The distribution and ecology of morphological types of Betula pendula Roth and Betula pubescens Ehrh., in Great Britain
The genus Betula occurs worldwide and is morphologically very variable; this variation is reflected in the two arborescent taxa which occur in Britain. In this work morphological data from birch were recorded from sites throughout Britain, with a view to defining morphological types derived from multivariate analyses such as cluster and discriminant analyses. Once these types had been defined, their distribution was investigated in relation to broad complexes of environmental factors. The multivariate analyses gave three morphological types which were equivalent to B. pendula, B. pubescens ssp. pubescens and B. pubescens ssp. odorata as described in published floras. Of the original characters used in the data base however, a total of 19 separated the three types, although there remained much overlap. A distinct separation, however, occurred between the type with pubescent twigs and hairs (B. pubescens ssp. pubescens) and the other glabrous types, but when these attributes were removed from the analysis, much morphological overlap was evident. Removal of these attributes was though necessary as they were 'masking' the ability of other characters to discriminate between the glabrous types. Each of the morphological types was associated with slightly different, but overlapping, complexes of environmental factors. B. pendula had an affinity with relatively cold winters, warm summers, dry areas towards the south and east, and was not found in the far north-west of Scotland. In contrast, B. pubescens ssp. pubescens was associated with relatively warm winters, cool summers, wet areas and did not show any relationship with areas of low rainfall or latitude. B. pubescens ssp. pubescens showed less environmental specificity, but was associated with wetter areas, relatively high altitude, towards the west, and also showed no association with low rainfall. The response to different photoperiodic treatments varied differently between taxa at different latitudes, but was similar at similar latitudes.