Hemiparasite – host plant interactions and the impact of herbivory: a field experiment

Abstract

Schizachyrium scoparium Michx. (Nash) growing naturally with varying numbers of the perennial hemiparasite Pedicularis canadensis L. were randomly assigned to one of four clipping treatments (none, early, late, early and late) to determine how parasitism and herbivory affect the grass and whether herbivory of the host indirectly affects hemiparasite growth. Any clipping eliminated subsequent flowering by S. scoparium in year 1 and reduced the number of plants that flowered in year 2, when no clipping occurred. Only hosts clipped early exhibited depressed growth after one summer. The following year plants that had been clipped twice the previous year produced the least shoot mass, and plants that were never clipped produced the most. Hemiparasite load was negatively associated with host shoot mass, especially in year 1, but did not alter the host’s compensatory response to clipping. The effects of host size and host clipping on the nearest hemiparasite were determined in year 1. Pedicularis canadensis shoot mass declined with host size if the host was clipped late, but increased with host size when clipping occurred once early in the season. Although the impact of hemiparasites and clipping on host growth are independent, clipping can alter the value of the host for parasites.