Community composition, demographics, and competitive responses in three prairie grasses.

Abstract

My thesis research focused on the influence of successional changes on community structure, life history trait expression, and competitive responses of three codominant prairie grasses, Andropogon scoparius, A. gerardii, and Sorghastrum nutans, growing in a 22-year old field and an undisturbed native tallgrass prairie. Results of a comparison of community composition between the two habitat types (Chapter 1) demonstrated that species richness was highest in the old field, but that percent coverage by the three target grass species was lower in the old field. These three species predominated in the native prairie habitat but were replaced by other species of perennial grasses and perennial forbs in the old-field site. The presence of a target plant species was correlated with a decrease in species richness and species diversity in the old field but not in the native prairie. The influence of successional stage on the expression of life history traits was investigated in Chapter 2. All three species grew at significantly different rates; native-prairie plants generally grew faster. All three species were taller in the native prairie in 1994, but had greater plant basal area in the old field in both years, suggesting that plants in the two habitats differ in their investment into particular types of vegetative growth. All three species showed different patterns of allocation to reproduction in the two successional stages. The proportion of plants flowering and reproductive effort was higher in the native prairie for A. gerardii and S. nutans, while individuals of A. scoparius flowered in a significantly higher proportion and had greater reproductive effort in the old field. Results of a common garden experiment using S. nutans from both habitat types showed little difference in the expression of three life history traits. This suggests that differences in these traits exhibited by plants in the field are the result of phenotypic plasticity by similar genotypes in response to the two different environments. In Chapter 3, I examined the effects of release from competition on S. nutans in the field. Experimental plants in both the old-field and the native-prairie sites increased by vegetative expansion into the competitor-free zone, however, the effect was only significant in old-field plants. Plants in both sites produced fewer reproductive structures on the competitor-free side. Results of this study suggest that resource-allocation trade-offs between vegetative and reproductive growth have enabled these species to locally adapt to their successional stage. All three species have flexible life history strategies and supercompetitive abilities that lead eventually to successional dominance in the tallgrass prairie community.