The boreal biome represents a large portion of the world’s forested area and is a major component of global carbon (C), water and energy cycles. In Alaska, observations of declining growth and increased mortality of black spruce (Picea mariana) and white spruce (Picea glauca) have led some investigators to suggest the boreal forest may be transitioning from a spruce-dominated region to one increasingly populated by temperate tree species and grasslands. Drought is often identified as the leading driver of declining spruce growth. However, long-term trends in radial growth of the dominant deciduous species (Alaska paper birch [Betula neoalaskana] and trembling aspen [Populus tremuloides]) and their sensitivity to drought remain relatively poorly understood. We took advantage of a large and spatially extensive sample of tree cores from interior Alaska to compare long-term growth trends among contrasting tree species (white and black spruce, paper birch and aspen). In addition, we used a century of temperature and precipitation observations to examine the strength and sign of climate-growth correlations.
Results/Conclusions
All species showed a growth peak in the mid-1940s, although growth following the peak varied strongly across species. Following an initial decline, growth of white spruce and birch showed little evidence of a trend, while black spruce growth showed a slight decline from ~1970 to present. Aspen growth was much more variable than the other species and showed a steep decline from ~1970 to present. We detected positive correlations between the standardized precipitation evapotranspiration index (SPEI) and growth of birch and both spruce species, but not in aspen. Instead, the significant growth decline in aspen since the mid-1970s was closely correlated with a rise in the population of a defoliator, the aspen leaf miner (Phyllocnistis populiella) in long-term monitoring plots near Fairbanks. Our results indicate that birch, black and white spruce are sensitive to moisture limitation, but recent conditions in interior Alaska may not be driving growth of these species to historically low levels. Potential insect-driven declines in aspen growth highlight the complexity of non-climatic drivers of boreal tree growth. Our analysis provides important historical context for modern growth rates among the dominant coniferous and deciduous tree species in boreal Alaska, and show that recent climate and biotic drivers of tree growth have not been more favorable for deciduous species than for spruce.