Abstract
Hedgerow trees are a valued and highly visible feature of the British landscape. However, the trends in their abundance and distribution suggested by various surveys during the past 50 or more years, indicates that their total numbers and, more important, the proportions of mature hedgerow trees is declining. This was recognised particularly by the Countryside Survey 2000 which revealed that in 1998 there was an estimated 1.8 million individual (isolated) hedgerow trees in Great Britain, of which 98 % were located in England and Wales; the total was approximately 3 % less than in 1990, although some areas showed greater declines (e.g. 8 % in the eastern lowlands of England). In relation to potential ecological value, most trees recorded in the survey were over 20 years old, while those 4 years or less declined by about 40 %. However, there were significant increases in the lengths of lines of trees (increased by 55 % overall). This poses a question of whether the rate of replacement of trees in hedgerows is sufficient to maintain their status, both visually and as key ecological resources. This is recognised by Defra in its Habitat Action Plans, especially the target for ancient and/or species rich hedgerows which has the aim to Maintain the overall national number of individual hedgerow trees (estimated by CS2000 to be 1.8 million in Great Britain in 1998), by maintaining the number of such trees within each county or district, through ensuring the balanced age structure. A key question which will guide the approach and outcomes in the current study is the how to measure and predict the nature of the balanced age structure necessary to maintain the target number of hedgerow trees and, thence, to use this information to inform future policy on this valuable habitat component. The present study will employ a range of information sources, including the Forestry Commission National Inventory of Woodland and Trees (NIWT) 1995 - 1999 survey and the Census of Woodland and Trees 1979 – 1982, comparing these data sets to the first Hedgerow and Park Timber and Woods under five acres report published in 1951. This information will be compared with that in the Countryside Survey datasets, which have detailed breakdowns of the structure and distribution, nationally, of hedgerow trees and linear features by Environmental Zones. Using this information, supplemented by expert consultation, the study will develop an expert decision choice model designed to predict both the viability of lines of trees compared with individual trees, both within a length of hedgerow and in complete isolation (e.g. parklands), and their invertebrate and other group values using appropriate scoring systems representing various criteria, such as ecological value, rare or endangered species, visual impact, etc. A particular focus of the modelling approach will be to describe and predict the rate of change in hedgerow trees to test the validity of earlier assumptions (from the 1950s) that the ratios of trees in different size/age classes in hedgerows should be heavily biased to young trees to support succession towards mature trees of greatest ecological value. The expert decision choice model represents a practical and achievable alternative to the traditional approach of data accumulation and prediction derived from sampling the numbers and ages of hedgerow tree species, which is a difficult and slow process. As such, the model approach will give a practical and testable definition of the percentage of hedgerows that are sustainable and which carry optimal invertebrate or other abundance and diversity or, by contrast, the percentage of hedgerows whose tree recruitment rates indicate that mature trees are likely to disappear. To measure ecological value, the study will use literature review, accessing scientific journals, databases, published and unpublished reports from (e.g. from DEFRA, EN/Natural England and county conservation bodies) and as many other sources as possible, to gather information relevant to assessing the capacities of hedgerow trees to support invertebrates and other key ecological groups. Focus will be on tree species, tree age, aspect, habitat & management related to isolation/connectivity of hedgerows compared with other trees within the landscape. As an exemplar, different tree species host widely different numbers of insect species, and their natural enemies. For example, oak supports more than 420 insect species, whereas ash supports 60-70 species and sycamore only 40-50 species. Older trees are also of greater value for invertebrates and other ecological groups (e.g. birds, mammals) associated with them compared with young trees because they provide greater resources (eg flowers, fruits) and a variety of ecological niches, especially standing dead wood and the fungi and other organisms associated with decay. Dead wood, in particular, supports nationally rare, highly specialised beetles and flies, which often have low powers of dispersal and require a mixture of habitat features (e.g. spring blossom). The ecological importance of ancient trees in parkland, wood pasture sites can be quantified by the Saproxylic Quality Index and the potential of using this index or a similar system for hedgerow trees will be part of the current investigation. Taken together, the analysis and modelling of trends in numbers and required replacement strategies for trees in hedgerows, as well as analysis of their ecological and landscape values, will provide advice to inform future strategies by Defra and other stakeholders to secure this valuable visual and ecological habitat component.
United Kingdom