Exile or asylum for Britain's immigrant maple?                   

by Ian Monger

This article was prompted by the discussion 'Sycamore: Tolerate or terminate?' on the AIE forum begun in April 2008 by james covenant.

What are we talking about?

The Sycamore in Britain is Acer pseudoplatanus. The Latin 'acer' means 'sharp', as maple wood was commonly used for spears. 'Pseudoplatanus' means 'false plane', as the leaves resemble those of the Plane trees.

A huge dome to 38m where it does best – southern Scotland and Kent. Twigs are short and twisting on straight limbs. The pinkish grey bark is smooth at first, then with small grey plates after 80 years. Stout, green, grey-pink shoots hold large green buds, which burst out large (18x26cm), dark green leaves with five lobes and coarse teeth. A tree bears seeds after about 25 years.

The are many named variants with purple, bright or variegated foliage. f. purpureum and 'Atropurpureum' are frequent in gardens. 'Brilliantissimum' leaves unfold pink-white and fades to yellow-green by summer. 'Prinz Handjery' has leave with dark mauve undersides. Variegated forms include f. variegatum, 'Nizetii', 'Leopoldii' and 'Simon-Louis Frères'.

Bear in mind that:

• The name 'sycamore' originally belongs to a species of fig – Ficus sycomorus – native to the Middle East and East Africa. It is the sycamore referred to in the Bible.

• The sycamore of Britain and Ireland is known commonly in Scotland as the Plane tree.

• Trees of the genus Platanus are known in North America as sycamores.

Why the debate?

Acer pseudoplatanus is not native to Britain. It is a naturalized species that was originally 'imported' by humans, rather than being one of the (only) 50 to 70 tree and shrub species that arrived here by natural means in prehistoric times – our native species. Its original home is on high ground in southern and central Europe, from France, east to Poland.

When and who first brought it here continues to be debated. Research is hindered by the difficulty in distinguishing its pollen from that of Field Maple. The Romans are often cited as the prime candidates.  But Keith Kirby, of Natural England, reminded that the first definite record of the species is from the early medieval period:

“There are claims for earlier occurrences... however some of the evidence of ‘early occurrence is [open to interpretation]. The leaf carving on St Frideswide’s tomb (Oxford) could easily have been carved by a French mason, or a local mason who had been to France; the fourth century Roman bucket made from sycamore occurs at a site with other finds of silver fir and quern stones from Germany. If it did occur prior to the medieval period it would appear not to have been abundant enough for anyone to notice (perhaps surprising for such a potentially useful tree), compared to more regular records from the seventeenth century onward.”

The oldest sycamore with a known planting date was the Newbattle Abbey Sycamore near Dalkeith, Scotland. Planted in 1550, the 85 foot high tree had a girth of 18 feet. (Unfortunately, the tree blew down in May 2006).  Whatever the case, sycamore was still something of a newcomer in the 16th century, and it has only really got established during the last 200 years.

It's not just sycamore that was introduced by man. One of the earliest trees thought to have been introduced is, surprisingly, English Elm (Ulmus procera), which might have been brought in by pre-Iron Age tribes from southern France. The Romans are often credited with introducing Sweet Chestnut (Castanea sativa) and Walnut (Juglans regia) to Britain. In fact, the name 'walnut' comes from the Old English 'wealhhnutu' which literally means 'foreign nut'. Sweet Chestnut has become so much part of our wildlife that only pollen analysis has proven it not to be native, whereas walnut shows no signs of becoming wildlife. And of course, Horse Chestnut (Aesculus hippocastanum), which was introduced from the mountain valleys of Albania and Greece in the 16th century, has become an icon of the 'British countryside' in popular imagination.

Sycamore is much resented by many land and property owners, gardeners, arboriculturalists, conservationists and foresters because of its success at seeding itself, its sheer speed of growth, the damage it can cause to property, and fears about the spread of the species through our sensitive habitats.

