Origins of Forest Patch Structure in Mountain Birch (Betula pubescens ssp. czerapanovii) in Sub-Arctic Northern Sweden; The Effects of Introgressive Hybridisation, Small Scale Genetic Selection and Individual Responses to Freezing Shock.

Abstract

The origins of observed morphological differences between windward, central and leeward sites within forest patches of Betula pubescens ssp. czerapanovii in Abisko, Northern Sweden were investigated. Samples from each of these sites from three forest patches of Betula pubescens ssp. czerapanovii and surrounding shrubs of Betula nana were analysed genetically. The findings showed no overall genetic differences between sites within a patch but did show differences between patches. Studies of an individual gene did however show evidence that selection may occur on individual genes at different sites. B. nana and B. pubescens ssp. czerapanovii were shown to be highly introgressively hybridised to the extent that B. nana were more closely related to neighbouring patches of B. pubescens ssp. czerapnovii than to B. nana adjacent to more distant patches This indicates that the hybridisation in Abisko is so great that separating these species is no longer justified from a molecular standpoint. Laboratory grown seedlings showed that morphological differences seen in B. pubescens ssp. czerapanovii have a genetic basis. Seedlings were exposed to different cold acclimation periods under summer photoperiod conditions before a freezing shock. Acclimation time had no effect on survival. Polycormic individuals showed evidence of slightly greater survival after freezing. Otherwise morphology had no effect on survival chances. The potential significance of these findings on understanding forest patches responses to global warming are discussed along with potential future experimental procedures required to further elucidate the origin of the observed morphological differences between sites in a patch. This includes whether the degree of polycormy is being genetically selected for at these sites, why this selection may be occurring and if developmental plasticity or mechanical damage are also affecting observed morphologies.