Stand age structure, mortality, and regeneration dynamics across a forest gradient in southwest Alaska

Background/Question/Methods

Regional warming over the last several decades is thought to have contributed to widespread mortality in spruce forests of southwest Alaska, but also to variable growth in trees at the western forest-tundra ecotone.  Whereas mortality may impact the structure and dynamics of these stands in the near-term, recruitment events are expected to be a greater control on future forest distribution.  In this study, we used tree-ring and plot-level data to reconstruct stand dynamics, disturbance and climate interactions in white spruce (Picea glauca) across a 300-km latitudinal gradient. Using a combination of mature open spruce forest (n=10) and spruce woodland (n=20) sites, we described the structural characteristics of the stands, including seedling recruitment and tree death dates, and examined whether recruitment and mortality events are associated with warmer periods in the historical record. 

 Results/Conclusions

Forest and woodland sites were generally of comparable age, and seedling densities did not differ between the two.  However, in woodland sites, seedling densities increased at the southern end of the gradient (P<0.01), consistent with ongoing expansion of white spruce near western treeline.   A similar increase was not detected in forested sites.  A composite of age structure derived from tree-ring chronologies across forest and woodland sites indicated that periods of tree recruitment have occurred in different decades in the two stand types, presumably due to different mechanisms.  Recruitment events in forest sites coincided with warmer than average periods (positive/warm phase PDO) and tended to lag stand-level canopy disturbances by roughly 10-15 years.  Recruitment in woodland sites has been more or less continuous since the 1850s, with the greatest establishment occurring during or immediately after warm PDOs.