Tree-ring reconstructions of spruce bark beetle outbreaks in Lake Clark National Park and Preserve, Alaska

Background/Question/Methods: Tree-ring studies have been used to gain information regarding past disturbance (insect outbreaks, fire) and long-term climate variability in temperate and high-latitude forests.  In southwest Alaska, a gradual warming since the 1970s is thought to be responsible for the recent spruce bark beetle outbreaks that have killed over 1.2 million ha of forest in south-central Alaska.  Tree-ring data are being used to reconstruct spruce bark beetle outbreaks in the Lake Clark National Park and Preserve (LACL) region over the past 200-300 years.  Our reconstructions are based on the assumption that spruce bark beetles kill the larger trees, leaving behind the smaller pole-sized trees.  When the smaller trees are released from competition, they grow rapidly for decades until the canopy closes again and competition for light slows their growth.  We collected over 800 tree cores from live and dead trees at eight sites in LACL, and at two sites on adjacent native land near Lake Iliamna.  At all sites we observed one or more growth releases (2x growth release for 10 years compared to previous 10 years) during the last 200+ years.  Following the identification of individual release years, we calculated the probability of observing a release event in each stand using the binomial model for years that exceeded the 2x growth threshold.  Results/Conclusions: The number of significant release periods attributed to beetle activity ranged from 1 to 6 per site.  Across all sites, the mean interval between significant release periods was 51 years, almost identical to the average for the Kenai Peninsula (52 years).  Regional stand-thinning events (1810s, 1830s, 1870s, 1910s, and 1970s) attributed to spruce beetle activity in LACL match the regional outbreak dates on the Kenai Peninsula.  The decades in which regional outbreaks initiated tended to have low moisture conditions due to warm winter-spring temperatures and dry late summer conditions.  Low moisture years are known to produce stress in white spruce, which greatly increases the probability of successful attack and reproduction of bark beetles.  Results from both the Kenai Peninsula and LACL suggests a history of extensive, synchronous spruce beetle outbreaks across south-central Alaska that may have been similar in spatial scale to recent outbreaks.  The conclusions from this study suggest that moderate- or higher-severity spruce beetle outbreaks were common in the past.  However, recent spruce beetle outbreaks may be of higher severity (i.e., higher mortality) than previous spruce beetle outbreaks in some areas.