Historical trends in tree growth inform the future of Alaska’s boreal forest

Climatic and biotic drivers shape expected changes in the vegetation of interior Alaska

About one-third of the world’s forested area is boreal forest, making it important in global biogeochemical cycles. Boreal forests are undergoing rapid climate change, and numerous studies in Alaska have found that the growth of spruce is declining as a result of warmer temperatures and drought. Combined with increased fire frequency, this favors the encroachment of deciduous tree species.

Researchers with the Pacific Northwest Research Station and their colleagues compared the growth and drought sensitivity of birch (Betula neoalaskana) and aspen (Populus tremuloides), two dominant deciduous tree species, with that of the coniferous black and white spruce (Picea mariana and P.  glauca) that they may replace in future boreal forest of interior Alaska.

All four dominant tree species in boreal Alaska were sensitive to drought, but aspen growth declined rapidly

Using tree-ring data from tree cores, the researchers found that all four tree species exhibited drought sensitivity, but only aspen demonstrated a rapid decline in growth, primarily driven by an herbivorous moth outbreak. Climate change is expected to increase wildfires, which will favor deciduous species but will also create warmer, drier conditions that may slow the growth of mature birch and aspen.  

These findings provide important historical context for understanding the response of these dominant tree species to climate change and have implications for the future productivity of the boreal forest of interior Alaska and global carbon cycle.