Tuesday, 6 November 2012

Mapping the distribution of Ash trees (Fraxinus excelsior) in Great Britain

A newly arrived disease is threatening one of Britain’s most common trees, the Ash. According to the Forestry Commission, “Chalara dieback of ash is a serious disease of ash trees caused by a fungus called Chalara fraxinea (C. fraxinea). The disease causes leaf loss and crown dieback in affected trees, and it can lead to tree death.”
CEH has been asked to prepare a summary of data on ash tree distribution for the relevant government departments and agencies. The analysis was carried out using one of our existing long-term, Great Britain (GB) wide data sources, the Countryside Survey (CS). Countryside Survey is a study or ‘audit’ of the natural resources of the UK’s countryside, which has been carried out five times between 1978 and 2008.
The ash tree distribution analysis, which can be read here [PDF], uses CS data to estimate how prevalent ash is across the countryside. It confirms:
  • that Ash is the most common hedgerow tree species (i.e. species growing as a full standard as part of a hedgerow)
  • that there are an estimated 2 million individual ash trees (outside of woodlands which contain tens of millions of ash trees) in GB
  • that Ash is the second most common individual tree species in GB (after the Oak)
  • that there is approx 214.4 (‘000 hectares) of ash woodland in GB
  • that Ash trees increased in number in linear features, which include hedgerows, between 1978 (the first survey) and 2007 (the fifth survey).
The analysis was used to produce maps of the areal extent of Ash trees and individual distribution, including this map:
Areal extent of ash based on % cover in Broadleaved woodland habitat parcels,
mapped using landclass means. Shading relates to % landclass containing
ash woodland.

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