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Vaccinium oxycoccos L., Cranberry

Account Summary

Native, occasional to frequent, but rather local. Circumpolar boreal-montane.

1902; Abraham, J.T. & McCullagh, F.R.; Feddan Bog.

April to January.

Growth form and preferred habitats

A tiny loose, spreading subshrub whose wiry stems need to be actively searched for, often on Sphagnum tussocks around the margins of pools in the wettest parts of raised bog surfaces. V. oxycoccos appears to need strongly acidic, wet growing conditions, but it cannot tolerate constant immersion. Thus it colonises wet bog hollows, only after they have already begun to fill with Sphagnum moss, principally the common yellow-green species, S. cuspidatum (Lusby et al. 1996).

Fermanagh occurrence

V. oxycoccos and Andromeda polifolia (Bog-rosemary) are the two dwarf ericaceous shrubs which best manage to colonise the transition zone between hummock and hollow on acid peat bog surfaces. However, while Cranberry is quite frequent, though easily overlooked because of its small size and inconspicuous growth form, the more upright Bog-rosemary is a much rarer and declining species, at least in NI (FNEI 3). In the whole of Fermanagh, there is just one relatively small, recently discovered Andromeda polifolia population on a lowland Fermanagh bog, but V. oxycoccos has been recorded over 130 times in the VC, in a total of 48 tetrads. This presence represents Cranberry in 9.1% of Fermanagh tetrads and 40 of them contain post-1975 records. As the distribution map indicates, Cranberry is thinly and quite widely scattered in boglands across the county.

Bog plant leaf adaption to nutrient starvation

The relatively long, slender, wiry stems of Cranberry allow it to keep pace with the rapid growth of the Sphagnum moss on which it sits around bog pools. It bears small, hard, evergreen leaves (ie sclerophylls) which are covered with wax. Like Calluna vulgaris (Heather), Erica spp. and other genera of ericaceous subshrubs, Cranberry is apparently drought resistant (ie the sclerophylls show 'xeromorphic' adaptation). However, these types of leathery, reduced leaves are now regarded in a wider ecological context than desert conditions and they are understood to be a reflection of nutrient starvation, an adaptive structural feature called 'peinomorphism'. This term meaning, 'a hunger or starvation induced form', was introduced by Weissenböck (1969) in connection with salt-tolerant halophytes, but it is becoming more widely applied to all nutrient-starved sclerophylls (Seddon 1974; Ellenberg 1988; Lusby et al. 1996).

Raised bogs by their nature and development are isolated from underlying mineral soils by a layer of fen or sedge peat. Thus plants growing on them receive only an extremely limited supply of mineral nutrients from blown dust and materials brought down onto their surface by rain. There is strong evidence from numerous very different ecological situations (including Tropical Rain Forest in Australia) suggesting that the apparent xeromorphic anatomy of this and many other bog plants, and especially the sclerophylly of their leaves, reflects tolerance of very low levels of available phosphate (Müller-Stoll 1947; Loveless 1961, 1962; Beadle 1966; Small 1972).

The small, hard, thick-cuticled, waxy, evergreen leaves of V. oxycoccus have down-rolled margins and their pores (stomata) are confined to the lower surfaces. The leaves also persist for two years, so that altogether, the species provides a classic example of the extremely stress-tolerant, highly conservative strategy of acquisition and use of mineral nutrients which typifies peinomorphic sclerophylly.

Root system

V. oxycoccus has a very shallow fibrous root system, hardly penetrating more than a few centimetres below the living parts of the moss layer on which it grows, so that it must have to compete with the Sphagnum for scarce nutrients. Like other ericaceous bog subshrubs, all species of Vaccinium produce dense root systems which end in fine absorbing rootlets, termed 'hair roots', ie rather than the normal, but anatomically different, 'root hairs'. Hair roots merely consist of one to three layers of cortex cells around a narrow central conducting stele (Pearson & Read 1973). Because of its very shallow root system, V. oxycoccus relies on the excellent water-conducting and retaining capacity of the Sphagnum mosses it grows over for its water supply (Malmer et al. 1994). Thus the growth of Cranberry can be limited by either the prevailing chronic shortage of mineral nutrients (especially nitrogen and phosphate), or by a lowered or fluctuating water table. Surface drying of the bog is very deleterious to V. oxycoccus and it has been shown that growth of the subshrub is best when the average groundwater level is 25-30 cm below the root-stem junction (Gronskis & Snickovskis 1989).

Flowering reproduction

The tiny, pink, waxy flowers resemble a miniature Turk's-cap Lily or a Cyclamen sp. and have a similar pollen release mechanism to the latter (visited by bumblebees). The open flowers are actually slightly less conspicuous than the flower buds, which display a more vivid crimson colour when still closed. Flowering begins in June and extends into August, the individual flower being unusually long-lived, an average of 27 days. Fruit development is very slow, taking between two and three months after pollination (Jacquemart 1997).

Occasionally, V. oxycoccus can be found fruiting in great abundance and the bright red, or red speckled with brown spots, very sharp-tasting berries, which slowly ripen over the winter period, are either transported locally by streams, or are eaten and dispersed in the spring by birds such as robins, thrushes, blackbirds and red grouse and probably also by mammals. The most important Cranberry collecting mammal in this part of the world is possibly the fox, but voles and mice may also feed on them if they can negotiate the wetter parts of the bog (Jacquemart 1997; Mundell & Povey 2002). The number of seeds per berry averages 7-8 and the estimated annual seed production per m2 is regarded as low, at around 480 ± 510 (Vander Kloet & Hill 1994).

Vegetative reproduction

Studies of successful recruitment from Cranberry seed in several parts of the Northern hemisphere strongly indicate that this type of plant establishment must be a rare, or even an extremely rare event. The dominance of vegetative over sexual reproduction is very prevalent in this genus, as it is in other ericaceous subshrubs, including most common Erica spp. (Jacquemart 1997).

British and Irish occurrence

Widespread in suitable bogland in Britain NW of a line between Cardiff and Hull, north into Scotland south of the Great Glen and with a few thinly and widely scattered sites beyond these limits. In Ireland, it is widespread but has declined in the Midland bogs due to very extensive, industrial peat extraction and in the W due to drainage and afforestation (M.C.F. Proctor, in: Preston et al. 2002).

European and world occurrence

V. oxycoccus is widespread in N & C Europe extending locally to south-central France, N Italy and SE Russia. It then extends through northern Asia to Japan and N America. It is circumpolar in the Boreal region of the N Hemisphere although there are distribution gaps in N Atlantic and Pacific areas (Hultén & Fries 1986, Map 1458; Sell & Murrell 2014).

Threats

Peat cutting and forestry and associated drainage operations still pose real threats to many Irish populations of Cranberry, including some in Fermanagh.