Interspecific competition for resources among plant species has often been assumed to be a dominant process controlling species composition and abundance. Although the occurrence of competition has often been demonstrated, the mechanisms underlying competitive outcomes have seldom been investigated. In these studies, I determined the competitive mechanism for two co-occurring moss species, Polytrichum commune and P. juniperinum, and how light and water availability interact to control which species has a competitive advantage. Spatial patterns of abundances in natural mixtures and in natural monocultures over light and evaporation rate gradients suggested that P. commune increases its competitive effect on P. juniperinum as light and/or evaporation increase, whereas P. juniperinum decreases its competitive effect on P. commune as light and/or evaporation increase. P. commune has a higher net photosynthetic rate than P. juniperinum, which may contribute to its competitive advantage over the gradient. In contradiction to expectations, P. juniperinum had a competitive advantage over P. commune in field experiments. P. juniperinum suppressed recruitment of P. commune whereas P. commune had no effect on P. juniperinum recruitment. P. juniperinum also increased in undisturbed mixtures during this time. Recruitment was found to be dependent on light reaching the soil surface and intact moss canopies deplete light to low levels at the soil surface (1-3% of incident). Thus, competition for light is probably the primary mechanism determining the recruitment and abundances of these species. The ability to deplete light depends on the maintenance of tissue water content: P. commune loses water more rapidly but replaces it more effectively from the soil, whereas P. juniperinum loses water more slowly and cannot replace it as effectively. Thus, if soil moisture is available, P. commune should have a competitive advantage whereas if soil moisture is not available, P. juniperinum should have a competitive advantage. Therefore, the long-term outcome, coexistence or competitive exclusion, should depend on the frequency of moderate droughts.Ph.D.Biological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105059/1/9023581.pdfDescription of 9023581.pdf : Restricted to UM users only.