccinium myrtillus (Guitián & Bermejo 1989). In Finland, berries are also eaten by herbivorous and carnivorous mammals; arctic hares (Lepus timidus) have been observed eating them as well as the semidomestic reindeer (Rangifer t. tarandus), the fox (Vulpes vulpes), pine martens (Martes martes) and the racoon dog (Nyctereutes procyonoides) or other mammals (Pulliainen 1978).

Blackbirds (Turdus merula) are the main dispersers of S. aucuparia fruits in the British Isles, accounting for 78% of the total records. Song (Turdus philomelos) and Mistle (Turdus viscivorus) thrushes as well as starlings (Sturnus vulgaris) take the fruit comparatively little. Robins (Erithacus rubecula) usually sally to pluck a fruit, then take it to a perch before swallowing it (Snow & Snow 1988). Snow & Snow (1988) also recorded blackcap (Sylvia atricapilla), garden warbler (Sylvia borin), lesser whitethroat (Sylvia curruca), magpie (Pica pica) and jay (Garrulus glandarius). In Finland, Pulliainen (1978) reported various ‘generalist’ feeders: Bombycilla garrulus, Carduelis carduelis, C. chloris, C. flammea, C. monedula, C. spinus, Coccothraustes coccothraustes, Columba livia domestica, Corvus cornix, Dryocopus martius, Erithacus rubecula, Fringilla coelebs, F. montifringilla, Nucifraga cryocatactes, Parus caeruleus, P. major, P. montanus, Passer domesticus, Perisoreus infaustus, Pica pica, Picus canus, Sturnus vulgaris, Turdus iliacus, T. merula, T. philomelos, T. pilaris, T. torquatus, and T. viscivorus.

Rowan is well known to be one of the most important bird fruits in northern Europe, as indicated by its German name vogelbeer (bird-berry) as well as the French sorbier des oiseleurs (bird-catcher sorb). The size of the rowan crop in Fennoscandia affects the migration of fieldfares (Turdus pilaris) and the eruptive movements of waxwings (Bombycilla garrulus). In years when there is a good crop, part of the fieldfare population remains in Finland after the normal time of migration (Tyrväinen 1975). Similarly, waxwings stay in the north all winter when S. aucuparia fruits are available (Siivonen 1941), and these fruits are much the most important food of waxwings wintering in Germany (Schüz 1933) and in Poland (Harmata 1987). In a highland glen in Inverness, northern Scotland, Swann (1983) has found that redwings (Turdus iliacus) pass through rapidly on autumn migration, with the peak passage usually 10–24 October. Individual birds remain for less than 24 h, during which they feed mainly on S. aucuparia.

Bullfinches (Pyrrhula pyrrhula) and sometimes blue tits (Parus caeruleus) are seed-predators of S. aucuparia, the number of records exceeding that for all of the dispersers except blackbird (Snow & Snow 1988). Some trees have a large fraction of their total seed crop destroyed by bullfinches. In Finland as well as in Spain, bullfinches also eat the fruits during winter (Pulliainen 1978; Guitián 1985). Also, Loxia curvirostra, L. leucoptera, L. pytyopsittacus and Pinicola enucleator extract the seeds and discard the other parts (Pulliainen 1978). These specialist birds are adapted to feeding on the seeds of S. aucuparia which improves the birds' chances of overwintering successfully in northern Finland (Pulliainen 1978).

There are also many references to S. aucuparia being eaten by birds in central Europe (Blaschke 1976). In Germany, starling (Sturnus vulgaris) was the most important species, next was waxwing, then blackbird, fieldfare and redwing. Six species were irregularly or rarely recorded, four or five of them presumably as seed-predators: crossbill (Loxia curvirostra), bullfinch (Pyrrhula pyrrhula), greenfinch (Carduelis chloris), brambling (Fringilla montifringilla), greater spotted woodpecker (Picoides major) and blue tit (Parus caeruleus).

(ii) Insecta

Sorbus aucuparia supports a relatively species-poor insect fauna (Wratten et al. 1981). The tree appears to possess a mechanism of inducible defence against foliar predation by insects (Edwards & Wratten 1985; see VI f). Delayed inducible resistance was also experimentally demonstrated after artificial defoliation or defoliation by Epirrita autumnata (Lepidoptera: Geometridae) (Neuvonen et al. 1987).

