NWDG/WT workshop to discuss “sycamore: the future for base-rich woods?”, Woodhill Wood, Alva, Clackmannanshire, 1st July 2017
Attendees: Matt Elliott (WT), Alan Crawford (RDIA/WT), Alasdair Firth (RDIA/WT), Eamon Wall, Simon Stuart, Andy Acton, Brian Coppins, Sandy Coppins, Richard Toleman
Written by Alasdair Firth, July/August 2017
Woodhill wood is a largely broadleaved woodland on south-facing slopes of the Ochil hills on the north side of the lower Forth valley. The base rock is basalt, which overlies sedimentary rock and breaks down to form rich brown earth soils. The woodland canopy is composed mainly of sycamore and ash, along with minority species which include hawthorn, holly, hazel, oak, beech, larch and remnants of wych elm. By far the largest areas of single species domination in the wood is the central area which is composed of high, medium-aged sycamore, with very occasional ash. In areas like this with high forest sycamore canopy there are no young trees but there are many seedlings, almost entirely ash; there are no sycamore seedlings. In other parts of the wood there are some younger trees, including sycamore and hawthorn. The woodland floor has a scattering of species at moderate cover, especially Mercurialis perennis dog’s mercury and the grass Brachypodium sylvaticum wood false-brome along with other species indicative of rich soils and shade like upland enchanters nightshade Circea x intermedia. Roe deer are present in the wood in low densities. There is a network of paths through the wood. The wood (c74 hectares) is owned and managed by the Woodland Trust.
The site visit and workshop were undertaken to discuss the implications of the potential changes in the woodland canopy from Chalara ash dieback. Chalara is present in the wood and is already killing some trees.
The visit consisted of a series of stops at relevant points of discussion. The main topics discussed were the historic factors that led to the current woodland composition and structure, the characterisation of trees like sycamore and beech as non-natives and the implications for biodiversity, the impacts of tree diseases and objectives of woodland management at Woodhill wood and other base-rich woods in general.
Sycamore Acer pseudoplatanus is not usually considered a native species in Scotland. It is thought to be either an archeophyte (introduced by humans before 1500) or a neophyte (introduced by humans after 1500). In some situations (for example Glen Urquart) it has been removed from woods as a non-native invasive species. Acer campestre field maple, the only Acer native to the British Isles, is considered native in England but not in Scotland. Sycamore trees support a large number of invertebrate species and their bark is suitable for colonisation by a wide range of lichen species. Sycamore is suited to similar conditions to ash, but is slightly less tolerant of wet conditions and tends to suffer from butt rot when grown in wet places. Casts heavier shade than ash, the wood is not quite as versatile, but has some specialist uses and is generally a good timber. It generally seeds freely and spreads, especially into enriched or disturbed ground where there are sufficient light levels. There is some evidence of alteration of generations between ash and sycamore. Sycamore is a minor component of many continental beech woods. In Woodhill wood sycamore - and in fact most species other than ash - are not currently regenerating. Much of Woodhill wood already has a dense canopy and there is little ground disturbance in the open areas. Generally in Scotland, sycamore is undergoing range expansion mainly through natural regeneration along with some planting.
After entering the wood, the group discussions started with thoughts on the general historical and ecological context of this wood. We discussed the background of sycamore in Scotland and whether its spread should be thought of as having a negative or positive impact. Was this spread ‘natural’, ‘an invasion’ or maybe ‘semi-natural’? We moved on to discuss other ‘non-native’ tree species (there was some debate about what native/non-native actually meant), tree diseases, future change in woodlands and silvicultural management. We often returned to discussing the objectives of management.
In some previous management recommendations (for example for woodlands in the Clyde valley) an upper limit of 40% sycamore in the canopy was proposed. This percentage has also been used to assess designated woodlands (SSSI/SAC) for favourable or unfavourable condition. We discussed why this had been suggested and whether this was appropriate. We also talked about whether such an upper limit should be proposed for other species, for example oak in western woodlands and pine or birch in more eastern and upland woods. The discussion revolved around resilience and whether, as in other parts of the world, forests in Scotland with a greater number of species in them would be more likely to survive in the face of threats from tree disease, climate change and other long-term impacts.
