A 5000-year record of disturbance and vegetation change in riparian forests of the Queets River, Washington, U.S.A.

Abstract

We used fossil pollen, charcoal, and sediment stratigraphy in three small hollows to investigate disturbance events and changes in the composition of riparian forests on a small section of the Queets River floodplain, Olympic Peninsula, Washington. The records ranged in age from approximately 500 years at two sites 300 and 550 m from the river, to 5000 years at a site 800 m from the river. Approximately 400–600 years BP, the two sites nearest the river were either inundated by a very large flood or covered by the active channel, which would have occupied a substantially different position than its present course. Following inundation or channel movement, the pollen record suggests that Alnus rubra Bong., the primary mesic forest colonizer in the Pacific Northwest, increased and was then replaced by Picea sitchensis (Bong.) Carrière and Tsuga heterophylla (Raf.) Sarg. At the site farthest from the river, two fires occurred within the last ca. 4500 years. One of the fires was followed by a period of shrub dominance and succession to Tsuga heterophylla. The other fire did not cause a change in the pollen record. A recent unprecedented rise in Tsuga heterophylla pollen, which began ca. 1000 years BP, might be in response to cooling during the Little Ice Age. Overall, the small hollow records highlight the complex effect of floods, fire, and possibly climate change on riparian forests of the Queets River.