Responses of Pacific Northwest prairies to soil nutrient manipulations : implications for restoration of Castilleja levisecta and control of invasive species

Graduation date: 2015Pacific Northwest prairies have become significantly reduced in extent, and in the Willamette Valley of Oregon, less than one percent of native upland prairies remain. Many species have been impacted by this extreme loss of habitat, including Castilleja levisecta (golden paintbrush), a threatened hemiparasitic forb species endemic to the Pacific Northwest. Many of the prairie fragments that remain are poor quality, and face the threat of invasion from non-native species and a loss of biodiversity. These non-native species pose an obstacle to restoration and to the reintroduction of threatened and endangered species. Nutrient enrichment has been shown to promote increased invasion of communities, decrease success of native species, and decrease biodiversity. A way to counter these effects is through carbon addition to the soil which stimulates microbial activity and immobilizes nutrients, making them unavailable to plants. This strategy has potential as a restoration tool to improve conditions for native species, which are often outcompeted under high nutrient conditions.\ud We tested the efficacy of carbon addition for controlling non-native species and restoring C. levisecta in two Willamette Valley prairies. We created treatments of varying nutrient availability and measured the responses of the plant community and of C. levisecta. Experimental plots were established in autumn 2012 with either carbon (sucrose) addition, ambient soil nutrients (controls), or nutrient addition (NPK fertilizer). Nutrient treatments were reapplied throughout the length of the experiment. Crossed with these treatments in a fully factorial design were seeding treatments of C. levisecta and of thirteen other native species, to increase the diversity of the plant community, providing host plants for C. levisecta, a hemiparasitic plant. In the two growing seasons following treatment we conducted plant community surveys and counted and measured seedlings of C. levisecta to determine the effects of the nutrient manipulations and seeding treatments on the community overall and on this threatened plant.\ud Community composition differed significantly between the different nutrient treatments (perMANOVA, p