The value of modelling potential woodland cover
The purpose of the Native Woodland Model
An understanding of the potential of different areas within a landscape to support different woodland types is crucial to planning the restoration and expansion of native woodland. The NWM has been developed as a strategic tool to aid those involved in such projects, whether on currently unwooded land or in non-native forest plantations. It is suitable for use at scales above 1:50 000.
The modelled pattern of potential-natural woodland types provides a template for the development of FHNs. Using a Geographic Information System (GIS), the model can be used in conjunction with other datasets to highlight where there are opportunities for woodland expansion.
Natural regeneration is the preferred option for creating new native woods or restoring Plantations on Ancient Woodland Sites (PAWS). The advantages are likely to include best matching of species to site, patchy distribution of trees, structural irregularity and natural appearance and conservation of local genetic stock. In addition, many species characteristic of ancient woodland are slow to colonise new habitats. A new area of woodland is, therefore, likely to develop a more valuable community if it is close to appropriate colonising seed sources. PAWS adjoining surviving Ancient Semi-Natural Woodland (ASNW) are particularly valuable for restoration, especially where the ASNW is the same woodland type as that likely to develop on the PAWS site. Worked examples are provided later.
The NWM can also be used in association with digital woodland inventories to predict the likely current area and range of particular woodland types in Scotland. In particular it can be used in combination with the Scottish Semi-natural Woodland Inventory (SSNWI, Caledonian Partnership 1999)’s ‘semi-naturalness’ data to estimate the woodland type of any existing semi-natural woodland.
The value of establishing woodland appropriate to site conditions
The tree and shrub species predicted as being most suitable to the site should grow and regenerate most successfully and best incorporate existing biodiversity. The requirements of different woodland types are taken largely from Rodwell (1991) and Rodwell & Patterson (1994), with additional information taken in particular from McVean & Ratcliffe (1962), and to a lesser extent from other literature listed in the References. Any relict populations of fungi, plants or animals are likely to be able to re-establish better in woodland composed of site-appropriate species.
Species which are unsuitable for the site conditions may survive for the first few years but fail to thrive. For example, where alder has been planted on exposed shallow peats in the northern and western Highlands, it can often demonstrate good early growth only to die back after four or five years. Other species may grow well but fail to regenerate. This may be less important in traditional forestry, where new crops are planted after harvesting, but it is vital in native woodland, which must perpetuate itself through natural regeneration.
Even closely related species may have different site preferences. For example, downy birch and pedunculate oak (typical of the NVC communities W11 and W17) tend to occur together on moister and flushed ground, whereas silver birch and sessile oak (more characteristic of W10 and W16) would tend to grow on drier sites (Peterken 1998).
The NWM extends the range of tools available for planning the establishment of native woodland in an ecologically appropriate way.
Using historical information to plan new woodlands
Where land is known to have been wooded in the past, it has been suggested that the composition of historic woodland is a key element of planning new woodland. In theory it is possible to use pollen analysis to reconstruct the composition of former woodland. However, this is not a straightforward process and not necessarily the most appropriate, as explained below.
First, the relationship between pollen frequencies and tree species abundance is not simple, although for very small woodland hollows there are strong linear relations between the two. Pollen analysis depends on trees producing flowers and on the relative frequency of all pollen producing plants in the catchment. Any trees which do not flower (e.g. many species when growing in the understorey) will not be represented, and wind-pollinated species produce larger quantities of lighter pollen than those pollinated by insects (e.g. fruit trees). Consequently, it can be difficult to be sure about the true composition of past woodland from pollen analysis alone. Further, sites may receive pollen from large areas which makes it difficult to reconstruct a good impression of former local vegetation cover (Tipping 1998).
Catkins
Second, for much of Scotland, the degree of spatial resolution of pollen analysis information which can be produced at present is too coarse and fragmented to serve as a guide to native woodland restoration, although more detailed reconstructions are possible in specific areas where detailed work has been carried out, such as the Southern Uplands (Tipping 1998).
Third, even where pollen information is good, it may not be appropriate to use it as a template for designing a new native woodland, because most pollen analysis reconstructs vegetation from several thousand years ago and this may not be appropriate (climatically or edaphically) for the present-day establishment of woodland cover on a site.
The concept of woodland ‘naturalness’
Peterken (1996) defines five degrees of woodland naturalness, of which original-naturalness, the state which existed before people became a significant factor, is the type which can be reconstructed from most pollen diagrams.
Present-naturalness is the hypothetical state which would prevail now if people had never become a significant factor. Such forests would have continued to develop in response to climatic change, long-term soil maturation and continued migration of species. Therefore, they could be very different in many ways from the original-natural forests.
Potential-naturalness is the forest that would develop now if human influence were removed completely, and the resultant succession took place instantly. Such forest would be different again, because soils have changed due to the removal of forest cover and subsequent land-use, and they may also have been affected by eutrophication and other forms of pollution.
The NWM, in matching potential woodland types to existing soil types, predicts potential-naturalness. Rather than attempting to recreate an idealised concept of woodland at the post-glacial climatic optimum or any other historical time, the NWM enables the prediction of woodland types which would develop under current climate and soil conditions.