Microsite and seed limitation in two communities dominated by contrasting grasses
The relative importance of microsite and/or seed limitation on grassland community structure can be influenced by community context. To investigate how two dominant grasses affect these limitations, we conducted a factorial microsite and seed limitation experiment in a sand prairie grassland in northwestern Illinois, USA. In two communities, dominated by either Schizachyrium scoparium, a native warm-season grass, or Bromus inermis, a non-native cool-season grass, three experimental manipulations were applied to 30 x 30 cm plots. New microsites (=gaps) were created by removing vegetation and roots, then replacing the soil (and seed bank) into the plot and allowing re-colonization via vegetative and local seed sources. Seeds of 21 native species were added to both undisturbed vegetation plots (=seed) and an additional set of gap plots (=seed + gap) to test for concurrent microsite and seed limitation.
In gap plots, species richness increased in the Schizachyrium, but not Bromus community. Recruitment from the local species pool was derived from all life histories and from both seed rain and seed bank. In seed addition plots, species richness did not differ from controls in either community. In seed + gap plots, species richness was greatest and species composition diverged most from control plots in both communities. Percent cover generally followed patterns of species richness. Individual sown species differed in being either microsite or seed limited, but most species had their greatest frequency in seed + gap plots. Low recruitment in gaps in the Bromus community may indicate inferior microsites and/or greater seed limitation in that community. In contrast, the low establishment of sown species in undisturbed vegetation suggests that traits common to both dominant species, such as high production of biomass and dead vegetation, may make communities difficult to invade, regardless of dominant species identity. Finally, results for combined gap and seed addition plots indicate removal of multiple limitations is necessary to achieve the greatest change in community structure and species composition, independent of dominant species identity.