A Tale of Two Spruce

How do black spruce and white spruce respond to a warming climate? White spruce seem to respond well to warming temperatures and thawing permafrost, but black spruce responds well to a wetter climate.

Over the past ten thousand years, the dominant vegetation types in Alaska have changed in concert with the climate. As a result, the dominant vegetation types have mediated the effect of climate on fire regimes and formed the habitat for other terrestrial plants and animals. White spruce forests expanded into Alaska about 9.5 million years before present and around 4 to 5,000 years before present, black spruce replaced white spruce as the most dominant tree in interior Alaska. It is speculated that the shift to the more flammable black spruce-dominated forest was at least one of the causes of increased fire frequency on the landscape at this time.

Black and white spruce are the two most common tree species in interior Alaska today. Yet, the two species rarely grow together, with black spruce more common on nutrient poor or cold, wet soils underlain with permafrost. White spruce is more common on productive sites, with warmer, drier, and more nutrient-rich soils. As the climate in Alaska warms at twice the rate of the global average and historically unprecedented warming is projected by the end of the 21^st century, the future composition of the boreal forest is an important and much debated subject. The interactive role of climate, permafrost, fire, and vegetation is complicated and intertwined.

Tree rings have been used to help understand how trees have, and maybe will, respond to climate changes by relating ring width to weather conditions. Because black and white spruce often grow in very different site conditions and site conditions moderate how trees response to weather conditions (e.g., trees on steep, dry slopes grow narrower rings in response to hot/dry conditions than trees on flat, moist terrain) the growth response of these two species to weather cannot be compared—unless you compare the two species when growing in the same exact same site conditions.

In a study published in Ecosphere in 2021, NPS researchers did just that. To directly compare the climate-growth response of black and white spruce, the NPS researchers selected all the plots from an extensive vegetation monitoring dataset from Yukon-Charley Rivers National Preserve, Denali National Park and Preserve, and Wrangell-St. Elias National Park and Preserve where both black and white spruce grew within a stone’s throw of each other. Researchers then extracted cores from both species (a sliver of wood extracted from the tree so rings from each year could be measured).

What they found was that the climate-growth responses of the two species hinged on the presence of permafrost, but especially white spruce. Increasing June-July temperatures decreased white spruce growth when growing on permafrost, but increased growth when growing on deeply thawed soils. Black spruce growth was less sensitive to June-July temperature than white spruce, but had a consistent and more positive response to summer precipitation. The less negative growth response of black spruce to summer warmth on permafrost compared to white spruce may explain why black spruce is more common in these lowland areas underlain with permafrost, which are also the hottest places in interior Alaska in summer. These findings further imply that widespread thawing of permafrost as the climate warms may foster some expansion of white spruce in this region at the expense of black spruce. However, black spruce had a more positive radial growth response to increased summer precipitation than white spruce. Thus, future changes in precipitation patterns will likely exert important influences on relative rates of growth in these two species and, in some landscape positions, black spruce may gain competitive advantage over white spruce in a wetter climate.

Divergent responses to permafrost and precipitation reveal mechanisms for the spatial variation of two sympatric spruce

Abstract

The ranges of black and white spruce are largely sympatric, suggesting both species have similar climate requirements. The two species, however, are highly segregated across the landscape with black spruce most common on nutrient-poor sites with cold, poorly drained soils and white spruce more common on productive sites with warmer, well-drained soils. Because site conditions influence tree climate–growth responses, it is difficult to compare white and black spruce climate–growth responses as these responses are confounded by the differences in site conditions in which the two species naturally occur. As the climate warms dramatically in northern latitudes, it is critical to understand how a changing climate and associated changes in permafrost and fire regimes will interact to shape future species composition and ecosystem functioning in the boreal forest. In this study, we examined the climate–growth responses of black and white spruce growing in the same sites. This approach eliminates the confounding factor of site conditions and facilitates our understanding of how these two species respond to climate. We included standardized thaw depth of the active layer in our analysis as a representation of permafrost, which is a key factor delineating these two species' habitat preferences and is actively warming and thawing as the climate warms. Our most important finding was that the climate–growth responses of the two species, but especially white spruce, hinged on the thaw depth of the active layer. Specifically, with increasing June-July temperatures white spruce radial growth increased in areas with deep thaw or no near-surface permafrost, but strongly decreased when growing in areas with near-surface permafrost. Black spruce radial growth was less sensitive to June-July temperature than white spruce but had a consistent and more positive response to summer precipitation. These findings point to a primary mechanism potentially driving the positioning of these two tree species within the landscapes of boreal interior Alaska and imply widespread thawing of permafrost may foster expansion of white spruce in this region at the expense of black spruce, but that in a wetter climate, black spruce may gain competitive advantage over white spruce in some landscape positions.


Nicklen, E. F., C. A. Roland, R. W. Ruess, T. Scharnweber, and M. Wilmking. 2021. Divergent responses to permafrost and precipitation reveal mechanisms for the spatial variation of two sympatric spruce. Ecosphere 12(7): e03622.