Field experiments were conducted to determine the effect of nitrogen fertilization on competition between wild oat (Avena fatuaL. # AVEFA) and spring wheat (Triticum aestivum‘Anza’). Nitrogen fertilizer treatments were applied over several wild oat-wheat density combinations. Wheat grain yield in wild oat-infested plots generally declined with fertilization while the density of wild oat panicles increased. Apparently, in competition with wheat, wild oat was better able to utilize the added nitrogen and thus gained a competitive advantage over the wheat. The increased competitiveness of wild oat resulted in reduced crop yields. Under the conditions of these experiments, nitrogen fertilization resulted in positive wheat yield response only when the wild oat plant density was below 1.6 percent of the total plant density.
Field experiments were conducted to measure the grain yield of wheat (Triticum aestivumL. ‘ANZA’) at various wild oat (Avena fatuaL. ♯ AVEFA) and wheat plant densities. Wheat yield declined as wild oat plant density increased. Wheat yield increased in wild oat-infested plots as wheat plant density increased. Regression models were developed to describe the combined effect of wheat and wild oat plant densities on wheat grain yield. Wheat yields were best described by a nonlinear regression model using the relative density of wild oat in the weed-crop stand as the dependent variable. Inclusion of crop stand as a competitive factor significantly improved the fit of all regression models tested. Wild oat were more competitive against wheat in these experiments than in experiments reported by others. Possible reasons for differences in observed competitiveness are discussed.
Using the NCAR CAM3 model in aquaplanet configuration, we perform a suite of simulations spanning a broad range of warm climates. The simulations show a spontaneous transition to superrotation, i.e., westerly winds at upper levels above the equator. The momentum convergence leading to superrotation is driven by eastward-propagating equatorial waves with structure similar to the modern MaddenJulian Oscillation (MJO), whose amplitude increases strongly with temperature. We analyze the moist static energy (MSE) budget of the model's MJO to identify mechanisms leading to its enhanced amplitude. Two such mechanisms are identified: a rapid increase of mean low-level MSE with rising temperature, as found in previous work, and reduced damping of the MJO by synoptic-scale eddies. Both effects imply a reduced gross moist stability and enhanced MJO amplitude. The reduced eddy damping is caused by the transition to superrotation, which allows a greater penetration of extratropical eddies into the equatorial zone; the dominant effect of this greater penetration is to flatten the meridional gradient zonal-mean MSE, which effectively impedes the generation of anomalous MSE divergence by MJO-modulated eddies. This mechanism implies a positive feedback between superrotation and the MJO which may hasten the transition into a strongly superrotating state.
Because of declining anadromous fisheries, resource managers are concerned about the timing and quantity of water flows in Northern California's Scott River. We analyzed 48 years of flow and precipitation data to improve our collective understanding of the Scott River fall flow regime and to provide information for current and future fisheries-restoration efforts. Fall flows are primarily controlled by the water content of snow and precipitation during the previous 12 months. These two factors account for nearly 80% of the variation in fall flows. Our analysis shows that downward trends in fall flows appear to be explained by a decrease in the water content of the snow falling on the Scott River watershed. This information will be useful in assessing the relative benefits of conservation and restoration strategies against the larger background of climate-caused changes in river flow.
Noncropland such as levees, roadsides, field borders, fencerows, and wildlife areas are vulnerable to weed invasion. Many sites have undergone frequent human disturbance, such as manipulation from surrounding land uses, and lack competitive, desirable vegetation. This study addressed the importance of revegetation in an integrated weed management program including revegetation for noncrop areas. The study evaluated 14 cool-season perennial grasses (seven native species and eight introduced species) for their establishment, vigor, and ability to suppress weeds. It also evaluated the impact of herbicides on weed control and grass establishment. Treatments were applied at three noncrop sites in Northeast California that were heavily infested with weeds. Chemical weed control during the year of seeding and the following year was critical for perennial grass establishment. Weed cover was greater than 50% whereas average seeded grass cover was less than 6% in untreated plots at all sites 2 yr after seeding. In contrast, average seeded grass cover at all sites was 22 to 31% 2 yr after seeding for treatments where herbicide use resulted in wide-spectrum weed control and grass safety. Increasing perennial grass cover decreased total weed cover across perennial grass species 1and 2 yr after seeding. Individual grass species' cover differed among sites. Two introduced grasses (tall wheatgrass and crested wheatgrass) and three native grasses (western wheatgrass, bluebunch wheatgrass, and thickspike wheatgrass) showed broad adaptation and had > 20% cover at all sites 2 yr after seeding. In herbicide-treated plots, these grasses reduced total weed cover by 43 to 98% compared to unseeded plots 2 yr after seeding.
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