Coleoptera

Byturidae. In May and June, some inflorescences or even whole trees may be infested by Byturus fumatus (Fab.), which feed on    flower buds or, later, on flower parts—the styles and stamens were frequently completely eaten (Raspé 1998).

Hemiptera

Cicadellidae. Edwardsania crataegi (Douglas) and E. rosae (L.) (Alford 1991).

Aphididae. Blackman & Eastop (1994) reported the following species: Aphis spiraecola, Brachycaudus helichrysi,    Dysaphis sorbi, Eriosoma sorbiradicis, Macrosiphum pyrifoliae, Muscaphis drepanosiphoides (including ssp. Irae), Myzus   ornatus, Nearctaphis californica, Ovatus insitus and Rhopalosiphum insertum. Alford (1991) also mentions Aphispomi Degeer.

Lepidoptera (Carter & Hargreaves 1986; Alford 1991)

Incurvariidae. Incurvaria pectinea Haworth.

Lyonetiidae. Lyonetia clerkella (L.).

Gracillariidae. Phyllonorycter corylifoliella (Hübner).

Sesiidae. Caterpillars of Synanthedon myopaeformis Borhausen eat the leaves.

Yponomeutidae. Sorbus aucuparia is the primary host of Argyresthia conjugella Zell. This fruit moth is also known to infest    apple fruits, but if a moth has the opportunity to choose between fruits of S. aucuparia and Malus spp., it will consistently    choose the former (Ahlberg 1927). Sperens (1997b) has shown that in Sweden the number of fruits produced in a population    determined the number of A. conjugella individuals in the same population. A. sorbiella (Treitschke) is strictly linked to S.   aucuparia (Alford 1991). A race of Yponomeuta padellus (L.) infesting S. aucuparia has a local distribution (Alford 1991;    Kooi et al. 1991).

Tortricidae. Pandemis cerasana (Hübner), Acleris sparsana (Denis & Schiffermüller).

Geometridae. Caterpillars of Venusia cambrica Curtis feed rapidly each August on the leaves. Acasis viretata Hübner is also    found in Britain. In northern Sweden, larvae of Epirrita autumnata feed on the leaves (Neuvonen et al. 1987). Caterpillars of    Operopthera brumata L. can cause severe defoliation in Fennoscandia (Tikkanen et al. 1998).

Arctiidae. Arctia caja (L.), Callimorpha dominula (L.).

Drepanidae. Caterpillars of Cilix glaucata Scopoli eat the leaves.

Hymenoptera (Alford 1991)

Cecidomyiidae. Contarinia sorbi (Kieffer) causes light thickening and folding of leaflets (Lawalrée 1958).

Tenthredinidae. Dineura testaceipes (Klug) and Pristiphora geniculata (Hartig) are strictly linked to S. aucuparia. Caliroa cerasi   (L.), Priophorus morio (Lepeletier) and Croseus septentrionalis (L.) may also be found.

The seeds are subject to attack by several species of chalcid flies (Anonymous 1963).

(iii) Acari

Phyllocoptes goniothorax sorbeus (Nalepa) induces the production of hypertrophied hairs (Alford 1991). Aculus aucuparia Liro may severely damage the leaves. Eriophyes pyri sorbi (Nalepa) commonly causes galls on the leaves (Alford 1991). In their study of phytoseiid mites (Phytoseiidae) in apple orchards and surrounding vegetation in S. Finland, Tuovinen & Rokx (1991) reported the following species on S. aucuparia: Phytoseius macropilis (Banks), Paraseiulus triporus (Chant & Shaul), Anthoseius rhenanus (Oudemans) and Typhlodromus richteri Karg. The most abundant species was P. macropilis, representing c. 45% of all phytoseiid mites found on S. aucuparia.

(iv) Others

In experimental conditions, leaves show a high palatability to the snail Helix aspersa Muller (Edwards & Wratten 1985).

(b) plant parasites

(i) Spermatophyta

Sorbus aucuparia has been reported as an host to Viscum album L. (Barney et al. 1998).

