• Competition between cheatgrass and two native species after fire: Implications from observations and measurements of root distribution

      Melgoza, G.; Nowak, R. S. (Society for Range Management, 1991-01-01)
      During 1987 and 1988, a study was conducted in northern Nevada to examine root growth of cheatgrass (Bromus tectorum L.) and 2 native species, needle-and-thread grass (Stipa comata Trin. & Rupr.) and rabbitbrush (Chrysothamnus viscidiflorus (Hook.) NW.), after fire. Profile wall maps were used to determine the distribution of roots in the soil profile for the 2 native species. Root morphology differed between the 2 species: needle-and-thread grass had a flabelliform root system, whereas rabbitbrush had a main tap root with 2-4 major lateral roots. Although total root biomass differed between the species, more than half the root biomass was in the top 0.2 m of soil for both needle-and-thread grass and rabbitbrush. Measurements of root length density were used to evaluate the interaction between root systems of cheatgrass and the native species. Root production of plots with only the native species was not significantly different from that of plots with both the native species and cheatgrass for the first 2 years after fire. Furthermore, root production of plots in a recently burned area was also not significantly different from that in an area burned 12 years prior to our study. Thus, root systems of these species rapidly occupied the belowground space and competed for soil resources after fire, and the presence of cheatgrass partially reduced the root systems of the native species.
    • Photosynthetic Characteristics of Crested Wheatgrass and Bluebunch Wheatgrass

      Nowak, R. S.; Caldwell, M. M. (Society for Range Management, 1986-09-01)
      Light and temperature dependencies for net photosynthesis and stomatal conductance were generally very similar between foliage on crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult.) plants and that on bluebunch wheatgrass (A. spicatum (Pursh) Scribn. and Smith) plants. The similarity of these gas exchange characteristics between the 2 bunchgrass species was true for foliage on unclipped plants as well as on partially defoliated plants. However, light and temperature dependencies of senescing leaf blades that were exserted in late-spring were significantly different for unclipped plants of these 2 species. Photosynthetic rates and stomatal conductances of senescent late-season blades on bluebunch wheatgrass plants were greater than those on crested wheatgrass plants at light intensities greater than 0.8 mmol photons m-2 s-1 (photosynthetic photon flux density) and at all foliage temperatures between 18 degrees C and 41 degrees C. These greater photosynthetic rates and stomatal conductances do not mean that bluebunch wheatgrass tillers gained substantially more carbon or lost substantially more water than crested wheatgrass tillers. If both the photosynthetic area composition of tillers and the environmental conditions of the northern Utah study site were considered, carbon gain and water loss for individual bluebunch wheatgrass tillers would be very similar to those for individual crested wheatgrass tillers despite the significantly different responses to light and temperature during mid-summer.