Resource limitation and the role of a hemiparasite on a restored prairie.

Hemiparasitic plants tend to thrive in and significantly affect plant communities in low-nutrient, high-light environments. Hemiparasites are assumed to be weak competitors for light but strong parasites, leading to the prediction that effects on hosts and communities should be a function of resource supply. We investigated the effects of light and mineral nutrients on hemiparasite-host relations in two experiments. Removal of the hemiparasite, addition of fertilizer, and full sun significantly increased total aboveground dry mass in small plots on a restored tallgrass prairie. After 3 years, removal of Pedicularis canadensis almost doubled the mass of grasses and had smaller effects on forb species, but the impact of the parasite was independent of resource level. Fertilizer increased grass growth only in full sun, increased non-legume forb growth only when shade was applied, and tended to depress legume growth when shaded. Light manipulation did not affect the hemiparasite across 4 years of manipulation but fertilizer increased P. canadensis shoot mass. A complementary greenhouse experiment with Andropogon gerardii as host produced qualitatively similar effects and showed that shade reduced root growth of both the host and the parasite. These results do not support common assumptions regarding hemiparasite-host relations under field conditions but indicate that a small hemiparasite can significantly affect prairie productivity regardless of resource supply.