Coming undone: Hemiparasite effects and presence in a prairie diminish over time

Background/Question/Methods Root hemiparasites are green plants that tap into neighbors’ vascular systems to extract resources and this parasitism strongly reduces host growth and depresses community productivity. Hemiparasites can exert keystone effects if parasitism differentially depresses dominant species. Hemiparasite effects are predicted to be strongest in habitats where hosts are nutrient-stressed, resulting in light levels adequate for low-lying hemiparasites. In 2006, we began a field experiment testing whether hemiparasite effects on communities are context dependent. Three years of hemiparasite removal and nutrient manipulation in plots on a reconstructed prairie demonstrated: (a) the clonal, perennial hemiparasite Pedicularis canadensis significantly reduced productivity, (b) that effect of hemiparasitism was expressed most strongly in graminoids, (c) the impact of the hemiparasite was independent of light or fertilizer, but (d) fertilizer increased growth of the hemiparasite, non-legume forbs, and especially, graminoids. In this talk we present results after 7 additional years of hemiparasite removal and fertilizer treatment of experimental plots. We again sampled dry biomass to determine if the previous conclusions still held. In addition, we evaluated cover in the 48 non-removal plots and calculated community metrics (Shannon-Weiner, Evenness, Floristic Quality Index) as an alternative means of evaluating the hemiparasite’s effects on the prairie.

Results/Conclusions In 2015, community productivity no longer responded to hemiparasite removal. Pedicularis canadensis had disappeared from 24 of the 48 non-removal plots, and thus its influence on community productivity diminished. Fertilizer still increased productivity but now non-legume forbs responded most strongly. Mean hemiparasite biomass declined in fertilized plots but not in control plots, although this effect varied spatially. Hemiparasite decline was associated with diminished light due to increased growth of surrounding vegetation. Hemiparasite cover significantly affected the community indices, with diversity (Shannon-Wiener) responding most strongly and positively. The Floristic Quality Index, a measure of “floristic integrity” of a site, was not significantly associated with hemiparasite cover. These observations indicate that hemiparasites have strong effects on the community and respond to abiotic factors. The distribution and impact of this keystone species can be significant but also temporally dynamic.