Methods

The one kilometre grid has been adopted as the standard unit of recording. It was felt that the more usual tetrad unit (2km x 2km), adopted for the 1997 pilot study, was too coarse for urban situations. The kilometre square has also been used for other city surveys, most recently that of Belfast (Beesley & Wilde, 1997). By adopting a grid coincident with the National Grid it is then convenient to reference a square by its bottom left corner, e.g. [TA 03 13]. This convention has been used throughout.

Plant nomenclature has been based on Stace (1991).

Although the survey was conducted over a three year period, we have not had the resources to be as systematic about recording as we would have liked. Ideally each square would have been visited in the spring, summer and autumn. Unfortunately this has not always been possible and some squares will be lacking in one or more of these seasonal visits, particularly the spring one. We have also tried to minimise recorder bias by group visits and rotation of recorders. Experience has shown that even a well worked square will produce new taxa if visited by a different recorder. This phenomenon is well known and has been noted and investigated recently (Rich & Smith 1996).

One of the most taxing problems with botanical mapping, for which there does not seem to be a satisfactory answer, it that of which plants should be recorded and which should not. For this study we have tended to err on the side of inclusion rather than exclusion. The process has been regarded as that of a botanical "audit" with a view to assessing the biodiversity of the city. There has been so much intervention within the region that there are few, if any, natural areas left.

Native plants have been recorded even if known or suspected introductions.
Well established or naturally regenerating non native introductions have been recorded.
If in doubt it has been recorded - it is possible to ignore records but not to interpolate them.

The natural vegetation of the region has been adequately described by Crackles (1990). Animals dependent upon a particular food plant will not make the destinction between native, accidental or deliberate plantings.