Tree densities of black and white spruce in the Brooks Range, Alaska

Although black spruce is the dominant treeline species in the eastern boreal forest, its distribution stops several kilometers short of treeline in the Brooks Range in Alaska, and white spruce is the dominant treeline species. The explanation for this distribution is not known, but two hypotheses are plausible. First, black spruce may be less tolerant of climatic conditions near treeline than white spruce. Second, black spruce may be unable to regenerate successfully near treeline due to long intervals between fires. We are establishing permanently marked study plots along a transect from the Yukon River basin, where black spruce is the dominant species, to the foothills of the Brooks Range, where it reaches its distributional limit. We are reconstructing recruitment history of both black and white spruce at our study sites, and are reconstructing recent fire history from analysis of fire scars and stand age structures. These data are being used to parameterize matrix population models, with which we are describing patterns of population stability.