Variation and cytotype distribution have been studied in secondary birch woodland where intermediates between Betula pendula and B. pubescens appear to predominate. Trees at three sites in lowland eastern England, with contrasting soil conditions, were examined.
Thirty-eight of the trees studied proved to be tetraploid (2n= 56) and the remaining 12 were diploid (2n= 28); no triploids or aneuploids were detected.
Principal Components Analysis of up to 20 biometric characters of leaves, catkins, fruits and achenes revealed an essentially continuous pattern of variation. The diploid trees displayed rather limited variation and corresponded fairly well with B. pendula. In contrast, the tetraploids showed very great variation which included that normally associated with both species and most of the possible kinds of intermediate. The same pattern of variation was evident in several discontinuous variables and also in measurements of pollen diameter and guard-cell length made on 20 trees at one site (Holme Fen).
Variation was in part site-dependent, suggesting that the different environments were influencing phenotype. Twenty trees at one site (Dersingham) were located on a topographic gradient from bog to heath; here there was a strong correlation between the loadings of individual trees on the principal axes of variation and the elevation of those trees on the gradient. This indicates a morphological cline which might be determined by site-wetness. The cline was not associated with polyploidy, as it was mostly attributable to variation within the tetraploids.