Document Type

Thesis

Date of Degree Completion

Summer 2020

Degree Name

Master of Science (MS)

Department

Biology

Committee Chair

Clay Arango

Second Committee Member

Karl Lillquist

Third Committee Member

Paul James

Abstract

Disturbances by herbivores can drive nutrient cycling in forest ecosystems by adding frass, carcasses, and molts to the forest floor which are broken down into nitrogen, phosphorous, and other elements to be recycled into biomass. Western spruce budworms are defoliators native to the central Cascades and their herbivory could increase the decomposition rate of forest materials by adding essential nutrients and/or by increasing light and rainfall penetration to the forest floor by thinning the forest canopy during outbreaks. Budworm defoliation events are expected to increase in severity as the climate warms, potentially altering forest ecosystem function. The purpose of this study was to measure how budworms influence nutrient availability in forest throughfall, decomposition rate on the floor, and soil nutrient concentrations by sampling 4 sites with low budworm activity and 4 sites with high budworm activity from September 2015 through November 2016. Budworms appeared to accelerate ammonium and nitrate loss from the canopy via throughfall during the spring 2016 feeding season, yet those forms of nitrogen did not increase concurrently in soils suggesting rapid immobilization of nitrogen. In contrast, budworms did not influence throughfall phosphorous yet soils in budworm sites had significantly higher phosphorus concentrations, possibly due to frass addition or dead adults. Decomposition rates were unexpectedly faster in the low budworm sites in the Teanaway suggesting that the nitrogen losses and/or canopy changes by budworms were not strong enough to influence forest floor organic matter cycling. Overall, my study shows that budworms can influence nitrogen movement from the canopy to soils during feeding, but much of that nitrogen appears to be immobilized in soils. Other environmental factors must control litter decomposition rates. Due to the short term nature of this study, I cannot conclude whether or not the ecological effect of budworms changes during the course of an outbreak; however, if budworm activity increases due to continued fire suppression and climate change, budworms could accelerate nitrogen losses from the canopy which could be lost to downstream ecosystems with a large or prolonged outbreak.

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