The large, leathery foliage of sycamore casts a dense shade over the woodland, reducing available light for understorey and ground flora. Keith Kirby, of Natural England, explained that a distinct feature of English woodlands, over their continental counterparts, is the prominent role of relatively shade-intolerant trees and shrubs, because certain other tree species failed to recolonise (or arrived only late) after the last glaciation. For example, our northern limestone woodlands are distinct because, being dominated by ash, they lack a dense shade-casting tree. Sycamore alters this aspect. “Therefore in important ancient woods, particularly Sites of Special Scientific Interest and National Nature Reserves where it has become established only in the last century, it is likely to detract from the special characteristics of these woods.”

This shade, plus the sugar-rich sap of the leaves (maples... think 'maple syrup') provides the perfect habitat for massive populations of aphids, which release a downpour of 'honeydew' (aphid excrement - sugar solution) onto the leaves and cars below. Black fungi ('sooty moulds') grow on this energy-rich deposit - which makes leaves look pretty tired and dirty - even by mid summer. The build up can slow photosynthesis in the leaves of other trees, and so stunt growth. Car owners also complain about deposits on their vehicles. But this high insect biomass does support a higher bird population.

A cut sycamore stump will regenerate freely, and so in the urban environment, trees that have established themselves in inaccessible places can be very difficult to eradicate.

Sycamore is prone to 'tarspot' fungi (Rhytisma spp.), which cause raised black spots to appear on the upper leaf surface. Although it's pretty-much harmless to the tree (except perhaps a bit of early leaf drop), for some reason it does look fairly unattractive to many of us humans. Sycamore is also, almost exclusively, affected by 'sooty bark disease', caused by the fungus Cryptostroma corticale. First detected in 1945, the fungus is widespread on deadwood, but during hot dry summers it can lead to large numbers of local tree deaths following symptoms of foliage wilting and death, staining of wood and sooty, dead and peeling bark. Outbreaks are local and short-lived.

The sugar-rich thin bark of young sycamore trees seems irresistible to the Grey Squirrel (as it was to the Red Squirrel when populations were still widespread). Squirrels strip phloem and cambium from stems and branches between April and July. They are probably after the same sugar as the aphids, but the damage they do can be bad enough to kill off whole crowns or mature trees. Research suggests that trees between the ages of 10 and 40 years are most often attacked, and it is usually the young males that do the damage. The discarded bark is found at the base of the tree.

Does sycamore have any good points?

Sycamore is one of only three very common trees in Britain (along with Field Maple and Limes) that have insect pollinated flowers - small pale green flower clusters that hang in spring. These are a vital pollen and nectar source for bees.  Einstein is famously misquoted as saying “If the bee disappeared from the surface of the globe then man would only have four years of life left. No more bees, no more pollination, no more plants, no more animals, no more man.”  It turns out he never actually said these words, and knew no more about bees than you or I, but pollination remains one of the most important processes in the global ecological system.

Sycamore thrives in exposed sites where other native species struggle. Sycamore is hardy in lowland Britain to about 500m, but is far more resilient to exposure, industrial pollution and coastal salt than any of our native species - so it is prized as a windbreak tree in these areas. The Orkneys' largest and oldest tree, the Big Tree of Orkney, is a sycamore living on Kirkwall's Main street. Believed to have arrived as a windblown seedling several centuries ago, it has survived this windswept environment where other species fail. Although the hollow stem has now been pollarded to a 3 metre stump, it is sprouting again with gusto. (See Forestry Commission website for a photo of the tree).

The cream-white timber of sycamore is odour and taste-free once seasoned, and so is ideal for making items that are to be used with food. The wood also gives a very smooth finish and so is used widely in furniture making, joinery and flooring and other indoor uses. The 'fiddle-back' wood found in some trees - with a wavy grain - is prized by cabinet makers, and for fine violins and veneers.

