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).

Existing native woodland is often largely confined to gullies (A.J. Hester)

Existing native woodland is often largely confined to gullies (A.J. Hester)

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.

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:

This booklet explains how the model was developed, what it shows and how it can, and cannot, be used. Case studies illustrate its value as a tool in the development of strategies for expanding woodland cover. Finally, the attached CD contains maps of model output for all the Scottish uplands, together with a sample of the model itself for a small area of the Highlands.