Patterns of Twentieth Century Treeline Advance in Alaska: Insights from Dendrochronology and Permanent Plot Studies

Tuesday, 16 December 2014: 10:50 AM
Andrea H Lloyd and Christopher L. Fastie, Middlebury College, Middlebury, VT, United States
Warming over the last several decades has led to an upward or poleward shift in the distributional limit of trees at sites around the circumboreal north. In our own research in three regions in Alaska, we found evidence for an upward shift in treeline at every site, in the form of a decline in population age with increasing elevation. The prevalence of that particular signature of change suggests that continued expansion of forest vegetation at treeline is likely. But how fast, and how sustained are those changes likely to be in the future? Changing disturbance regimes, non-linear growth responses to warming, and unexplained regional variability in the rate of historical responses to warming all introduce significant uncertainty to projections of future change at treeline. In an effort to compare historical with current patterns of change, we set up permanent plots in each of the sites at which we had reconstructed forest history, tagging each tree and seedling. We resampled these plots more than a decade after the initial measurements, and thus have an opportunity to compare the actual trajectory of change in each plot with the trajectory that we inferred from our dendrochronological reconstructions. At most sites, the observed pattern of change over the last 10-15 years is consistent with the trajectory inferred from dendrochronological studies. Seedling density at treeline and above treeline increased significantly over this time period-- almost doubling (98.6% increase) at treeline plots, and increasing more than 189%, on average, in above treeline plots. The expansion of forest vegetation at treeline has thus continued at an extremely rapid pace. The remeasurements also, however, yielded surprises that could not have been anticipated from the reconstructions at forest history. A severe fire at one of our sites killed most of the seedlings that had established above treeline, and in the years since the fire, aspen-- which is not a common treeline species in Alaska-- has become the dominant woody vegetation both at and above treeline at this site. This suggests that changes in disturbance regime at treeline could have unexpected consequences-- leading to rapid establishment of vegetation types that are not currently common at treeline.