New pollen cores from southwest Alaska show variation in the timing of ecosystem change

Background/Question/Methods

The study of pollen assemblages recorded in lake sediments is a widely used indicator of past environmental changes. To date, the late Quaternary vegetation history of southwest Alaska has not been well described. We have had little information regarding the development of birch and alder communities that are currently rapidly expanding, or about the southern migration of spruce that can be observed on the Alaska Peninsula today.  Two new pollen records obtained from sites near Lake Clark, Lake Clark National Park and  Preserve, and Nonvianuk Lake, Katmai National Park and Preserve, respectively, record changes in the dominant vegetation on the Alaska Peninsula over the last  ~11,000-14,500 years. 

Results/Conclusions

At the northern site, near Lake Clark, birch expansion occurred rapidly between 15 and 14 ka BP and persisted in the record as a dominant (~80%) ‘Birch Zone’ assemblage until about 10 ka BP.  This period of birch expansion ended abruptly at ~10 ka BP with a rapid rise in alder and concurrent decrease in birch pollen, to ~40%. At the southern site, near Nonvianuk Lake, birch was present on the landscape at the beginning of the record, ~11 ka BP, and did not change significantly though time.  Thus, the Nonvianuk record lacks the distinct “Birch Zone’ observed in the Lake Clark record.  The alder rise in the south was more gradual, and the record became co-dominated by Birch and Alder after approximately 9.4 ka BP.  Spruce did not arrive at the northern site until approximately 3.5 ka BP.  Spruce was likely present in the southern (Katmai) region by about  2.5 ka BP, but did not increase in frequency until about 500 cal yr BP. The late occurrence of spruce at these sites suggests a slow migration of spruce from the north, although an increase in spruce and alder pollen at the southern site after the 1912 eruption of Mt. Katmai also suggests that vegetation can respond rapidly to stochastic events. Variation in the timing and nature of ecosystem change (e.g. alder rise ~9.4-10 ka BP) between these two sites suggest that different climatic and geographic factors could have influenced ecosystem evolution and the distribution of major vegetation types on the landscape.