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IntroductionAspen, (Populus tremula), is one of the most widely distributed tree species in the British Isles. It is Scotland's only native Poplar and a characteristic species of the Scottish Highlands, but it is only in Strathspey, Deeside and parts of Sutherland that Aspen-dominated woodlands occur. Elsewhere, Aspen tends to be found only in isolated small stands clinging to steep crags and hillsides in remote Highland glens. Aspen also adds a unique charm to the landscape being one of the last trees to come into leaf in the spring and has very distinctive yellow autumnal colours. Today only 160ha of Aspen woodland remains, a scarce and very threatened resource, comprising of only 18 stands that are larger than 1ha, of which only 25ha has any statutory protection, (MacGowan 1992). These larger stands are true remnants of the boreal woodland that colonised the Highlands soon after the last ice age. Invertromie Wood on the RSPB Insh Marshes Reserve is one of these stands. The RSPB and AspenThe Society's interest in Aspen began in the 1990s when it became lead partner for three UK Biodiversity Action Plan Priority Aspen dependent species:
The niche of these three species is extremely different, each one representing a distinctive stage in the succession of an Aspen woodland. Epione requires low scrubby regenerating Aspen, Orthotrichum only grows on mature trunks, whilst Hammerschmidtia likes Aspen dead wood. All three species should be seen as flagship species, each representing a wide range of species across the taxa that similarly depend on Aspen. The future of Aspen conservation in the UK currently lies with these species and this process (Cosgrove, this volume). Invertromie WoodThe RSPB's Insh Marshes Nature Reserve in the central Highlands is primarily renowned as an internationally important wetland (Prescott 2000). However, it is also gaining recognition for its Aspen dominated woodland at Invertromie, Scotland's fourth largest stand of Aspen. It is around 50ha in size and is predominantly a Birch wood with an extensive Aspen component comprising around 20%. Other species include Hazel, Bird cherry and Rowan. Invertromie Wood came under RSPB management in 1987 and one of the first steps was to remove the winter sheep grazing. Much of the Birch and Aspen regeneration dates to this time. However, much of the site is overrun with Rabbits, the light sandy soils being ideal for burrowing and the adjacent fields providing excellent feeding. Roe deer are also common and browse the Aspen regularly. Thus, there are few signs of regeneration and the wood is slowly dying on its feet. Management for HammerschmidtiaOne of the key species at Invertromie is Hammerschmidtia, the larvae live in the wet decaying cambium layer under the bark of recently dead Aspen feeding on the micro-organisms in this pungent and gooey decaying layer. Hammerschmidtia requires a regular input of fresh dead wood for its continued survival, but only Aspen stands greater than 4.5ha have a sufficient resource. In 1999, with the support of Scottish Natural Heritage (SNH), the Malloch Society undertook a full survey of the site for Hammerschmidtia (Rotheray 1999). This study estimated the larval population at just 65 individuals, surviving in just two pieces of fallen timber and recommended the following specific management proposals.
