Growth trajectories of diploid and tetraploid trees of the Betula pendula/B. pubescens complex (Betulaceae): a 38-year record of trunk circumference

Growth in trunk circumference in a natural, uneven-aged stand of 20 trees in the Betula pendula/B. pubescens complex at Holme Fen, Cambridgeshire, UK was monitored over a period of 38 years, from 1977-2014. At the beginning of the study, their chromosome numbers were determined, and trees were aged by counting the rings in radial cores. Circumferential growth trajectories with increasing age of five diploid and 13 tetraploid trees were modelled using three-factor sigmoidal regression. As with previous morphological and molecular studies of the same trees, the tetraploids were considerably more variable in growth trajectory than the diploids; tetraploids included both the slowest- and fastest-growing individuals. Diploids behaved more coherently, having more similar trajectories. Greater variation in tetraploids probably reflects their allopolyploid origin, with subsequent unidirectional introgression. There were indications that diploids may grow faster and reach greater asymptotic circumferences than tetraploids but the differences in this small sample were not statistically significant. There was no evidence that the two cytotypes have different life expectancies.