Linking field based studies with greenhouse experiments: the impact of Centaurea stoebe (=C. maculosa) in British Columbia grasslands

The grassland community of Lac du Bois Provincial Park in the interior of British Columbia has become increasingly invaded by Centaurea stoebe (=Centaurea maculosa; spotted knapweed). Allelopathy, through the production of the phytotoxin (±)-catechin by C. stoebe roots is believed to be partly responsible for knapweed's invasive success. We used field sampling and greenhouse experiments to examine two questions: (1) Is increasing abundance of C. stoebe negatively associated with the abundance of specific native grassland species? (2) Do species that exhibit a negative correlation with C. stoebe abundance in the field demonstrate increased levels of susceptibility to application of (±)-catechin during germination? Thirty-eight plots were sampled in the grassland, encompassing areas of low--high knapweed abundance. Seeds from eight native species, exhibiting positive, neutral or negative correlation with knapweed abundance, were treated with three concentrations (0, 0.5, 2.0 mg/mL) of (±)-catechin. Root growth and percent germination were measured over a 6-week period. The results indicate that C. stoebe abundance is negatively correlated with native plant species abundance and may alter plant community composition. Moreover root radical growth was significantly negatively affected by treatment with (±)-catechin in all four native plant species that exhibited a negative correlation with knapweed abundance in the field. Past studies have failed to conclusively link greenhouse results with plant community patterns. Here, we provide a correlative link between plant community composition and tolerance to a phytotoxin.