COS 87-3
Responses of Pacific Northwest prairies to soil nutrient manipulations: Implications for restoration of Castilleja levisecta and control of invasive species
Pacific Northwest prairies have become significantly reduced in extent, and in the Willamette Valley 99% of native upland prairies have been lost. Many species have been impacted by this loss of habitat, including Castilleja levisecta (golden paintbrush), a threatened species. Remnant prairie fragments also face the threat of invasion from non-native species and loss of biodiversity. Nutrient enrichment has been shown to promote increased invasion of communities, decreased success of native species and decreased biodiversity. Carbon addition can counter these effects by reducing the available nutrients for fast growing weedy invasives. This strategy has the potential as a restoration tool to improve conditions for native species that may be outcompeted under high nutrient conditions. To test this potential, we created treatments of varying nutrient availability to determine the responses of the native plant community and of C. levisecta. At two sites, experimental plots were established with either carbon (sugar) addition, ambient levels of soil nutrients (controls), or nutrient (NPK fertilizer) addition, and plots were seeded with C. levisecta. We conducted plant community surveys and seedlings counts of C. levisecta to determine the effects of nutrient manipulation on the community overall and on this threatened plant.
Results/Conclusions
Mean total plant cover was significantly reduced by carbon addition compared to controls, and increased in nutrient plots. A significant difference was also observed in the plant composition of the different treatments (MRPP, p<0.001). Carbon addition plots had the lowest proportion of invasive species cover, while nutrient addition plots had a higher proportion of invasive cover compared with controls. Richness of non-native species was also significantly reduced by carbon addition compared with controls, whereas native species richness was unaffected. Some invasive species were found to be highly affected by the nutrient manipulation, while others were not, suggesting that carbon addition may be an effective control for only certain invasive species. At both sites, C. levisecta emergence was lower in carbon addition plots compared with controls. In nutrient addition plots, C. levisecta emergence was higher at one site but lower at the other compared with controls, suggesting that nutrient addition may only be beneficial under certain conditions. These results suggest that although carbon addition decreased seedling emergence of C. levisecta, it could be useful in the control of invasive plants and prairie restoration.