Sycamore has a long cultural history, and many trees have matured into important and historic veteran trees. They accounted for 20 of the 150 candidates for the Heritage Trees of Scotland – that's over 13 percent (And sycamore was only one of over 28 species represented in the shortlist).

Sycamore was a common choice for 'dool' trees in Scotland. These natural gallows were used for hanging criminals - in prominent positions for all to see - on many estates until the mid 18th century. It was the strength of timber (which was less likely to break at the crucial moment) that made sycamore the preferred choice. The ancient sycamore at Blairquhan Castle, Ayrshire (now completely hollow, and pollarded in 1997), is thought to have been planted during the 16th century, and is one of the few remaining dool trees. (See Forestry Commission website for a photo of the tree).

Thomas Packenham highlights the 250 year old Tolpuddle sycamore in 'Meetings with Remarkable Trees'. It was under this Dorset tree that, in 1834, six exploited farm labourers formed Britain's first ever trades union to bargain for better pay and conditions. They were arrested, found guilty of sedition, sentenced to seven years imprisonment, and transported to Botany Bay. The case caused such a public outcry that the Tolpuddle six were brought back, after three years, to live in their village, where the sycamore grows to this day. Now heavily pollarded, the tree is owned by the National Trust (one of their smallest properties – but still one of the largest sycamores in Dorset).

Should the species be tolerated, terminated, or - what?!

It is undeniable that sycamore has traits which make it completely unsuitable for growing in certain situations – especially, perhaps, in the urban environment, where human activity is so much in conflict with trees, and the natural world in general. Seedlings happily establish in cracks in stonework, pavements or hard standings, or in gutters and on roofs. Saplings thrive amongst decaying buildings and in neglected areas, and young trees are hacked into 'weird and wonderful' forms in an attempt to beat them into submission. Wherever there are structures to be damaged, sycamore has the capacity to damage them, and with time and a lack of management it probably will. Sycamore is one of the most successful pioneer tree species wherever it grows, and our towns and cities are no different.

Having said that, the urban 'wasteland' in which sycamore is usually seen growing is an extremely rich habitat, that has become essential for the continued survival of many bird, butterfly and plant species; such as the Black Redstart and the Dingy Skipper. This 'urban commons' is often identified as a valuable habitat within UK Biodiversity Action Plans (BAPs) (which came out of the 1992 Rio Convention), to be retained and included as 'biodiverse' natural assets. (See the Birmingham and Black Country BAP as an example).

But it is in our woodlands and green spaces - defended as the last reservoirs of our native species – that many fear this unnatural foreign invader is doing damage. To those, it threatens to suffocate our wild spaces and squeeze out our native trees. They want the species controlled and eventually eradicated, so that these places can be returned to an earlier, more 'natural' state. 'Tolerate' or 'terminate' seem, to many, the only options available against this 'malignant weed'.

AIE has asked some of the largest conservation organisations (and largest woodland owners) about their approach to sycamore management in their woodlands. (Opinions were sought from the Royal Forestry Society, and the Forestry Commission Scotland, and Wales, but were not received in time).

The Forestry Commission England

Rebecca Isted, Biodiversity Advisor for Forestry Commission England, said:

“...within [Forestry Commission England's] estate we don’t have a specific policy on sycamore, but where it occurs naturally we actively incorporate it into the rotation and manage it as part of the broad-leaved timber resource. We rarely plant it, but recognise its potential as a high value timber tree. We have recently achieved a high price at auction for the sale of sycamore for sawmilling. We also recognise sycamore’s biodiversity potential.”

And within the English Woodland Grant Scheme, she indicated that, as sycamore is not considered a part of the native woodland resource, grant aid would not be given for planting it within new native woodland. But the Forestry Commission England would consider it under wider restocking grants for broadleaved trees.

