Stipa

Abstract: Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.

Abstract: Summary Six open-top chambers were installed on the shortgrass steppe in north-eastern Colorado, USA from late March until mid-October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO2. Three chambers were maintained at current CO2 concentration (ambient treatment), three at twice ambient CO2, or approximately 720 μmol mol−1 (elevated treatment), and three nonchambered plots served as controls. Above-ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C3 and C4 grasses. Mid-season and seasonal above-ground productivity were enhanced from 26 to 47% at elevated CO2, with no differences in the relative responses of C3/C4 grasses or forbs. Annual above-ground phytomass accrual was greater on plots which were defoliated once in mid-summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO2 on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C3) and Bouteloua gracilis (C4) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO2-grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO2, but only Stipa comata (C3) plants exhibited significant reductions in N under elevated compared to ambient CO2 chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above-ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO2 enriched environments.

Abstract: The spatial distribution of some chemical and physical soil properties was determined in the microenvironment of individual plants of three matorral species (Rosmarinus officinalis L., Stipa tenacissima L., Anthyllis cytisoides L.) in an area of patchy vegetation in south-east Spain. The influence of isolated plants on the chemical and physical status of the topsoil decreased significantly from the plant axis to the bare inter-plant area, giving rise to “islands” of improved topsoil properties beneath each plant (i.e., increased organic matter content and aggregate stability, and decreased bulk density, penetration resistance, shear strength and rock fragment content and cover). The degree and lateral extent of topsoil modification was significantly more pronounced under Rosmarinus and Stipa plants than under Anthyllis shrubs, mainly because of the differential influences of species morphology (i.e., above-ground structure) and components (i.e., canopy and litter covers) on soil properties and erosion/sedimentation processes.