Resource Competition between a C3 and a C4 Grass Species: Individual and Population Level Responses
Farrer, Emily Christine
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Although plant competition is generally accepted as one of the major determinants of species composition in a community, there is still much debate surrounding the actual mechanisms of plant competition, especially what determines a good competitor and how competition is influenced by environmental stress. These questions were addressed in the context of two different photosynthetic pathways, C3 and C4 photosynthesis, each conferring different inherent traits to the grasses Danthonia spicata and Schizachyrium scoparium, respectively. This study consisted of three different sections. The first was analysis of percent cover data from year 2000 of the two grass species in experimental plots in Allegan State Game Area, in Michigan. The plots consisted of control, fertilizer, and early spring disturbance treatments and their effects on the percent cover of Schizachyrium and Danthonia were analyzed. Danthonia, a spring-flowering species, was greatly set back due to the early spring disturbance, and Schizachyrium exhibited a large growth response in the disturbed plots, most likely due to the lack of Danthonia competition. The second section of this study was a field experiment in 2001 in Allegan, measuring the effects of the same treatments on population size and individual traits such as growth, reproduction, and resource allocation. Danthonia, at population and individual levels, was still feeling the effects of the spring disturbance, while Schizachyrium growth was still elevated in the disturbed plots. The third section of this study was a greenhouse neighbor experiment, which had more power to isolate competitive effects than the field studies. In some cases, Schizachyrium was found to be a better competitor than Danthonia, but in other cases, their competitive effects were similar. These results are discussed in the context of the two opposing competitive theories, Grime's triangular model of primary plant strategies and Tilman's resource reduction hypothesis. Mechanisms of coexistence of the two grasses, such as temporal resource partitioning are also discussed.