The effects of Spotted Knapweed (Centaurea stobe L.) on grassland plants and soils in British Columbia

In British Columbia, grasslands provide critical habitat for a wide variety of species and are a significant forage base for BC’s ranching industry. Grasslands in BC are threatened from several human-caused stressors, including urban development, over-grazing, climate change and the introduction of non-native invasive species. Spotted knapweed (Centaurea stoebe L.), is an invasive plant introduced to North America from Europe. It can establish large monocultures in BC’s grasslands which can cause a reduction in wildlife and livestock forage production, a lowering of native biodiversity, and an alteration in soil nutrient composition. I investigated the carry over effects immediately following removal of large and small spotted knapweed patches on soil mineral nutrients (nitrogen (N), phosphorus (P), carbon (C) and volumetric water content) and the growth of an important native grass, rough fescue (Festuca campestris Rydb.), in grasslands within Lac du Bois Provincial Park, located to the northwest of the city of Kamloops, British Columbia. The results of field experiments showed that soil total nitrogen, total carbon, volumetric water content and biomass of rough fescue were lower in former spotted knapweed patches, but there was no significant difference in soil total phosphorus. In addition, there was no measureable difference between large and small spotted knapweed patches in soil mineral nutrients and plant growth. In a greenhouse experiment I manipulated N:P ratios (1:1, 15:1 and 30:1) and biochar (10g/pot or none) to test their effect on competitive performance between spotted knapweed and rough fescue. The experimental design included five plant combinations: spotted knapweed alone, rough fescue alone, two spotted knapweed, two rough fescue, one spotted knapweed and one rough fescue. The results of the greenhouse experiment showed that total biomass of spotted knapweed was greater than rough fescue at each N:P ratio when they were grown alone or grown under intraspecific condition. Also, results showed that the competitive effect of spotted knapweed was -0.136 ± 0.052 SE; while rough fescue has a facilitation effect of 0.020 ± 0.007 SE. Biochar addition had no effect on plant growth of spotted knapweed or rough fescue, or competitive interactions within and between the two plant species.