Pedicularis canadensis (Scrophulariaceae) obtains the materials needed for its survival by both autotrophy and parasitism, thus interacting with other plants on two levels. A greenhouse experiment was designed to determine the effects of parasitism on the growth of three species, Big Bluestem (a grass), Tall Goldenrod (a composite), and Showy Tick Trefoil (a legume), under conditions of both inter- and intraspecific competition. Pedicularis canadensis reduced the shoot mass of Tall Goldenrod, marginally affected Big Bluestem shoot mass, and had no effect on their root mass; the shoot/root mass of Showy Tick Trefoil was not affected by the presence of the hemiparasite. Competitive interactions among host species were not affected by the hemiparasite and the hemiparasite did not have a greater effect on host mass than a competitor. However, a much smaller hemiparasite had an effect as large as a much larger competitor. We attribute the failure of this experiment to produce more significant results to nutrient availability. Rather than parasitism and autotrophy being alternative strategies to employ under different condidtions, our working hypothesis is that the conditions under which maximum gain is realized by parasitism are the same as those where maximum gain is realized by autotrophy. Hemiparasitism may function both to obtain limited resources from hosts, and as a mechanism to improve its competitive interactions under conditions of nutrient limitation. When nutrients are not limiting, gain realized by autotrophy and parasitism would be reduced.

Key words: competition, hemiparasite, prairie, Scrophulariaceae