The key action is the input of fresh dead wood into the system annually. Ideally such wood should have a diameter greater than 15cm, as the decaying cambium layer is too thin to support Hammerschmidtia in thinner timber. This happens naturally by wind-blow, either of whole trees, branches or tops. However, if this is not the case the second option is to sever the root-plates of wind-blown trees that are still alive and/or fell Aspen columns and trees whose tops have blown out. If such timber is left in-situ, fungi slowly attack the live wind-blown trees whilst the standing columns quickly dry out. Felling the timber in contact with the ground slows both the fungal attack and the drying out process, thus making the best use of this scarce resource. An annual survey at Invertromie is conducted each spring to determine the amount of suitable fresh dead wood that has entered the system naturally and the location of new wind-blown trees and columns. If two pieces of suitable fresh dead wood are found then no further action is required that year. If no suitable fresh dead wood has entered the system naturally then the severing of one or two root-plates and/or felling of columns is required. If both of these are not present, then the final course of action is the felling of live trees. This latter scenario must be seen as a last resort and is only sustainable providing there is sufficient regeneration. It is often necessary to protect this fresh dead wood from browsers keen to strip-off the bark, thus destroying the larval nurseries. This is easily achieved by stretching rabbit wire over the fallen timber without the expense of erecting a full rabbit fence. Such netting need only be temporary as it probably takes 2-3 years for the fresh dead wood to become suitable for Hammerschmidtia and it is then only suitable for a further 2-3 years before the bark cracks and the cambium layer dries out. The provision of 'new' dead wood is an emergency measure and obviously not sustainable in the long-term without regeneration. So, during the winter of 2000/2001, a rabbit control programme was instigated with the erection of around 6km of rabbit fencing and installation of 25 rabbit box traps, funded by the Forestry Commission (FC), through a Woodland Improvement Grant. The box traps have proved to be a very effective way of controlling the Rabbits and comprise of an underground box, beneath a tunnel through the fence, with a treadle floor. When unset, Rabbits have free access via the tunnel through the fence and when set, Rabbits fall into the retaining box through the false floor. The boxes are sited on existing runs between 100-500m apart dependent on the density of Rabbits. The traps are initially set weekly and as the catch reduces the period extended to fortnightly or more. The boxes are always set in the late afternoon and checked first thing the following morning. Regular checking of the fence is also vital to ensure that Rabbits are using the tunnels and not creating their own access points by digging under the fence. In 2000 a survey, jointly funded by RSPB and SNH, was undertaken by the Malloch Society of all the larger Aspen stands in northern Scotland to determine the size of the UK Hammerschmidtia population (Rotheray 2000). This was estimated at just 300 larvae, living in just 12 pieces of suitable dead wood, an extremely low population for an insect. Specific management recommendations were produced for each site. Management for Epione and OrthotrichumEpione larvae (the Dark-bordered beauty moth) feed on low shrubby regenerating Aspen up to around 1m in height. There are currently only three known sites for this moth in Scotland (Young, this volume). The adult moths often fly weakly during the day and are not strongly attracted to light. RSPB and Butterfly Conservation are currently overseeing a survey of the main Aspen stands for this species in the hope that more colonies await discovery. One of the targets in the Species Action Plan is to ensure that there are 10 viable populations within the historical range by 2010, by enhancing population sizes at known sites or by re-introducing populations to suitable locations (UK Biodiversity Steering Group). Epione has not been recorded at Invertromie though parts of the wood look ideal. As Aspen regenerates the area of ideal habitat will increase, making the site a priority candidate as one of the suitable re-introduction locations. One of the main management prescriptions will then be to coppice the Aspen to maintain a continuity of low shrubby Aspen of a suitable height. In 1999 Orthotrichum obtusifolium, was discovered on the bark of a single Aspen tree on the edge of Invertromie Wood. It was then known from only one other site in Scotland but has subsequently been found on four further Aspen trees in adjacent Aspen stands (Rothero, this volume). Mature trees adjacent to these sites will be retained in the hope that Orthotrichum may colonise them in the future. Research and survey work at InvertromieInvertromie is now becoming an important study site for Aspen and its dependent taxa and several exciting and important discoveries have already been made. Knowledge gained and implemented at Invertromie could prove to be invaluable at other sites where the pressure on Aspen and its associated species are far greater. One of the most significant discoveries was of the Aspen specific bracket fungus Phellinus tremulae, a first for Scotland though it is now proving to be fairly widespread on suitable Aspen trees in the area (Emmett and Emmett 2001; Emmett and Emmett, this volume). As a result of the Aspen seminar, a great deal of interest has been shown in Aspen by lichenologists (Street and Street, this volume) and a recent survey has revealed four species new to Britain and one previously believed