The Native Woodland Model
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Scottish Natural Heritage is a government body The Macaulay Institute, based in Aberdeen, Scotland is responsible to the Scottish Executive and through them the premier land use research institute in the UK. We are Scottish Ministers. uniquely equipped to carry out research to meet the needs of sustainable rural development in Scotland, in Europe Our mission statement: Working with Scotland’s people and elsewhere internationally. to care for our natural heritage. The potential for native Our mission: We aim to be an international leader in Our aim: Scotland’s natural heritage is a local, national research on the use of rural land resources for the benefit and global asset. We promote its care and improvement, of people and the environment. woodland in Scotland: its responsible enjoyment, its greater understanding and appreciation and its sustainable use now and for future Our remit: To undertake research on the environmental, generations. economic and social aspects of rural land use and on the the native woodland model consequences of potential changes in policy, management, climate or pollution for sustainable rural development and the management of natural resources. Natural Heritage Management ISBN 1 85397 390 4 Price £22.50 Scottish Natural Heritage/The Macaulay Institute Visit our websites at http://www.snh.org.uk and http://www.macaulay.ac.uk Page 59 Scottish Natural Heritage 2004 ISBN 1 85397 390 4 A CIP record is held at the British Library NP2.5K0304 Acknowledgements Authors: Willie Towers, The Macaulay Institute; Jeanette Hall, SNH; Alison Hester, The Macaulay Institute; Ann Malcolm, The Macaulay Institute; Duncan Stone, SNH. Grateful thanks to Pat Carnegie (Macaulay Institute) for design and formatting work on an early draft. Thanks to Helen Gray, Jenny Bryce and Irvine Ross for providing case studies on the use of the NWM, and to Jan Breckenridge for permission to quote from her thesis. Thanks to Eleanor Charman, Claire Seymour and Lee Douglas for valuable assistance with GIS analysis and presentation and especially to Elaine Dunlop for valuable editing and ensuring that the publication process kept to schedule'. Apart from the major funder of the project (Scottish Natural Heritage), other financial contributions were made by the Macaulay Development Trust, the Forestry Commission, the Cairngorms Partnership, the EU Leader II programme and Tayside Native Woodlands. All photography Lorne Gill, SNH, except where individually acknowledged. The maps in this publication are based upon Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office © Crown copyright 2004. Any unauthorised reproduction infringes Crown copyright and may lead to prosecution or civil proceedings. © Scottish Natural Heritage 100017908 (2004) Further copies of this booklet and other publications can be obtained from: The Publications Section Scottish Natural Heritage, Battleby, Redgorton, Perth PH1 3EW Tel: 01783 458 530 Fax: 01783 458613 The potential for native woodland in Scotland: the native woodland model Contents 5 Executive Summary 6 Introduction 7 Assumptions and general principles underlying the Native Woodland Model 8 The value of modelling potential woodland cover The purpose of the Native Woodland Model The value of establishing woodland appropriate to site conditions Using historical information to plan new woodlands The concept of woodland naturalness 11 Methodology Base data Dataset integration The predicted woodland types Allocation of woodland types to different site conditions Potential woodland cover in different NWM categories 16 How accurate is the NWM? Potential sources of error and uncertainty in the NWM The NWM’s prediction of site conditions Validation of NWM predictions against existing native woodland Accuracy of the NWM - Summary 20 How potential woodland types correspond to HAP types 22 Use of the Native Woodland Model – case studies The Scottish Forestry Grant Scheme and Forest Habitat Networks Restoring and expanding Upland Ashwoods in north Mull Estimating the extent of old sessile oakwood Developing the Cairngorms Forest and Woodland Framework Practical Application of the Native Woodland Model 38 References 42 Appendices 42 Appendix 1 The base datasets 43 Appendix 2 The National Vegetation Classification 45 Appendix 3 Allocation of woodland types to different site conditions 54 Appendix 4 Correspondence between NWM and HAP types Page 5 Executive summary 1. The Native Woodland Model (NWM) has been developed to assist in the planning of native woodland expansion. 2. The base datasets are the 1: 250 000 scale national soils map (MISR 1982) and the 1: 25 000 scale Land Cover of Scotland 1988 (LCS88) dataset (MLURI 1993). Because of the scales of these datasets, the model should not be used at scales below 1:50 000. 