Sorbus aucuparia L.

A very variable sexual diploid (2n=34) species. By far the most widely distributed species in the whole genus in the interpretation adopted in this work. Since isolated trees fruit heavily and yield viable seed, and yellow fruited cultivars tend to yield yellow fruited offspring, it might be deduced that most individuals are self compatible. However, the excellent summary of all aspects of the biology and ecology of S. aucuparia provided in the UK ‘Biological Flora’ account (Raspé et al., 2000) describes conflicting results in different studies while emphasising the better fruit and seed set with outbreeding as expected for most trees (Petit & Hampe, 2006).

As interpreted here, Sorbus aucuparia occurs from Madeira, North Africa and Iceland in the west across Europe, Central and Northern Asia to North China and Kamchatka, but does not reach Japan or other offshore islands in East Asia. It probably only overlaps with other species of the genus in Northern China where S. discolor also grows, the hybrid between them being S. × pekinensis.

Although it is very variable, S. aucuparia can be distinguished from all other species of the genus by its blackish, ovoid, non-sticky buds with usually predominantly whitish hairs, rather untidy, non-leathery foliage, leaflets papillose beneath (x 70), and orange-red fruits with densely white-tomentose, free carpel apices forming a conical protuberance within the calyx.

In the mountains of Central Europe and in North Europe the more glabrous trees are named ssp. glabrata (=S. gorodkovii). At least in the north these trees are probably no more than a transition to the even more glabrous trees farther east in Siberia, often referred to S. sibirica, which also tend to have more delicate, more finely pointed and more deeply toothed leaflets and more conical, less hairy buds. This type of tree extends south into Mongolia and North China. It seems likely that these rowans are more or less continuously distributed from Europe across Asia and may well be continuous with populations of smaller trees on the North Eastern coast of Asia (Hulten 1971) which have been referred to S. amurensis, S. anadyrensis and S. kamschatcensis, and larger trees with very white-hairy hairy buds from North China which have been referred to S. pohuashanensis. An examination of type specimens of these species and of other specimens from the Amur region, Anadyria and Kamtschatka shows them to have papillose leaflets and to be almost indistinguishable, at least on the herbarium sheet, from European S. aucuparia. No character or character combination could be found which could distinguish these taxa from one another or from S. aucuparia. Taken together these taxa seem to form a single large variable population throughout Europe, North Africa and the Northern parts of Asia south to the Altai Mountains and are perhaps best treated as belonging to the single species S. aucuparia with the more distinct variants being recognized as subspecies.

It is interesting that this is the only geographically extensive, more or less continuously distributed species within the genus Sorbus, and that it is diploid and sexual. All other species are much more restricted in geographical distribution.

S. pohuashanensis of horticulture is an attractive tree characterised by large trusses of fruit, large, white-hairy buds and large broad leaflets, quite unlike the narrow, deeply cut leaflets of the trees from the Lake Baikal region of Siberia. Cultivated plants raised from seed collected in the wild in north-east China have the broad leaflets and white-hairy buds characteristic of the trees already in cultivation, so it seems quite possible that the trees in cultivation derive directly from Chinese seed and are not of hybrid origin as has been suggested (Clarke 1980.). Hybrids between S. aucuparia and S. esserteauiana, which are superficially similar to S. ‘pohuashanensis,’ can be distinguished from any of the variants of S. aucuparia by the more leathery texture of their leaflets and larger and more persistent stipules, stouter stems, shorter, fatter buds and smaller, harder, broader fruits which ripen late, all characters inherited from the S. esserteauiana parent.

As S. aucuparia is a common European tree there is much literature on its ecology (Grime et al. 1988, Raspé et al. 2000). It hardly ever forms pure woodlands as do species of other northern genera which grow with it (eg. Betula, Pinus, Salix, Alnus, Quercus, Fraxinus, and the conifer genera Pinus, Picea, and Larix.). Instead, S. aucuparia tends to occur scattered through woodland, often in clearings, and above the treeline, but is also frequent as suppressed small plants in denser woodland. It is found mainly on lighter soils in cool, moist climates. Its prolific production of bird dispersed fruit enables it to colonise suitable habitats, especially dwarf shrub heaths, cliffs, and disturbed ground in woodlands, over a very wide area. Its colonising ability make it a weed tree of forestry on any soil type within its distribution area. In woodlands on clay soil it roots primarily in the surface humus and is therefore unstable in high winds, which is in marked contrast to its usual extreme tolerance to exposure in mountain cliff habitats. The distribution of Pinus sylvestris is almost identical to that of the common rowan S. aucuparia and also to that of Prunus padus (bird cherry) and Populus tremula (aspen).

Though it can grow quickly in favourable conditions when young, it is slower growing than species of the above genera when mature and rarely grows as large, perhaps explaining why it is a poor competitor (Raspé et al. 2000). Also, unlike species of all these other genera, it produces fleshy fruit which probably accounts for a significant proportion of a year’s photosynthetic production which cannot therefore be put into extension growth.

Seedlings are quite shade tolerant, often existing in large numbers in the ground layer of woodland, growing very slowly and only producing a rosette of leaves each year. In an experiment under low light conditions Betula and Quercus seedlings showed hundred per cent mortality where S. aucuparia had zero mortality (Pigott 1983). When the death of a canopy tree allows more light to reach the ground layer such seedlings grow rapidly to fill the gap. In this characteristic S. aucuparia is similar to Fraxinus excelsior in depending on the presence of suppressed seedlings to take advantage of gaps which appear in the canopy in closed canopy woodland (Wardle 1959) rather than depending on subsequent seed dispersal to the site.

