GSA Connects 2021 in Portland, Oregon

Paper No. 211-4
Presentation Time: 8:50 AM


BUSH, Chelsea1, BENSON, Mary Alice2, DAVIS, Elizabeth J.3, TROOST, Kathy4, TAVENGAR, Varqa G.3 and MALONEY, Devin A.5, (1)Aspect Consulting, LLC, Seattle, WA 98104, (2)HWA Geosciences, Inc., Bothell, WA 98021, (3)Earth and Space Sciences, University of Washington, Seattle, WA 98195, (4)Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195-1310, (5)David Evans & Associates, Tacoma, WA 98402

Forested slopes above a low-elevation terrace on the northwest Pacific coast of Washington State host a mature Sitka spruce forest with stands of red alder. The predominantly Sitka spruce forest is commonly replaced by red alder in areas of more recent landsliding. Nineteen trees surveyed across the terrace and one landslide date to the same 57-year interval. The purpose of the survey was to gain insight into this complicated geomorphic setting by cataloging the diameter of trees, age, location on the slope, relative amount of sunlight, and substrate: whether soil or nurse log. While the original experiment sought to assign a minimum age for the underlying landslide, results instead demonstrated that all the trees are likely too young to have grown atop a fresh slope failure. Radiocarbon dating from the landslide deposit and the roughness of the slide surface suggest the landslide occurred several hundred years prior to tree growth. Across the entire slope, we found trees to range from 43 to 100 years old, with none older than 100. Sixteen of the surveyed trees were older than 70 years. Despite a wide range in tree diameter, from 33 to 174 cm, there was no significant correlation between tree diameter and age. This slope has never been logged. We suggest the current forest emerged in the aftermath of the Great Olympic blowdown event, a large-scale windstorm in 1921 that razed a 30-km wide swath of forest from the mouth of the Columbia River to Vancouver Island, resulting in loss of 20% of the trees along the coastline. Historic records indicate that few, if any, mature trees remained standing in the Rialto Beach area after the event. Trees killed in the event may remain on the slope as the many nurse logs that host the current forest. Complicating the story is the fact that an established Sitka spruce forest on the terrace is currently dying due to terrace erosion and increasing coastal exposure. This survey catalogs the recovery of a forest 100 years following a dramatic perturbance and may provide a model for forest recovery after future disturbances.