We also discussed woodland management and silviculture. The detailed history of Woodhill wood was not known to the group, and it was unclear whether the main sycamore areas of the wood were a result purely of planting or of a mixture of planting and natural regeneration. In the main sycamore areas the trees are even-aged (probably around 70 years old) and of low silvicultural value for their timber, having small crown diameters and often warped or crooked main trunks. Some are multi-stemmed. Perhaps sycamore was originally planted as a resource for the silver mines nearby, but as the mine operated nearly 200 years ago this would not explain why there were stands of sycamore of this more recent date. The last felling here was probably around the time of the Second World War. Extraction on the steep slope would be difficult. It seemed unlikely that there would have been an alternation of generations between ash and sycamore on this site.
A stone dyke at the top edge of the sycamore marked a vague transition to more mixed woodland where ash and silver birch were more dominant as well as some oak, elm and hawthorn along with sycamore. This mixed woodland above appeared to be of similar age to the sycamore, with the exception of some much older open-grown oak. We debated whether Woodhill wood was previously much more open, like many of the small stands of trees on the southern slopes of the Ochils. If so, it probably infilled with trees once grazing was removed.
Chalara ash dieback is already seriously affecting trees in the wood although it was only confirmed here this year, and we discussed this disease in detail.
Matt Elliott gave some details of disease pathology. Ash dieback is a fungal infection that disrupts water transport in the tree, causing dieback. It is often visible in the cambium layer of younger shoots. The fungus is a disease of Manchurian ash, but this species of ash is very tolerant of it and it causes only minor symptoms. In European ash, the fungus has much more serious consequences. It causes a cyclical infection, the fungus entering the leaves, replicating on the rachis after the leaves dropped, and spreading through young leaves again in the spring into the wood of the tree itself. Because re-infection occurs annually from leaf litter, places where leaves survive longer, like damp places, will have higher infection rates year-on-year. Ash dieback will have very high costs but it is not clear who will pay these. Many ashes are wayside and hedgerow trees, and tree safety will be an issue in the longer term.
With the buffer zones proposed when the disease was first found in Great Britain having failed to stop the spread of the disease, it does not seem that is will be possible to slow the rate of spread. The fungal spores are bourn on the wind and can travel at least 10km. Road corridors have become corridors for spore dispersal because vehicles move air along them. With the high spore production rates due to the large amount of infected leaf litter it is unlikely that any ash in Scotland will remain free of ash dieback in the long term. The fungus is also transported by blown leaf litter and probably through the movements of birds and other animals.
Ash dieback is different from Dutch elm disease because elm can survive, although since older stems are killed, it is suppressed. Chalara ultimately kills ash. Chalara has more unknowns and tolerance may vary over the country, but some places are likely to lose ash sooner than others. There are still uncertainties and it is a case of waiting to see what happens in the long term.
Other tree diseases are like Dutch elm disease and Xylella are also a threat to base-rich woods. Although there is no potential buffer zone for ash there is potential to develop a buffer zone against Dutch elm disease in wych elm around or north of the Great Glen. Dutch elm disease is moving south-west along Loch Ness side, but there are substantial gaps with little or no elm between there and the west coast where the largest elm populations in Scotland remain. Proactive management for elm has worked in the past, for example Edinburgh still retains elms because the city council actively removes diseased-looking trees.
The import of plants is a potential source for the spread of tree disease. Xylella is a major disease of olives and has the potential to affect a wide range of plants, including Acers like sycamore. With the dual threats of ash dieback and Dutch elm disease, it seems that base-rich woods in Scotland are already more seriously threatened by disease than most. However, globally, the most threatened forests or stand-types are monocultures. For example Phytopthora lateralis has affected commercial harvests of Lawsons cypressus, which is planted as a single species crop in North America. Where there are more tree species, disease spread is potentially slower and less devastating.
On the point of whether sycamore is more of a ‘threat’ than beech or western hemlock or rhododendron we came to the conclusion that the effect of the dominance of sycamore in a wood is different than that of these other species, although ideally perhaps no single species should be dominant.
Non-native species can certainly add something to a wood (and perhaps, as a non-native species ourselves we should be grateful for that). But they can also significantly alter a wood from what was there before. Questions about what is desirable and what is not should revolve around the objectives for the wood rather than the native or non-native status of the species concerned.
