Light intensity alters the allelopathic effects of an exotic invader

Abstract

Background: Conditionality in the occurrence or strength of interactions among species yields insight into the relative importance of different ecological factors and provides a deeper understanding of the mechanisms through which organisms interact. Aims: We explored light-dependent conditionality of the allelopathic effects of Centaurea stoebe, an exotic invasive forb in North America, on Koeleria macrantha, a North American native bunchgrass. Methods: We compared the total biomass of K. macrantha competing with C. stoebe in high- and low-light treatments and in substrate with and without activated carbon. Results: Koeleria biomass did not differ in low- and high-light treatments when grown alone and was highly suppressed by C. stoebe in both low and high light; however, high-light activated carbon treatments designed to ameliorate root-mediated allelopathic effects resulted in an almost seven-fold increase in Koeleria mass in comparison to no carbon treatments. Activated carbon had significant but much weaker positive effects in low light. Activated carbon had no effect on Koeleria in the absence of C. stoebe. Conclusions: Other research shows that C. stoebe decreases sharply in abundance under tree canopies, and under canopies its typically strong competitive effects on natives in open grasslands are reduced. Our results provide a hypothesis for these patterns, suggest a possible mechanism for light-dependent conditionality in the way plants interact, and suggest that in vivo greenhouse experiments at moderate light levels may provide conservative evidence for allelopathy.