Factors affecting spruce establishment and survival near western treeline, Alaska

Background/Question/Methods

Regional warming and increases in tree growth in western Alaska have resulted in increased productivity at the western forest margin (western treeline).  The effect of increasing temperatures on seedling recruitment in this area has received less attention, in spite of forecasted forest expansion.  Here, we used stand structure and environmental data from white spruce (Picea glauca) stands (n = 70 plots) sampled across latitudinal and productivity gradients to explore factors influencing establishment and mortality.  Using tree-ring chronologies developed from a subset of the plots (n = 30), we estimated establishment dates for trees of all age classes at forest, woodland, and high elevation treeline sites.  We hypothesized that establishment events, if they occurred, would be relatively synchronous across the 300-km latitudinal transect.  We expected that seedling and sapling densities would be greatest at the southern sites, near western treeline, where high mean annual precipitation should presumably buffer the effects of warming. Ongoing work will compare periods of establishment at these sites with independently-derived climate chronologies to test the hypothesis that establishment events are associated with multi-year climate anomalies. 

Results/Conclusions

Tree-ring chronologies from forest, woodland and treeline plots (n=1302 cores) showed periods of high establishment that were generally consistent across sites.  Establishment years in forest and woodland plots centered on the 1880s, 1910s, and 1930s, with northern and southern forest sites also showing a pulse of establishment in the 1840s.  Across high elevation treeline plots, establishment dates were highly synchronous: seedling establishment peaked in the 1940s-1950s and accounted for up to 70% of trees present.  Seedling densities measured in lower elevation plots were greatest in the southern open forest and woodland sites near the forest-tundra ecotone, but were not strongly correlated with any of the environmental variables tested.  In contrast, sapling densities did not vary with latitude.  The proportion of saplings in a plot was negatively correlated with maximum July temperature (r=-0.40; P<0.001) and positively correlated with mean annual precipitation (r=0.33; P<0.01) and snow season length (r=0.40; P<0.001), suggesting that seedling survival may be greatest in areas where water is not limiting.