However, a species that is a benefit in one site type might be considered a threat in another. For example, there are differences in nutrient cycling rates in acidic and base-rich woods, so leaf litter like beech litter that decomposes fairly quickly in soils with high nutrient turnovers (base rich woods) may persist for longer in woods with slower turnover rates (acidic woods).
We briefly compared the characteristics of beech and rhododendron. Both species can fundamentally alter the ground flora, acidify the soil and eliminate other species through preventing their regeneration. The main difference between them is that beech is less insidious. Rhododendron is vigorous in most conditions where it is damp enough. Beech, in woods like Woodhill wood, has very low regeneration rates (evidenced by the lack of seedlings). Beech litter breaks down fairly quickly in soils with high nutrient turn-over, but rhododendron leaves are resistant to decomposition. Beech is probably not a great threat in base-rich woods, because although individual beech trees can grow well, and grow to very large sizes, they tend not to regenerate as freely as in more acidic conditions; here they would add diversity rather than altering the wood completely. There were some differences of opinion in the group over whether movement to beech dominance in the long term would be positive or negative.
We discussed the relationship between trees and lichens. Woodhill wood has lower lichen numbers and diversity than other comparable woods for example further west. The main cause is historic air pollution and possible discontinuity of habitat. Ash trees are a good habitat for lichens, but as well as the species of host tree, the age, position and the form of the tree are important. Lichens need to find the appropriate niche habitat and isolated older individual trees provide a larger number of niches, especially in terms of light levels, than do most trees in woodland. Patchiness is beneficial for biodiversity. Monitoring of the wood can be very helpful to identify particular areas or patches in woods which have high biodiversity or potential for this to develop. Management might also aim to create suitable host trees for lichens by allowing open-grown structures to develop.
We stopped at an open area dominated by bracken. There was a little regeneration of ash around the edges of the bracken, but nothing else. Near to this the woodland was more diverse with ash and silver birch and some very old oaks. We discussed whether bracken glades like this were beneficial for biodiversity or whether bracken should also be considered an invasive species, albeit native. Would planting be appropriate here and might a tree canopy naturally suppress the spread of bracken? It might be appropriate, if possible, to manage woods at a scale that takes things like this into consideration, for example, opening up the canopy in areas away from bracken colonies where regeneration may occur without the threat of bracken invasion.
The final area we looked at was an area near the bottom of the wood where the canopy had previously been opened up by the removal of a coup of moderate size, possibly around twenty years ago. Regeneration had been abundant, mostly of ash, with sycamore, hawthorn, cherry and regrowth of sycamore from cut stumps. The ground disturbance at time of felling may have aided sycamore regeneration since we had seen no seedlings in the rest of the wood.
The Woodland Trust’s general objectives for the woods they manage relate mainly to improving woodland biodiversity and increasing peoples’ understanding and enjoyment of woodland. Although the most of the trees in Woodhill wood are non-native sycamore, the management objectives for the wood mainly relate to recreation within a woodland setting and the differences between ash and sycamore are not likely to affect this much. Woodland Trust management guidelines refer to a long term vision of restoring or transitioning non-native plantations and secondary woods to woods of predominantly native species composition and semi-natural structure. It is not clear how this could be implemented at Woodhill wood where native species are a minority and the structure will continue to be affected by impacts of tree disease. The complexities of this relatively small woodland indicate that increasing a woodland’s long-term resilience to changes imposed by tree disease and climate may be at least as important as specific objectives or targets for native species and biodiversity in determining future composition and structure in base-rich Scottish woods.
How to Replicate the Functions and Biodiversity of a Threatened Tree Species? The Case of Fraxinus excelsior in Britain. Mitchell et al. Ecosystems. 2016
Sycamore. Townsend. Woodland Trust position statement. 2008
Urquhart Bay. Management Plan 2013-2018. Woodland Trust. 2013
Wood Hill Wood – Management Plan 2016-2021. Woodland Trust Scotland. 2016
A history of the Native Woodlands of Scotland, 1500-1920. T.C.Smout, Alan R MacDonald & Fiona Watson. Edinburgh university Press 2005
Stand dynamics in Tilio-Acerion woodlands of the Clyde Valley. Thompson & Peace. Highland Birchwoods. 2005