Invasive Alien Species in Northern Ireland

Acer pseudoplanatus, Sycamore

Acer pseudoplanatus
© Fiona Maitland
Acer pseudoplanatus
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Acer pseudoplanatus L.
Acer pseudoplatanus L. has been grown in Great Britain for many centuries, and is believed to have been introduced to Ireland around the 15th century. It is an imposing, large tree up to 40m tall, with a rounded-pyramidal head and dark green palmate leave. It can probably be seen at its best in northern Britain, where its great stature and spread make it a notable addition to the field borders, mixed woodlands and parklands in, for example, north Yorkshire. It freely flowers and produces keyed seeds which germinate very readily, making it, in suitable environments, a very successful coloniser. It has a pale wood which lends itself to turning, and which was formerly much used for the manufacture of goblets, plates and other domestic objects, as well as staves for butter barrels and other dairy items. The sycamore has produced many varieties showing variations in both colour and form, a large number of which are also represented in gardens and plant collections throughout Ireland.
Tree to 40m tall:
trunk - to 7-8m in girth; bark greyish, peeling in large flakes; branches hairless;
leaves - dark green and hairless above, paler and dull grey-green beneath, with pale brown hairs on the veins or in the vein axils, turning yellow-brown in autumn, often with tar-spot fungus blotches; 16-20cm wide, heart-shaped at base, with five oval lobes which are coarsely toothed; fruiting twigs often bear 3-lobed leaves;
flowers - in large drooping racemes, yellow-green, often flushed bronze-pink, and often branched at base;
fruit - a round seed with 2 keys 2-5cm long, the wings hairless and forming an angle of c. 60°.
Country of origin
Acer pseudoplatanus is a native of Europe and western Asia, but not considered a true native of Great Britain; it is believed to have been introduced to Ireland round the 15th -16th centuries.
Location in Ireland
Having been planted as a parkland tree in many places, and widely as a garden tree, the sycamore can be found almost anywhere, including as shelter-planting on exposed mountain farms and on the coast at the shoreline, exposed to salt spray. When left in woodland and scrub, it will quickly establish thickets of seedlings, for example, in the grounds of Belfast Castle on the slopes of the Cave Hill, where it has become a pest species.
Life cycle
Produces viable seed which germinates very successfully; the especially tenacious root system is established very early in the life of the seedling.
Wildlife and habitat impacts
As is usual with invasive plant species, the sycamore will, in suitable environments, speedily establish thickets of saplings which will progressively outcompete other tree and shrub species with which the parent tree was planted, forming a dense canopy which excludes light from the understorey, and thereby inhibiting the germination of other seeds, including those of native trees. There will therefore be an alteration in the habitat, including the food supply, affecting those animal species dependent on native trees and other vegetation.
It has been claimed that sycamore can, however, exist as a member of mixed woodland without seriously affecting insect populations. The greater danger being where it is the only species.
Human impacts
Near buildings, sycamore can be a particular problem for householders because of its invasive root-system, which can penetrate walls and disturb drainage. Once established, the seedlings can be physically difficult to uproot if not tackled when the plant is very young.
Key vectors
Distributed readily by the wind-borne keys, which can disperse the seed some distance from the parent plant; the germination rate is extremely high. Although the survival rate by percentage for very young seedlings may not be large because of grazing by rodents and fungal attack. The seeds are still produced in sufficiently large numbers for these survivors to provide a significant invasive population.
Management measures
Eradication programmes may have to be put in place, with clear-felling of stands, followed by treatment subsequently with an appropriate herbicide. As is common with invasive species, careful monitoring and follow-up applications of herbicides may be necessary.
There are also suggestions that sycamore could become a useful member of managed, commercial woodland.
Further information
Many papers detailing studies on sycamore by, for example, Binggeli, Rushton and others can be found on the Woody Plant Ecology web site.
Text written by:
Catherine Tyrie, Curator, Botany, Ulster Museum