Scott D. Bridgham

The Role of Edaphic Factors in Determining Plant Community Structure in Upland Prairies in the Pacific Northwest
           

Collaborators:  Bitty Roy and Bart Johnson (University of Oregon)

Graduate Student:  Laurel Pfeifer-Meister (University of Oregon)

Undergraduate Honors Student:  Esther Cole

            Bitty Roy and Bart Johnson began an experiment to examine the response of a fairly high quality upland prairie in Buford County Park on Mt. Pisgah near Eugene to fire as a potential restoration tool to enhance native plant diversity.  They set up eight plots that were burned in the spring of 2003 and eight plots were left as unburned controls.  Their results indicated that the effects of burning on restoring native plant diversity were highly dependent on differences in soils across the site.  Thus, Laurel Pfeifer-Meister and I characterized seasonal soil moisture and nutrient mineralization dynamics, as well as other soil geochemical characteristics, in the plots relative to plant community composition and productivity.  Our goal is to determine how soil factors determine the relative competitive success of native vs. exotic prairie plant species.

            We performed paired field and greenhouse experiments to evaluate the competitive dynamics between two native (Danthonia californica and Deschampsia cespitosa) and two exotic (Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions at this site (Pfeifer-Meister et al., submitted).  In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated.  In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum,produced more aboveground biomass when competition was reduced.  In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses.  Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses.  L. multiflorum dominated competitive interactions with all other grasses across treatments.  In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica.  D. californica, D. cespitosa,and S. arundinaceus all produced more biomass in high moisture, high nutrient environments, and D. cespitosa, L. multiflorum,and S. arundinaceus allocated more biomass belowground in the low nutrient treatment.  Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient rich, moderately wet environments.

            We also examined the seasonal importance of moisture and temperature as controls over nitrogen and carbon mineralization and phosphorus availability in soils from these sites (Pfeifer-Meister et al., in press).  Across the prairie, we collected soils seasonally and measured microbial respiration, net nitrogen mineralization, net nitrification, and phosphorus availability under field conditions and under experimentally varied temperature and moisture treatments.  The response variables differed in the degree of temperature and moisture limitation within seasons and how these factors varied across sampling sites.  In general, we found that microbial respiration was limited by low soil moisture year-round and by low temperatures in the winter.  Net nitrogen mineralization and net nitrification were never limited by temperature, but both were limited by excessive soil moisture in winter, and net nitrification was also inhibited by low soil moisture in the summer.  Factors that enhanced microbial respiration tended to decrease soil phosphorus availability.  A variety of edaphic factors (% moisture, total organic carbon, % clay, pH, and inorganic nitrogen and phosphorus) explained 76% of the seasonal and spatial variation in microbial respiration, 35% of the variation in phosphorus availability, and 29% of the variation in net nitrification.  Much of the variation in net nitrogen mineralization remained unexplained (R2=0.19).  This study, for the first time, demonstrates the complex seasonal controls over nutrient cycling in a Pacific Northwest prairie.

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