THE INTERIOR ALASKAN BOREAL FOREST AND HOLOCENE MOISTURE DYNAMICS

Current and recent observations show enhanced vegetation productivity (greening) in the low arctic that is linked to shrub expansion, whereas in some regions of the northern boreal zone there is evidence of reduced tree growth (browning). These opposite responses to warming require explanation. Biogeographically, the northern boreal forest is less exposed to immigration of new taxa as a result of climate change, compared with the tundra to the north (new shrub species) or the border of boreal and mixed forest to the south (that exists in many but not all regions—new cool-temperate species). Thus, responses to changes in temperature and precipitation must come from extant species, which may have bioclimatic requirements that fit the changing conditions less well than before. In interior Alaska, a survey of current boreal tree taxa distribution in climate space, particularly in relation to soil moisture (a function of P-E), shows spruce (Picea) is near to climate thresholds that may now be reached in years of extreme weather, and indeed productivity declines have been recorded. With this modern understanding of moisture thresholds for spruce distribution, questions arise about the role of P-E in determining spruce persistence, expansion and distribution across Beringia. A compilation of late-Quaternary spruce dynamics and new data on how P-E may have varied during the Holocene suggest that the temporal patterns of behavior in interior Alaska can be better explained by variation in critical aspects of paleohydrology rather than growing season temperatures alone. Given recent climatic trends related to arctic amplification, boreal ecosystems in interior Alaska may be particularly sensitive to the effects of further climate change on moisture.