Competition at the population level along a standing crop gradient: a field experiment in successional grassland

I measured competitive responses of experimentally-established populations of the perennial grass, Andropogon gerardi, across a complex gradient of st anding crop and species composition in the successional grasslands of south west Michigan. The goal was to assess whether long-term (three year) popula tion-level responses of Andropogon to competition matched the inferences ma de from a previous phytometer study that examined transplant responses to c ompetition across this same gradient over a single growing season. Replicate experimental populations of Andropogon were established at seven grassland sites by sowing seed into 0.5 x 0.5 m plots that had been denuded of all vegetation. During the first year of the study, all Andropogon popu lations were maintained as monocultures by hand weeding. At the end of the first growing season, half of the monocultures were selected for continued weeding and half were left open to invasion by competitors for three years. Invasion of the unweeded populations by neighboring plants varied strongly among sites and was positively correlated with standing crop. Increased su sceptibility to invasion and competition resulted in the extinction of the unweeded Andropogon populations at the two most productive sites, supportin g the hypothesis that Andropogon is restricted by competition to low produc tivity sites in these grasslands. The finding that the intensity of competi tion was positively correlated with standing crop is consistent with the pr evious transplant study, suggesting that short term experimental assays of competition on the growth of individual transplants may have predictive val ue for longer-term outcomes of competition at the population level.