3. These soil and land cover data have been related to the requirements of different types of native woodland, to predict potential woodland types – i.e. the woodland National Vegetation Classification (NVC) types – which would be expected to develop under current soil and vegetation conditions, with no or minimal ground intervention, including fertilisation, ground preparation and drainage. 4. The resulting maps represent the potential-natural extent of native woodland cover under current environmental conditions; they are not aspirations for final woodland cover. 5. The model covers upland Scotland, where soil types are generally semi-natural. Where soils have been heavily modified by cultivation and industry, the model is considered less useful in predicting semi-natural woodland distribution patterns because the highly modified soils could now support many types of woodland. 6. Comparisons of the NWM output with accumulated temperature, moisture deficit and exposure data suggest that the combined soils and land cover datasets provide a robust surrogate for the main climatic variables at the resolution of the model output. 7. A review of NWM outputs for the Highlands, and comparison with actual NVC surveys, suggest that the NWM accurately predicts site suitability for oakwood, ashwood and pinewood. Comparison with NVC surveys has yet to be carried out for birchwood, mixed broadleaves and wet woodland. 8. The NWM output has been classified according to Habitat Action Plan (HAP) types, to facilitate its use in biodiversity action planning. 9. The use of the NWM in developing Forest Habitat Networks is shown in a worked example, and a summary is given of how it was used in the development of the Cairngorms Forest and Woodland Framework. Page 6 Introduction Expansion of Scotland’s native woodland is increasingly a priority. Today only 4% of Scotland’s land area is covered by semi-natural woodland (MacKenzie 1999), whereas the area of land capable of supporting woodland is considered to be closer to 50%. Expansion targets have been set within the UK Biodiversity Action Plan (UK Biodiversity Group 1998) and the development of Forest Habitat Networks (FHNs), which seek to alleviate the consequences of previous native woodland fragmentation, is a priority in the Scottish Forestry Strategy (Scottish Executive 2000). In order to achieve the greatest benefit from the strategic expansion of native woodland, we need to understand the potential distribution and extent of different woodland types. Surviving native woodlands generally provide only partial information, since they tend to be highly fragmented and their composition and distribution have often been radically altered by grazing, burning, timber extraction and under-planting, and invasion by introduced species such as Rhododendron Ponticum. Historical reconstructions from palaeobotanical studies are likely to be of limited relevance, since site conditions have been modified over the centuries by the removal of the original forest cover, agricultural cultivation of soils, climate change and environmental pollution. Existing native woodland is often largely confined to gullies (A.J. Hester) A more practical approach is to predict potential woodland distribution under current environmental conditions using site suitability models. The Native Woodland Model (NWM) has been developed to assist in the planning of native woodland expansion. Known site conditions are related to the requirements of different types of native woodland to predict potential woodland types – i.e. those which would be expected to develop under current soil and vegetation conditions, with no or minimal ground intervention, including fertilisation, ground preparation and drainage. The resulting maps represent the potential-natural extent of native woodland cover under current environmental conditions; they are not aspirations for final woodland cover. The model covers upland Scotland, where soil types are generally semi-natural. Where soils have been heavily modified by cultivation and industry, the model is considered less useful in predicting semi-natural woodland distribution patterns because the highly modified soils could now support many types of woodland. The NWM has been used to indicate the potential for the expansion and establishment of native woods in areas as diverse as the Western Highlands (Towers et al. 1998 a), the Angus Glens (Towers et al. 1998 b) and Rum (Hester et al. 1999). It was also used in the development of the Cairngorms Forest and Woodland Framework (Cairngorms Partnership 1999) to: • indicate the suitability of sites for natural regeneration • identify priority areas for the targeted and phased