Variation in Vesicular-Arbuscular Mycorrhizal Relationships of Two Sand Prairie Species

Abstract

We tested the hypothesis that western sunflower (Helianthus occidentalis) reduces abundance of prairie species within its colonies by producing allelochemics that inhibit seed germination or vesicular-arbuscular mycorrhizal (VAM) formation. Germination of seeds of prairie species was not significantly (P <0.05) inhibited by water extracts of whole sunflower plants or soil collected inside the colony. Comparisons were made of VAM inoculum potential between soil collected from within a sunflower colony and an adjacent area outside the colony dominated by sand lovegrass (Eragrostis trichodes). Significant differences in VAM inoculum potential were found due to season but not due to site of collection. Sunflower roots were significantly more colonized (75 vs. 34%) and more infectious in bioassay tests than lovegrass roots. Root mass (mg/cc) was significantly greater outside the colony than inside, possibly accounting for similarity in inoculum potential between the two areas. Differences in VAM colonization levels and infectivity between sunflower and lovegrass suggest different mycorrhizal relationships for these tapand fibrous-rooted species, respectively. INTRODUCTION Field observations suggested that many common sand prairie species had reduced abundances within dense colonies of western sunflower (Helianthus occidentalis) compared to their abundances in adjacent areas outside of the colonies. This work was initiated to determine the causes of these reduced abundances. It is possible that because of their rhizomatous growth patterns that once established, sunflower could eliminate other species from their colony through competitive exclusion. However, several workers have reported that sunflower species, including annual, ashy and prairie sunflowers, have allelopathic properties (Curtis and Cottam, 1950; Wilson and Rice, 1968; Rice, 1974; Anderson et al., 1978). We hypothesized that western sunflower produced allelochemics that either inhibited plant growth directly and/or interferred with vesicular-arbuscular mycorrhizal (VAM) formation. Interference with mycorrhizal formation in red maple by red pine has been demonstrated by Tobiessen and Warner (1980), and Kovacic et al. (1984) found that mycorrhizal inoculum was absent in the understory of ponderosa pine. Nearly all dominant sand prairie species are mycorrhizal. Sand prairie soils are droughty and contain low levels of available inorganic nutrients (Dickman et al., 1984); therefore, inhibition of mycorrhizal formation by sunflower could reduce the competitive ability of other species. Our study site is located in W central Illinois, Mason Co., in the Sand Prairie Scrub Oak Nature Preserve. The vegetation in the preserve is a mosiac of sand prairie, savannah and closed oak-hickory forest (Anderson and Brown, 1983). Western sunflower is an occasional species in the sand prairie. Our study was conducted using a single sunflower colony. METHODS The perimeter of the sunflower colony was mapped and the area of the colony estimated by drawing a scale diagram of the colony and determining the area of the diagram with a planimeter. Vegetation inside the colony and in an area immediately adjacent to the colony was sampled using a hundred 10 cm x 10 cm quadrats within and outside the colony. Quadrats were located approximately 1 m apart using a stratified random sampling procedure. Plant cover, irrespective of species, projecting over the quadrat was estimated, flowering and nonflowering rosettes of western sunflower rooted