(ii) Ascomycotina

Names of corresponding anamorphs (Deuteromycotina) also found on S. aucuparia are given in square brackets. Ellis & Ellis (1997) recorded the following discomycetes: Dermea ariae (Pers.) Tul. ex P. Karsten [Foveostroma sp.], Hyaloscypha quercina Velen. var. resinacea Dennis, Pezizellaster serratus (Hoffm.) Dennis, and Tympanis conspersa (Fr.) Fr. (also recorded by Dennis 1981 and Muskett & Malone 1983). Other discomycetes found on rowan include Dasyscyphus cerinus (Pers.:Fr.) Fuckel and Mollisia melaleuca (Fr.) Sacc. (Kirk & Spooner 1984); Lophodermium aucupariae (Schleich.) Darker (Holm & Nannfeldt 1992); Tapesia cinerella Rehm and T. fusca (Pers.) Fuckel (Aebi 1972); Tympanis alnea (Pers.:Fr.) Fr. (Ouellette & Pirozynski 1974).

Dennis (1981) and Ellis & Ellis (1997) recorded the following pyrenomycetes: Coronophora gregaria (Lib.) Fuckel; Diaporthe impulsa (Cooke & Peck) Sacc. [Phomopsis sp.]; Dothiora pyrenophora (Fr.) Fr. [Dothichiza state causes canker of small branches]; Eutypella sorbi (Alb. & Schw.) Sacc. [Cytospora sp.] and Leucostoma persoonii Höhnel [Cytospora leucostoma Sacc. (Truszkowska & Chlebicki 1983)]. Other pyrenomycetes found on S. aucuparia include Nummulariella marginata (Fr.) Eckblad & Grammo (=Biscogniauxia marginata (Fr.) Pouzar) and B. repanda (Farr et al. 1989; Ju et al. 1998); Botryosphaeria obtusa (Schwein.) Shoemaker (Farr et al. 1989); Hypoxylon multiforme (Fr.:Fr.) Fr. (Scheuer & Chlebicki 1997); Rosellinia subsimilis Sacc. (Petrini 1992), Valsa ambiens (Pers.:Fr.) Fr. (Truszkowska & Chlebicki 1983); Venturia aucupariae (Lasch ex Sacc.) Rostrup ex Lind. (Foister 1961) and V. inaequalis (Cooke) G. Wint. (Walton & Rich 1974; Sivanesan 1977; Farr et al. 1989). Nectria ditissima Tul. & C. Tul. and its anamorph Cylindrocarpon willkommii (Lindau) Wollenw. have been found on twigs but also on fruits (J.R. De Sloover & D. Brayford, pers. comm.).

Podosphaera aucupariae Eriksson (a powdery mildew) can be found mostly on young branches (Ellis & Ellis 1997). P. tridactyla (Wallr.) de Bary was recorded in the United States (Farr et al. 1989).

Scheuer & Chlebicki (1997) recorded two pyrenomycetes on S. aucuparia var. glabrata: Diaporthe impulsa (Cooke & Peck) Sacc. (mentioned above) and Rosellinia aquila (Fr.:Fr.) de Not.

(iii) Basidiomycotina

Exidia thuretiana (Lév.) Fr. (Tremellales) was recorded from England (Reid 1970). Basidioradulum radula (Fr.:Fr.) Nobles was found in Scotland (Kirk & Spooner 1984). In their study of the genus Polyporus in Finland, Niemelea & Kotiranta (1991) recorded the following species on S. aucuparia: Polyporus brumalis Fr., P. ciliatus Fr., P. leptocephalus (Jacq.:Fr.) Fr., P. melanopus (Pers.) Fr., P. squamosus (Huds.:Fr.) Fr. and P. tuberaster (Pers.:Fr.) Fr. According to these authors, P. squamosus Fr. is the only polypore species to cause significant harm as a pathogen. In Belgium and Luxembourg, S. aucuparia is the most common host, after Betula spp. and Fagus sylvatica, to Pycnoporus cinnabarinus (Jacq.:Fr.) P. Karst., a polypore which has shown a spectacular range expansion since 1960 (Thoen et al. 1998). Foister (1961) recorded Heterobasidion annosum (Fr.:Fr.) Bref. and Armillaria mellea (Vahl: Fr.) Kummer on S. aucuparia in Scotland. Spermagonia and aecia of the rust Gymnosporangium clavariaeforme (Jacq.) D.C. can be found on the leaves (Ellis & Ellis 1997; Wennström & Eriksson 1997). Wennström & Eriksson (1997) have shown that the fungal mycelia do not survive in the buds during winter. Infection results only from spore transmission in spring, from diseased Juniperus communis shrubs. Farr et al. (1989) also reported rowan to be an aecial host to G. libocedri (Henn.) F. Kern. Uredinia of Ochropsora ariae (Fuckel) Ramsb. can be found on the leaves (Ellis & Ellis 1997).

