Boreal Alaska aspen growth rate collapse and mortality from high temperatures and drought and insect attack

Background/Question/Methods

In the early stages of the analysis of possible climate change effects on Alaska forests, aspen, with its adaptation to the warmest and driest sites, was identified as a species that might experience relatively less growth reduction than other tree species as temperatures increased. We conducted a study of the climate sensitivity of aspen in boreal forest region of Alaska. Our study represents the first significant test of the hypothesis that aspen growth is relatively better under elevated temperatures than other boreal trees in Alaska. We collected, cross-dated, and measured 117 tree disks from 117 aspen trees in 7 widely separated stands across central and eastern Interior Alaska. We also collected 12 birch, and 15 white spruce disk samples to compare relative growth performance of those species with aspen in the same stands. The ring width data from the all trees was compared to mean monthly temperature and monthly total precipitation data from Fairbanks from 1912 through 2011. In addition to climate factors, growth of this population of aspen has been affected by a severe outbreak of the aspen leaf miner that began in 1998 and has continued through 2011.

Results/Conclusions

The earliest aspen ring in the sample was formed in 1795, approaching the earliest known for the species. Sixteen aspen ring width chronologies begin at 1829 or earlier, and a number originate in the 1950s. Radial growth of aspen on the sampled sites is under strong climate control (r² 0.6+; p < .01), with warm summer temperatures a negative predictor of growth and precipitation in specific months a positive factor. From the mid 1970s through the 1990s, strongly unfavorable climate index values were associated with a major growth reduction. Since 2004 aspen experienced a growth rate collapse, with some trees failing to form rings along a portion of the bole. The sustained leaf miner outbreak coincides with overprediction of growth by the climate index. Tree death from climate stress and insect leaf mining has become appreciable in the past 3 to 4 years. Our results suggest that as long as the highly unfavorable climate regime of the past several decades persists, aspen productivity will remain at historically low levels, and aspen are at risk of elevated mortality. With further temperature increases, aspen is likely to be eliminated on the warmest and driest sites, a process which may have begun.