to be extinct, (Coppins et al. 2001). Detailed autoecological work on the Aspen longhorn beetle, (Saperda carcharias) presented in these proceedings was based at Invertromie. What was probably Scotland's biggest ever moth trapping night was also held at Invertromie, when members of the British Natural History and Entomological Society ran over 20 light moth traps in Invertromie Wood. Several studies are in progress focusing on the saproxylic diptera, (dead wood dependent flies), trying to find answers to key lifecycle questions to help guide future management. In contrast to many flies, the requirements of the larval stages of Hammerschmidtia are well known compared to those of the adult fly. Little is known about the adult's behaviour or food requirements. The lack of nectar or pollen sources within the wood may be a factor limiting the population. This is currently being monitored through detailed observations in the field. Another crucial unknown concerns the adult's powers of dispersal. This information will prove invaluable in planning and prioritising Aspen connectivity projects and in determining locations for the targeted provision of fresh dead wood in other areas. The provision and subsequent annual monitoring of fresh dead wood at Invertromie will lead to a better understanding of the exacting larval requirements of the saproxylic fauna. This will help in the determination of timescales and a better understanding of the processes of decay. For example, how long does it take for new fresh dead wood to become a suitable larval breeding media? How long do these conditions persist? What density of larvae can the timber support? Experiments are also in progress trying to create artificial sap runs in Aspen. These occur naturally and are used by both adults and larvae alike. This is currently proving problematic as any sap flow induced is soon healed by the tree. Samples from natural sap runs are being tested to identify bacterial or fungal agents that may be preventing the sap clotting and introduced into artificially created runs in an attempt to keep them open. The age structure of Aspen at Invertromie is being determined through mapping and coring. This will shed light on the history of the wood as well as quantifying the future resource. The distribution of clones is also being mapped, using both leaf burst and autumn colours to identify clones. ConnectivityStrathspey Aspen stands occupy a distinct zone, a band that extends from the edge of the floodplain to the base of the heather moorland. The sympathetic management of the present Aspen resource is paramount, but this must only be seen as a short-term measure. The long-term and far greater challenge is to link existing stands, effectively creating Aspen corridors between sites. Several of these stands are isolated, probably with poor genetic variation, such corridors will allow both movement of species and genetic flow (MacGowan, this volume). Some stands are separated by dense conifer plantations that act as an impenetrable barrier despite the closeness of adjacent stands. So, the clearance of some conifers or creation of Aspen rides in these plantations is a high priority. The nearest adjacent Aspen stand to Invertromie lies across the B970 at Torcroy. A priority management objective at Invertromie is to link these two stands by planting an adjoining corridor of Aspen along the perimeter of the adjacent fields. The distribution of Aspen in Badenoch and Strathspey is presently being mapped and an Aspen database created. This exercise is an essential step if the aim of linking stands is to be achieved, as it will readily highlight sites for the creation of Aspen corridors as well as identifying barriers (MacGowan, this volume). These can then be prioritised and targeted action undertaken by encouraging regeneration or planting to link stands, or creating rides or restructuring plantations to remove barriers. AcknowledgementsMany people have contributed to the knowledge of Aspen and its associated species at Invertromie. In particular I would like to thank the following for their time, enthusiasm and expertise: Valerie and Ernest Emmett, Graham Rotheray, Iain MacGowan, Tracey Begg and Peter MacKenzie. Several of the projects described would not have been possible without the financial backing of various bodies including FC, Butterfly Conservation and SNH. I look forward to the continued use of the RSPB's Invertromie Wood as a research and survey site for all those interested in Aspen, its associated species and ultimately its conservation and discovering more about this wonderful and special habitat with such inspiring people. ReferencesCoppins, B. Street, L. and Street, S. 2001. Lichens of Aspen Woods in Strathspey. Unpublished SNH Report. Emmett, E.E. and Emmett, V.E. 2001. Phellinus tremulae, a new British Record on Aspens in Scotland, Mycologist 15:3 105-106. MacGowan, I. 1992. The entomological value of Aspen in the Scottish Highlands. Unpublished Malloch Society Report No. 1. Prescott, T. 2000. RSPB Scotland's Insh Marshes Management Plan (April 2000 - March 2005). Unpublished RSPB report. Rotheray, G. E. 1999. Current status of, and Management Recommendations for, Hammerschmidtia ferruginea at Invertromie. Unpublished RSPB Scotland report. Rotheray, G.E. 2000. Assessment of the current condition of breeding sites for the UK BAP priority hoverfly, Hammerschmidtia ferruginea, (Diptera, Syrphidae). Unpublished RSPB Scotland report. The UK Biodiversity Steering Group. 1999. Tranche 2 Action Plans. Volume IV - Invertebrates. Return to The Biodiversity and Management of Aspen Woodlands For further information about aspen, please go to the Aspen Information Resource Moths - related pages
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Published: 3 November 2002 | ||||||
