Since its creation in 1919, the Forestry Commission has periodically carried out surveys to determine the extent and condition of woodland and trees in Great Britain. Surveys were conducted in 1924, 1938, 1947-1949, 1951, 1965 and 1979-1982. The National Inventory of Woodland and Trees (NIWT) is the latest in this sequence and is designed to update the information gathered in the Census of Woodlands and Trees conducted in 1979-1982. The NIWT has the following goals:
- To produce digital maps of all woodlands in Great Britain over 2 hectares.
- To provide a basis for forecasting timber production.
- To assist the Forestry Commission in targeting advice and grant aid to woodland owners and managers.
- To provide information on the main forest types and tree species by County (in England and Wales) and by Region (in Scotland).
- To assess the condition of woodland and trees and provide. information on woodland as a wildlife and conservation resource.
- To provide data for use in other studies (e.g. land use change and biomass production).
The NIWT is divided into two sections: a main woodland survey covering woodlands of at least 2 hectares, and a small woodland and trees survey covering woodlands of less than 2 hectares, groups of trees and individual trees. In both cases sampling techniques have been employed to select target areas, which are then assessed in detail by field surveyors. Digital maps identifying woodlands of over 2 hectares are also produced.
The NIWT has been conducted in phases, starting with a pilot of the main woodland survey in the Grampian Region of Scotland in 1993-1994. The main woodland survey commenced in the remainder of Scotland in 1994, and was completed in October 1995. The small woodland and trees surveys started in Scotland in 1996, and were completed in 1997. In England and Wales, fieldwork started in 1996 and 1997 respectively; the target dates for the completion of fieldwork were March 2000 for the main woodland survey, and March 2001 for the small woodland and trees surveys. The Forestry Commission intends to publish the results of the NIWT as a series of inventory reports for each English and Welsh county, and for each Scottish region. National reports for England, Scotland and Wales, and a report for all of Great Britain will also be published. The Forestry Commission plans to conduct future updates of the NIWT on a cycle of about ten years.
The NIWT produced two main types of data:
- Digital map data held in a digital mapping system and linked to Geographical Information Systems. Digital maps are produced prior to fieldwork and identify woodlands of over 2 hectares. Each woodland consists of one or more 'polygons' distinguished by forest type (e.g. conifer, broadleaved, mixed). The area of each polygon is recorded, from which the total area of the woodland can be calculated.
- Data from the field surveys conducted for the main woodland survey and the small woodland and trees survey.
The main woodland survey data in the original database falls into six levels, which are related to the sampling methods used to select woodland for surveying:
- Wood level: data for each sampled woodland, derived from the digital maps before field surveying was conducted. This includes the grid reference of the woodland, an interpretation of the main forest types (based on the digital maps), the relevant local authority, and a polygon number linking the woodland to the digital maps.
- Ownership level: the name and contact details of the woodland owner (taken from Forestry Commission records), plus information derived from questions submitted to the owner: i.e. type of ownership (private, business, charity etc), 'management context' of the woodland (farm, woodland, mixed etc), and size of the woodland.
- Cluster level: an assessment by the surveyor of the management practices (e.g. 'agro-forestry', 'wildlife conservation') being conducted within a cluster of one to five sample squares. This is the first level of data derived from ground surveying.
- Sample square level: the location and square type of each sample square, together with the results of a 'structure assessment' of a 50 metre x 50 metre plot in the south-west corner of the first sample square in a cluster. The structure assessment is an 'attempt to describe the variation, complexity and density of a stand of trees' and records information such as canopy layers, canopy height, and the amount of standing dead wood.
- Section level: for each section within a sample square, details such as the actual forest type (conifer, broadleaved etc), whether the stand has been thinned, if natural regeneration is occurring, and information on any underwood species which are present.
- Element level: for each element within a section, details such as the primary tree species, productive area, potential of the wood for timber, tree health, and evidence of mammal damage.
The datasets in this series are available to download. Links to individual datasets can be found at piece level.
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