Using pollen and geochemical data from lake sediments to reconstruct spruce beetle disturbances

Background/Question/Methods

In recent decades irruptive populations of native bark beetles (Dendroctonus spp.) have damaged 47 million ha of coniferous forest in western North America. The scale and severity of this outbreak is unprecedented at least since Euro-American settlement. Climate warming is cited as the primary driver for two reasons: 1) beetle reproductive strategies accelerate during pervasive warm periods; and 2) mild winters promote greater survival of overwintering beetles. However, in subalpine forests stand-replacing disturbances, such as wildfire, often recur at time intervals that exceed historical observations, raising the possibility that outbreaks of comparable magnitude may have occurred prior to settlement. Our research investigates the recent outbreaks to improve our understanding of the drivers, outcomes, and long-term dynamics of bark beetle disturbances. We examined sediments from six subalpine basins in Utah from spruce/fir forests (Picea engelmannii/Abies lasiocarpa) that were severely affected by spruce beetle (D. rufipennis). Sediments were analyzed for pollen, sub-fossil plant and insect remains, loss-on-ignition, magnetic susceptibility, and stable carbon and nitrogen isotopes (δ¹³C, δ¹⁵N). We hypothesize that the post-outbreak mortality and subsequent defoliation of spruce: (1) decreased the ratio of spruce to fir pollen; (2) reduced canopy interception of precipitation, facilitating an increase in soil erosion and mobilization of terrestrial C; and (3) enabled leaching of foliar N thereby promoting increases in algal productivity. We tested our dataset using general linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak periods.

Results/Conclusions

The spruce/fir pollen ratio decreased significantly (p< 0.001) at all sites suggesting that this metric may be suitable for identifying past outbreaks (hypothesis 1). We found no support for influxes of minerogenic material and/or terrestrial C potentially due to differences in catchment structure (hypothesis 2). We found little evidence for increased N following outbreaks which is possibly attributable to human activities and/or uptake of N by the residual terrestrial community (hypothesis 3). To examine the prevailing assumption that climate is the primary driver of recent outbreaks, we compared the spruce/fir pollen ratio (hypothesis 1) with spruce biomass calculated using USFS stand inventory data and a pollen transfer function to reconstruct forest conditions in two watersheds on the Wasatch Plateau spanning the last 300 years. Our results suggest that land use history promoted the unprecedented scale and severity observed during the recent outbreak on this particular landscape.