Date of Award
Master of Science
An important ecological issue is determining how plants enter a community. The members of a community must pass through ecological filters to be present at a site. A plant must be present in the regional species pool and first overcome regional or propagule filters, then local or niche filters to establish. Therefore, if a plant is present in the regional species pool, but does not exist in a community it must be being filtered. An important filter that affects community composition is the recruitment stage (Tilman 1997). This stage can be used to examine if a species is limited by propagule or niche filters. If a seedling not present in the community is planted and establishes well, it has escaped its limitation and was previously propagule limited or niche limited at the germination stage. If a seedling not present in the community is planted but does not establish well, it is still filtered by niche limitations. Through this simple experimental design, much can be inferred by the addition of propagules. Propagules were added in the form of seed and seedlings to an old-field in southern Illinois. Mowing (spring and control) and fertilizer (annual and control) treatments had been applied to the old-field for 11 years resulting in different community types. Seedlings from three functional types (warm season grasses, cool season grasses, and legumes) and three abundance types according to presence in plots (abundant, present, and absent) were monitored to determine survivorship and relative growth rate. At the end of the season the seedlings were clipped to determine biomass. Seed traps were used to analyze the seed rain within the plots. Disturbance through mowing significantly increased the success of seedlings. There was no difference in soil nitrate between fertilized and non fertilized plots thus fertilizer had little influence on seedling performance and survivorship. Cool season grasses had the highest survivorship while warm season grasses had equivalent or higher relative growth rate than other functional groups. The establishment of legumes was unrelated to disturbance or fertilizer suggesting that their establishment was dependent on individual species characteristics more than those of the functional group as a whole. Present species performed better than species of other abundance types suggesting that they were the most propagule limited and supporting the idea of complementarity (that invader success is limited when functionally similar species are already present in the community). The study indicates that future composition in this old-field may be predicted with C3 grasses more successful in the long term as propagule limitation is overcome, but disturbance is needed to maintain the C4 grass cover. In order to increase evenness, present species (which are likely propagule limited) should be introduced into the community. Particularly aggressive abundant species that are not niche limited should be monitored closely and removed when possible to increase resources for other species since less abundant species have the potential to expand their population. However, an old-field community is most likely to be invaded by individuals whose traits are unlike the dominant species after disturbance releases available resources.
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