Aspen, adaptation, and climate change. Is Alberta aspen adapted to a fossil climate?

摘要

Trembling aspen (Populus tremuloides Michx.) is very widely distributed throughout North America. It is the predominant early successional tree species of the boreal forest. The species occurs throughout Alberta. It is small and shrubby in the dryer southeastern half of the province, but reaches full tree stature in the mixedwood forests of the north and west of the province. In these areas it is an important fibre source for pulp and oriented stand board production. Aspen is also an important species for wildlife, from beavers to bears, insects and birds. Aspen re-colonized the bare land surface of Alberta following the retreat of the Wisconsin glaciers about 12,000 years before present (BP). During the hypsithermal 10,000-8,000 BP the climate was warmer and moister than it is now. Since then, there has been progressive cooling and drying, so that current conditions are not as good for effective seedling regeneration. Aspen is also shade intolerant, and its seedlings are very sensitive to competition from pre-existing vegetation; it has been able to persist and spread largely because of its ability to reproduce asexually through root suckers. Following natural disturbance through agencies such as fire, windthrow, or insect defoliation, aspen has re-established itself as the dominant tree species in the early successional stages of mixedwood formations largely through asexual reproduction from remnant roots. The hypothesis presented here is that aspen is maladapted to current climatic conditions. This is because there has been relatively little opportunity for selection and adaptation to occur among the first colonizers after the retreat of the continental ice sheet in the absence of large-scale regeneration following sexual recombination. Itis proposed that, in this part of its range, aspen is adapted to a fossil climate, and that this adaptational lag could become more severe in the event of further climate change. Alberta aspens do flower and produce fertile seed, so the opportunity for genetic recombination exists. However, the frequency of successful sexual reproduction and stand regeneration events has probably been very much lower in aspen than in other boreal species such as balsam poplar (Populus balsamifera L.) or white birch (Betula papyrifera Marsh.) where successful sexual reproduction is more prevalent. This might provide an empirical test for the maladaptation hypothesis, since we would expect to find better local adaptation in species that have undergone a larger number ofcycles of genetic recombination and selection in the presence of environmental challenges in a changing climate. Range-wide provenance testing should provide some of the answers regarding the degree of local adaptation in aspen. Such trials would have the added benefit of guiding improved seed transfer guidelines, and they might also point to optimal seed sources for growth under current environmental conditions. Provenance testing should cover both the Canadian and American portions of this species range.