Populus tremula L., Aspen
Account Summary
Native, occasional, but also deliberately planted. Eurasian boreo-temperate.
1882; Barrington, R.M.; Lower Lough Erne.
April to February.
The fluttering leaves of this potentially large species moving in the slightest breeze make it such an attractive decorative feature that Aspen is often chosen as a specimen tree in demesnes, parks and larger gardens (thus avoiding trouble in urban areas with fluffy windblown seed). It is one of the last trees to break its buds in the spring, usually in late April after the oak (Hatfield 1957). It grows best when positioned near open water features, but must be sited well away from piped drains, which the roots have a tendency to invade and block. Roots and suckers can also affect building foundations by causing subsidence on predominantly clay soils. These organs can travel up to 30 metres, although 90% of damage is caused by trees growing within 20 m (Thomas 2000).
P. tremula is usually found in seasonally wet or damp, low-lying ground near lakes and rivers, occasionally in or on the margins of old or long-established woods or bogs, or in hedgerows where it may sucker along and further spread by self-seeding. Seed is liberated from the capsules of all fertile Poplars before the leaves are fully open, and it must settle on moist earth at once or it quickly perishes. Thus the species does not grow naturally in dry, infertile, stony or sandy soils, or under waterlogged conditions, or in shade (Hatfield 1957). Although this tree is indigenous it is often obviously planted in parks, gardens and along roadsides. On account of the unknown extent of past planting regimes P. tremula is definitely native only on the more remote parts of rocky lakeshores, uninhabited, small, lake islands, in gullies in mountain glens and on cliff faces.
Fermanagh occurrence
Around Fermanagh, Aspen is a frequent and widespread tree, or else a large, suckering shrub, having been recorded in 118 tetrads, 22.4% of those in the VC. P. tremula is the only native poplar species in Fermanagh.
Biology and ecology
Like its close relatives the willows, P. tremula is dioecious, the catkins on both male and female trees being pendulous. Likewise, it is not a long-lived species, perhaps surviving 50 years. Also, it readily forms hybrids with introduced members of the genus (Brendell 1990).
P. tremula is not regarded as being very competitive, especially with regard to light and shade. It is capable of rapid growth given full illumination, but
in deep shade it even fails to sucker let alone flower and fruit (Rackham 1980).
As a native species it primarily behaves as a pioneer colonist of seasonally wet or damp, open ground, which frequently involves unstabilised soil in upland situations, eg among boulders on scree, or as isolated individuals in cliff crevices and on ledges (Brendell 1990; Jonsell et al. 2000). Although Poplars have plumed seed that can travel long distances by wind, most P. tremula expansion occurs by vigorous suckering to form often dense, shrubby thickets (Milner 1992; Thomas 2000). On the other hand, seed and seedling mortality, even on bare soils, is described as very high (Jonsell et al. 2000). Some of these seedling losses will be due to an unfavourable physical environment, but predation is also highly significant (Milner 1992). In Fermanagh, apart from occasional trees growing on scarps, there really is no definite current evidence of this colonising ability and P. tremula has not been recorded in the typical habitats available for invasion, eg in old quarries, sand-pits, and damp disturbed waste ground.
British and Irish occurrence
The New Atlas hectad map shows Aspen is extremely widespread throughout Britain, being more frequent in the N & W. It is recorded even on the exposed conditions of the Scottish Isles, including the Outer Hebrides (where it is confined to cliffs) and on Shetland (Pankhurst & Mullin 1991; Preston et al. 2002). However, in Ireland the distribution is very much patchier. It is only really consistently recorded in the north of Ireland, and there principally in VCs W Donegal, Fermanagh, Tyrone and Londonderry (H35, H33, H36 & H40).
European and world occurrence
The world distribution of P. tremula is the widest of any Poplar, stretching from Iceland through Europe (except the S Iberian peninsula) to Greece and the Caucasus and across temperate Asia as far as China and Japan (Jalas & Suominen 1976, Map 264). It reaches from montane woodlands in the Mediterranean to the limit of tree growth in the Arctic tundra (Hatfield 1957; Meikle 1984). In Asia, it is largely represented by subsp. davidiana (Dode) Hultén, although the two forms (subsp. tremula & subsp. davidiana) overlap (Hultén & Fries 1986, Map 617; Jonsell et al.2000).
Threats
None.