INFRASTRUCTURE PROJECTS AND THEIR IMPACT ON MASS MOVEMENTS: DENDROGEOMORPHIC EVIDENCE FROM THE TRANS-ALASKAN PIPELINE, TONSINA, ALASKA

Mass movements affect roadways globally and are especially problematic in Alaska due to their sporadic and austere climate conditions and extreme rural nature. A particularly steep stretch of the Richardson Highway (mileposts 80-82) near Tonsina, Alaska was “flagged” as a continued problem for repairs by the Alaska Geological & Geophysical Surveys and Department of Transportation. Originally, this stretch of highway required extensive cleanup and a realignment (1973-74) to support the emplacement of the Trans Alaska Pipeline (TAP; 1975-77). Using dendrogeomorphic evidence, or evidence left in trees that adjusted their growth due to a tilting event, we evaluated the 4D stability of the site. This evidence, or reaction wood (in conifers), includes apparent changes in trunk tilt and wood-anatomy changes to wider, darker, and structurally more sound radial growth rings. We sampled 30 tilted black spruce trees (Picea Mariana Mill.; n=60 samples) for reaction-wood analysis at two different angles (downslope and 90º offset), digitized them and annotated reaction wood years, which were summarized in an event-response diagram. Of the 60 cores, 40 had reaction-wood present, 65% of which were cored from the downslope angle. Tree-and-site data revealed that the greater the slope steepness, the greater the tree tilt, which explains the higher reaction-wood response from the “A” cores. Throughout the 115-year chronology (1900-2015), reaction wood accounts for only 11% (±9%) of recorded radial-tree growth prior to 1977, but from 1978 onward accounts for some 87% (±9%) of radial-tree growth. The highway realignment in 1973-74 and the emplacement of the TAP in 1975-77 occurred just before this sharp increase in reaction-wood disturbance. These infrastructure projects and the accompanying construction traffic are likely factors in the lasting problems on this stretch of highway. Additionally, rapidly warming climate at these high latitudes exacerbates these slope-stability issues because of permafrost thaw, which can destabilize a landscape that has otherwise been relatively stable for centuries.