DENDROGEOMORPHOLOGY OF THE SILVERTIP MASS MOVEMENT, SEWARD HIGHWAY, ALASKA: INSIGHTS INTO SLOPE INSTABILITY AND PLAUSIBLE DRIVERS

Climate is warming 2-3x faster in high-latitude regions, leading to increased precipitation and permafrost thaw. Warming-induced slope instability elevates the frequency and severity of mass movements, imposing substantial financial burdens on affected communities. In conjunction with the AKDOT&PF, we selected for evaluation a troublesome, 0.5-hectare mass movement situated atop sporadic permafrost at MP 64.8 on the Seward Highway. This unstable slope, managed since 2000, impacts some 130 meters of roadway, where a gabion wall was emplaced to retain failure materials. The mass-movement maintenance, event, and repair costs are estimated at over $1,000,000 (AKSSE, 2021) with moderate traffic impacts expected. The combination of price tag and maintenance complexity make this site ideal for tree-ring evaluation. Twenty-five, tilted black spruce (Picea mariana) trees were sampled across the slope for subsequent reaction-wood analysis, a biomechanical response that facilitates geotropic correction of their posture towards verticality. These 25 trees resulted in a chronology (1749-2023, Stewart et al., 2024) and were evaluated for reaction-wood. Reaction wood (1850-2023) accounted for 29% (σ = 18%) of recorded annual growth observed, rising from about 5% in 1850 to about 55% in 2023. Trees were tilted in the down-slope direction with a mean tilt angle of 13^° (σ = 7^°). There is no distinct trigger time(s), as the site has been continuously unstable since at least 1850. Correlations between reaction wood and regional weather records show a distinct (P<.01) positive correlation with mean-annual temperature along with a positive correlation with annual earthquake activity (P<.01). Mitigating efforts, such as the gabion wall, may have provided relative stability for approximately a decade; however, ongoing slope failure suggests its limitations. Projections for this site indicate continued slope instability driven by a confluence of factors, including warming trends, earthquake activity, and the potential consequences of permafrost thaw.