The impact of Mt Mazama tephra deposition on forest vegetation in the Central Cascades, Oregon, USA

Abstract

The eruption of Mt Mazama, c. 7630 yr BP, was the largest North American volcanic event during the Holocene. High-resolution pollen and charcoal analyses were used to examine the impact of Mt Mazama tephra on forest vegetation and possible synergistic interactions with fire activity in the Central Oregon Cascade Range. We selected four small watersheds on a longitudinal transect north of Mt Mazama and recovered lake sediment that spanned the period of tephra deposition. Our sediment records had between 14 and 50 cm of tephra deposited, and we analyzed the sediment at centimeter resolution before and after the deposition horizon in each sediment record. Our analysis shows that nonarboreal pollen percentages and accumulation rates were depressed after Mazama tephra deposition. Recovery to pre-tephra deposition rates occurred after approximately 50–100 years. Arboreal pollen percentage and accumulation rates were less severely impacted, suggesting that the Mazama tephra deposition disrupted understory communities more significantly than overstory species, and that forest communities returned to their pre-tephra-deposition conditions after approximately 50–100 years. Fire events in conjunction with the Mazama tephra occurred in two of the four sites, suggesting that tephra deposition did not create conditions that precipitated a fire event in a consistent way. This research reinforces the notion that disturbance events may have cumulative effects on forest vegetation, but that the impacts of disturbance events need to be felt by similar constituents of the forest ecosystem to be truly additive.