The National Trust

The Trust's Forestry Policy states that:

“Native trees from local seed sources make a contribution to wildlife conservation, especially on ancient woodland sites but exotic trees may also be valuable. Introduced trees are part of our flora and are deeply embedded in many cultural and historic designed landscapes. It is our aim to protect them where, all things considered, they make a positive contribution to conservation.” (page 2)

 Natural England (formerly the Countryside Agency, English Nature, and the Rural Development Service)

Keith Kirby said that, for important ancient woods, particularly Sites of Special Scientific Interest and National Nature Reserves, Natural England's advice is generally to consider managing against sycamore.

“This management may, on some sites, with small quantities of recent invasion, aim at eradication; on other sites we may seek to coppice it to prevent seeding and further spread; we are likely to accept large old trees because of their potential as hosts for lichens and dead wood invertebrates; in some woods we recognise that sycamore has in effect already become the main tree species and to remove it would itself significantly damage the site.”

Outside of these areas though, Keith explained, landscape and cultural values may be of greater importance than nature conservation. “Natural England’s approach to sycamore must similarly broaden. My image of a Pennine farmhouse usually seems to have a sycamore or two around it.”

The Woodland Trust

Debbie Nicholls, in the Woodland Trust's Press Office, explained that:

“The Trust doesn't discriminate against sycamore. It provides an important habitat for many species. Having said that we don't tend to plant much of it as it will seed in anyway.”

The Trust's Position Statement on natives and non-natives states:

“When considering the need to control a species, its native or non-native status is unimportant. The key issue is whether it is causing significant ongoing habitat change or loss of other species, additional to that from climate change. However, even where this is the case, remedial action should only be undertaken if it could be sustained. For example:

• it will rarely be appropriate to eradicate beech planted outside of its historic range as climate space for this species is expanding rapidly north and west.

• sycamore control may only be justified and practical where it is not already established as a significant component of woods or in the surrounding landscape.”

Case study - The Avon Gorge Leigh Woods, in Bristol, includes 200 hectares of woodland on the south side of the famous Avon Gorge (across which stretches Brunel's Clifton Suspension Bridge). The wood includes former pasture woodland with old oak and small-leaved lime pollards, as well as ancient woodland of old coppice with standards. Rare trees include the Bristol Service (Sorbus bristoliensis) – found only in the Gorge – as well as the Wild Service Tree (Sorbus torminalis). The steep rocky slopes above the river are home to Bristol rockcress (Arabis scabra) – unique to the Gorge – as well as Ravens (Corvus corax) and Peregrine Falcon (Falco peregrinus). Over 300 species of fungi are found in the woods. The woods have been included in the Avon Gorge SSSI and has been designated a National Nature Reserve. The two main landowners in the woodland are the National Trust and the Forestry Commission. Lorne Campbell, of the Forestry Commission, said of the FC's approach to sycamore in the area: “Sycamore is treated as an invasive non-native species in the SSSI (Gorge area) section of Leigh Woods and is consequently cleaned from this zone (ongoing). Elsewhere in Leigh Woods we are more relaxed about it's presence. Management of broadleaves is by continuous cover systems. Our broadleaves also tend to be fairly intimately mixed and, currently, [sycamore] is accepted as a component capable of yielding products. During thinning operations they would not be favoured for retention and over the longer term we would look to control any significant expansion. However, this doesn't seem to be a problem where continuous cover is in place.”

Surprisingly to some, the main woodland conservation organisations in the UK are now accepting sycamore as an established part of the countryside, to be welcomed in all but the most sensitive sites as a tree with real ecological benefits. It supports vast populations of aphids which are an essential part of the food chain for birds, fish and other organisms, including the Dormouse. It provides a habitat for a wide range of lichen species. As one of Britain's fastest growing trees, the high quality timber continues to be a viable and valuable crop. And research suggests that, although sycamore is spreading in the UK, it thrives best where there is disturbance, and because it regenerates poorly under its own canopy, it cannot dominate woodlands for long periods.

© Text and photographs 1, 3 & 5 by Ian Monger May 2008  Other photos copyright-free.