Hemiparasitic plants are photosynthetic but acquire water and minerals from nearby plants through root haustoria. Consequently, hemiparasites may simultaneously compete with and parasitize neighboring plants. Based on observations of Pedicularis canadensis from two prairie sites we hypothesized that this hemiparasite is a strong parasite but a weak competitor for light, and so its impact on a community is a function of resource supply. We predicted: (1) that parasitism by P. canadensis, a short perennial growing in a restored tallgrass prairie, would reduce community productivity; (2) functional groups of species would differ in response to parasitism, and (3) reduction in productivity due to parasitism would be greatest under low nutrient conditions where vegetation is thin and light levels are consequently high (promoting parasite growth), and least under high nutrient conditions where hosts grow vigorously and shade the hemiparasite (impeding parasite growth). To test this hypothesis we conducted a four-year experiment in which 1 m2 quadrats were given one of eight treatments that were combinations of three factors: hemiparasite removal, fertilizer addition, or light reduction (shade). The aboveground dry masses of grasses, legumes, remaining forbs and the hemiparasite from the center of each plot were determined.
Results/Conclusions
Removal of the hemiparasite, addition of fertilizer, and full sun significantly increased total aboveground productivity but contrary to prediction, the impact of P. canadensis was independent of resource level. Removal of P. canadensis almost doubled the mass of grasses. Forbs were less affected, and removal had no impact on legumes. Effects of fertilizer depended on light levels and varied among functional groups. Fertilizer increased grass growth only with high light, increased forb growth only under low light, and depressed legume growth under low light. Surprisingly, light manipulation did not affect the hemiparasite but fertilizer increased P. canadensis shoot mass. These results indicate that a small hemiparasite can significantly affect prairie productivity and are in accord with studies of other systems. Further, because parasitism exerted a greater negative effect on the dominant grasses, parasitism may alter community structure. Our investigation is currently examining this effect.
