Treeline ecosystems are known to be sensitive to warming temperatures, and treelines have advanced globally in response to recent warming. Although widespread, treeline advance is not ubiquitous: moisture limitation, edaphic constraints, and disturbance can all limit treeline responsiveness to temperature. In Alaska, dendroecological studies have revealed widespread 20th century treeline advance, while also pointing to questions about future responses to warming. Will moisture sensitivity—suggested by declining tree growth at treeline sites—ultimately constrain treeline advance? How might disturbances, which are currently rare at treeline, alter response to a warming climate?
In an effort to monitor treeline dynamics, we established permanent study plots at treeline locations in interior Alaska in the late 1990s, in the same locations where we conducted dendroecological reconstructions of treeline dynamics. Seedlings and trees were tagged and mapped within study plots at and above treeline at each site. We revisited plots in four of the sites 15-20 years after their establishment and resurveyed tree and seedling populations.
The density of live white spruce (Picea glauca) seedlings increased over time at all but one site, at a rate higher than predicted from a projection of population growth based on dendroecological reconstructions of stand density. Mortality between the two sampling dates varied by site from 3% to 12.7%, but did not differ significantly between treeline and above treeline plots. The ratio of resampled seedling density to original seedling density ranged from 2 to 3.5, and was higher above treeline than at treeline (F=3.353, P=0.078, df=1). White spruce seedling density did not increase over time at one site, where a 2004 fire burned more than 70% of the sampled area and killed the majority of the trees and. Post-fire succession at that site was dominated by aspen (Populus tremula), which is currently rare at treeline in Alaska. Five years after the fire, the density of aspen at treeline was 3100 ± 1421.4 seedlings per ha. Density above treeline had reached 5250±2250 seedlings per ha. These densities were, respectively, 47 and 1050 times higher than the density of live spruce seedlings at the site. In summary, treeline forests in Alaska have continued to experience rapid change; observed changes over the last two decades have been more rapid than expected from retrospective studies. These findings also suggest that disturbances have the potential to rapidly switch treeline ecosystems into novel states.