In woodland rowans often grow as single-stemmed trees but in more open situations they frequently have several trunks. Individual trunks seem to have a lifespan of about 40 years in cultivation, but probably more in the wild in harsh conditions where growth is slower. There is, however, a record of a rowan reaching 28 m and being probably almost 100 years old (Young 1996). Old trunks often become infected with various fungi such as Pleurotus spp., Piptoporus squamosus, etc. and deteriorate gradually over several years and eventually die. Especially in open situations, such moribund trunks usually produce stool shoots from their bases and these produce a second generation of trunks. This process can be repeated indefinitely so that the lifespan of an individual could be many hundreds, if not thousands, of years. Though sucker shoots usually arise from the base of a trunk, in rare cases they may arise from roots some distance from the base of the trunk (personal observation of a shoot arising one metre from the base of a trunk in Prince’s Park, Eldwick, near Bingley, West Yorkshire in the 1980s) and up to five metres (Kullman 1986) and to 6.8 metres in Savernake forest, Dr. J.E. Oliver (pers. comm., 2006).

Although S. aucuparia is particularly characteristic of light, acid, sandy and peaty soils it is also frequently found on shallow soils over limestone and on limestone cliffs. A reciprocal transplant experiment using seed from trees on adjacent acidic and calcareous soils near Loggerheads in Clwyd, North Wales failed to show any ecotypic differentiation, seedlings from the trees on the acid soil growing as well on calcareous soil as on acid soil and vice versa (Gillham 1980). Like many pioneer species, S. aucuparia is thus tolerant of a wide range of soil pH. It is listed as an invasive alien in New Zealand and Patagonia, Canada, 27 northern states and Alaska in the US, On the Heights of Abraham just outside Quebec city the rowans are S. aucuparia, though the native S. decora occurs near the summits of Mont Royal in Montreal and Mt. St. Anne de Beaupré east of Quebec.

S. aucuparia is an attractive small tree and by far the most commonly planted species of the genus in Britain. It is cheaply raised from seed while almost all other species are grafted, usually onto stocks of S. aucuparia. It is seen at its best in native habitats in rocky mountain woods and on crags in northern Britain where we can perhaps understand Forrest’s comment that “In all my years in Yunnan I haven’t seen a member of the group to equal our own mountain ash when well grown” (quoted by Clarke 1980). It is in these situations that its golden autumn foliage colour is seen at its best and the fruits may hang on the bare branches till after Christmas. When it is cultivated in drier climates the autumn leaf colouration is rarely good and, particularly on heavy soils, the foliage may never look attractive due to marginal browning of the leaflets in summer. Even in the moist climate of the Isle of Man the leaves become a very unattractive grey-brown in autumn, never developing the attractive autumn colouration seen further north. However, it usually fruits well throughout Britain, and along roadsides and away from cover the fruit may remain on the trees for some time.

Many cultivars have been named (Hillier 1972, Clarke 1980) of which the yellow fruited ones are the least attractive to birds. As mentioned above, seed from yellow fruited trees gives rise to a high proportion of yellow fruited offspring so there are probably several, if not many, yellow fruited clones in cultivation.

Plants of European and Madeiran origin thrive in cultivation in Britain, though those from Northern Scandinavia are initially very slow growing. Specimens originating from continental North and East Asia (Siberia, Kamtschatka, Mongolia) break bud very early and are therefore susceptible to injury by severe spring frosts. They are slower growing in cultivation in Britain than European origins.

BEAN:

A deciduous tree, 30 to 60 ft high, of erect growth when young, becoming more spreading and graceful with age; trunk smooth and grey; branchlets downy when young, becoming glabrous later; terminal bud very downy throughout the winter, not gummy. Leaves pinnate, 5 to 9 in. long, with mostly six or seven pairs of leaflets, which are narrowly ovate-oblong, 1 to 21⁄2 in. long, smallest towards the apex, pointed, sharply toothed, downy beneath when young, becoming almost or quite glabrous by the autumn. Flowers white, unpleasantly scented, 1⁄3 in. across, produced very numerously in terminal, flattish corymbs 3 to 5 in. across; calyx and flower-stalks clothed with grey wool. Fruits in large showy clusters, 1⁄4 in. to 3⁄8 in. across, round or slightly oval, bright red.

The mountain ash is widely spread over the cool, temperate parts of Europe and Asia, and is abundant in most parts of the British Isles. It is one of the most beautiful of our native trees alike in leaf, flower, and fruit. Its beauty no doubt is greatest when the branches are laden with the large nodding clusters of ripe fruits in September, but where bird life is abundant that beauty soon passes. Of neat habit and never of large size, it is a useful tree in small gardens, for which, however, some of the varieties mentioned below might be selected, leaving the typical mountain ash for the larger spaces and woodland. It is easily raised from seed, and grows quickly when young. On this account young trees are much used as stocks for grafting varieties of this and allied species on. It likes a cool, moist situation and is by nature adapted to a growing season shorter than that of southern England, ripening its fruits well before autumn has set in and dropping its blackened leaves before the foliage of most other trees have started to turn colour. Where the winters are longer the leaves persist into autumn and colour yellow or red. It is said that in Scotland the colour depends on the soil (Dr H. Tod, Journ. R.H.S., Vol. 78 (1953), p. 105).