In the 1950s, the state of California approved an expansion of Highway 101 through Humboldt Redwood State Park (HRSP) in Northern California. While this expansion attempted to minimize its impact on old growth redwood (Sequoia sempervirens) forest, decades later, the spatial distribution of tree mortality and crown deterioration adjacent to the highway suggests that the road expansion has had a long-term impact on road-adjacent trees. To better understand how human disturbances affect redwoods, we investigated the physiological response that coast redwoods had to the highway expansion in the 1950s using tree-ring width measurements and carbon isotope discrimination (Δ13C) values. In 2018, we established 19 plots in HRSP in which we collected increment cores and data including each tree’s crown health (dieback or healthy crowns) and proximity to the highway and other areas of disturbance. After measuring ring widths in each core, we identified periods of growth suppression using radial-growth averaging and overall decreases in growth in road-adjacent plots in comparison to control plots. We also tested the hypothesis that Δ13C values decreased significantly following the road’s construction, which would indicate that the highway expansion reduced stomatal conductance.
Results/Conclusions
Radial-growth averaging results showed that trees near the highway experienced higher rates of growth suppression (65% for dieback trees and 40% for healthy trees) after the highway’s construction in comparison to our control plots (5% and 0%). Using the pre-construction period of 1900-1955 as a temporal control, we also determined that the trees in the highway adjacent plots and our control plots showed significant differences in growth from the time the road was constructed until about 1990. Furthermore, Δ13C values in dieback trees were lower than expected immediately following the start of the highway’s construction in the late 1950s and remained low until 2000. Healthy highway adjacent trees had a similar, but delayed response in which Δ13C values were lower than expected from about 1977 to 2000. Collectively, the tree-ring width and Δ13C analyses suggest that the highway impacted growth and stomatal conductance in highway adjacent trees for approximately 25-30 years after its construction. Consequently, we argue that future highway expansions should avoid areas of old-growth coast redwood, if possible, or special consideration needs to be taken into account if the long-lasting effects of road construction are to be mitigated.