Variation due to environment and heredity in birch transplanted between heath and bog

Variation in growth and morphology within the Betula penduia/B. pubescens complex has been investigated by means of a reciprocal transplant experiment. Phenotypic responses to adjacent bog and heath environments have been contrasted and the heritability of morphometric characters has been investigated. The extent to which selection by the two environments had occurred was also examined. Four tetraploid parent trees were selected: two from the bog resembled B. pubescens and two from the heath resembled B. pendula. Progeny were raised from seed and transplanted into the bog and heath field sites as established year-old seedlings. Survival, growth, leaf and twig morphology, and the responses to fire and deer browsing were monitored for up to three years after planting. Few of the transplanted individuals died, even after burning. Growth was determined largely by environment. Transplants grew much faster on the heath than on the bog; analysis of the annual radial growth increments in the boles of the parents and their neighbours revealed the same effect had occurred in the longer term. The slower growth of both parents and progeny on the bog was presumed to be due to the effects of soil anaerobiosis. After a fire on the heath, several main sterns regenerated from each burned stump, but the progeny of B. pendula-like parents produced taller, more competitive growths than those of B. pnbescens parents. Deer were highly selective; they browsed only the progeny of B. pubescens-like trees and only on the heath. Browsed trees became bushier and more branched. Principal Components Analysis of eight continuously varying leaf variables showed consistent displacements between bog and heath plantings and between parent and progeny for all four half-sib families. Regressions of progeny on parental values for the individual leaf variables suggested high heritabilities for all except base angle, even allowing for juvenile characters. Despite similar regression coefficients in the transplants to bog and heath, the relationships were displaced relative to one another between bog and heath by large differences of intercept for measurements of size; this reflected dwarfing of the leaves of bog transplants relative to heath transplants. Such displacements were much smaller for measurements of leaf shape, although apex angle was consistently more acute (i.e. more B. pendula-like) in the transplants into the heath than in the transplants into the bog. The significance of soil conditions, heath fires and deer browsing in interacting with heredity to mould the observed phenotypes of birch is discussed