(iv) Deuteromycotina

In addition to the Deuteromycetes given above, Ellis & Ellis (1997) recorded the following species: Corynespora cambrensis M.B. Ellis; Rhabdospora inaequalis (Sacc. & Roum.) Sacc.; Septoria sorbi Lasch and Septosporium bulbotrichum Corda, a rare species found in Scotland on inner bark (see also Kirk 1986). Other Deuteromycetes found on S. aucuparia include Cytospora rubescens Fr. (Farr et al. 1989); Dactylaria candidula (de Hoog 1985); Dothiorella pyrenophora (P. Karst.) Sacc.; Entomosporium mespili (DC.) Sacc. (Farr et al. 1989); Micropera cotoneastri Sacc. (Dennis 1986); Neta compacta G.S. de Hoog (de Hoog 1985); Phyllosticta sorbi Westend. (Foister 1961; Farr et al. 1989); P. sorbicola (Rab.) All. (Dennis 1986).

(c) plant diseases

Ringspot and variegation are very common on the leaves (Polák & Zieglerová 1996). In some cases the causal agents were identified as the apple chlorotic leafspot and the apple mosaic viruses, or both (Polák & Zieglerová 1996). Führling & Büttner (1998) reported chlorotic ringspots and mottling to be caused by pathogens belonging to the topso and tobamo virus group. The bacterium Erwinia amylovora, which causes fire blight on a variety of rosaceous species, can also develop on S. aucuparia (Zeller 1979; Schaefer et al. 1983; van der Zwet 1995). See also IX (b) above.

X. History

According to Jankun (1993), the primary centre of origin and differentiation of the genus Sorbus is Southeast Asia. It was from this region that migration of Sorbus species towards Europe and North America began. Secondary centres have developed in the Himalayas (especially of subgenus Aria), the Caucasus and Armenia (giving rise to numerous hybridogenous taxa) and Europe, where there has been much differentiation through hybridization and the stabilization of new forms by apomixis.

Boyd & Dickson (1987b) found S. aucuparia pollen in a sequence of lake muds and fen peat deposits from Loch a'Mhuilinn, North Arran, and Fossitt (1996) found S. aucuparia pollen in lake sediments dating from the early Holocene, in the Western Isles, Scotland. By around 6500 BP, S. aucuparia was present in the area. Regnell et al. (1995) report human use of S. aucuparia during the first settlement phase in southern Sweden, at c. 6650–6400 BP.

In France, Germany and Austria, fruits are sometimes distilled (Hegi Fl. 4, ed. 2; ‘kirsch’, Rameau et al. 1989). In Poland, the fruits are used to flavour vodka. Leaves are purgative and expectorant. Fruits are laxative, astringent, diuretic and antiscorbutic (Rameau et al. 1989) and antilipoperoxidant (Costantino et al. 1994). The strong and close-grained wood is sometimes made into tool handles (Anonymous 1963; Rameau et al. 1989).

Acknowledgements

Our thanks are due to Professor R. Iserentant for his comments on the first draft and for translations from German, to Mrs D. Champluvier for providing references on phytosociology and distribution, to Professor J.R. De Sloover for reviewing the fungal data, to Mrs M. Evrard for drawing Fig. 3, to Mrs J.M. Croft for preparing Fig. 1, and to Dr M.C.F. Proctor for writing most of the section II(A). We also warmly thank Arthur J. Willis for his patience and precious help. O. Raspé is ‘Chargé de Recherches’ and A.-L. Jacquemart is ‘Chercheur Qualifié’ of the Belgian National Fund for Scientific Research.

Ancillary