A wildfire in August 1984 eliminated big sagebrush from a shrub-steppe community in southeastern Washington. Bluebunch wheatgrass dominated the community in subsequent years. By comparing leaf area index and soil water storage on the burned site with an adjacent unburned site containing big sagebrush, we were able to test the hypotheses that plant community structure controls soil moisture dynamics and that wildfire can alter the water balance in these arid landscapes. Elimination of big sagebrush from the landscape greatly reduced the community leaf area index. Soil water storage computed to a depth of 125 cm was not affected by the change in community structure. However, storage computed to a depth of 275 cm was significantly greater on the burned site compared to the unburned site. This difference increased with time after the fire. Wildfire in shrub-steppe communities dominated by big sagebrush effectively removes the deep rooted, woody component from these ecosystems. Consequently, the ability of the vegetation to remove stored soil water is inhibited by a reduction in the ability to access deeply stored soil moisture.

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Downy brome (Brooms tectorum L.) is an alien grass that dominates disturbed ground in shrub-steppe ecosystems of the western United States. Responses of downy brome to added nitrogen and water were evaluated using intact soil cores obtained from an old field. Gas exchange data were gathered at the leaf and canopy scales. Stomata1 conductance and net photosynthesis rates were greater at the leaf scale than at the canopy scale, decreased with time from germination, and were weakly affected by treatments. Water-use efficiency was weakly related to time from germination and treatments. Biomass was greater in the nitrogen-plus-water (7.4 g) treatment, compared with water (3.6 g), nitrogen (4.5 g), and control (3.3 g) treatments. The leaf-area indes varied like biomass at the end of the esperiment. Shoot nitrogen was the same in the nitrogen (2.5%) and nitrogen-pluswater (2.5%) treatments, nearly twice the level in the control (1.5%) and water (1.3%) treatments. Nitrogen-use efficiency was highest in the control (67) and water (80) treatments and lowest in the nitrogen (41) and nitrogen-plus-water (43) treatments. The most significant conclusion of this work is that gas exchange was strongly related to the time from germination and little affected by water and nitrogen while growth characters were strongly affected only when water and nitrogen were added together. (Bromrrs tectorunt L.) have come to dominate resource-limited rangelands in the western United States (Klemmedson and Smith 1964, Mack 1981, Billings 1990). Downy brome shoot and root biomass increased with additional nitrogen in the field (Hulbert 1955). Biomass production was significantly related to stored soil moisture (Cline and Rickard 1973, Uresk et al. 1979). Cline and Rickard (1973) found that nitrogen was more limiting than soil water for shoot biomass production over several years of observation in the field. Past studies have investigated the response of downy brome production to nitrogen and water, separately. There have been no studies investigating the interaction of nitrogen and water on the physiology and growth of downy brome. The purpose of this study was to investigate the growth and gas exchange response of downy brome to separate and combined additions of nitrogen and water. We examined growth and gas exchange responses to water and nitrogen as a function of the time from germination. Gas exchange observations were taken at leaf and canopy scales.

Isolation of radioactive waste buried in unsaturated zones will require long‐term control of recharge and erosion. Soil covers control recharge at arid sites by storing rainwater close enough to the surface to be removed by evapotranspiration. Surface layers of rock or gravel control erosion at sites with sparse vegetation, but can also alter plant habitat and cause recharge through interred waste. As an alternative, gravel mixed into the uppermost soil layer may control erosion over the long‐term better than surface gravel layers. We postulated that gravel admixtures also would not influence plant establishment or soil water balance in waste‐site covers. We measured the interactive effects of gravel admixture concentration, vegetation, and precipitation on soil water content and plant cover at the U.S. Department of Energy's Hanford Site in Washington state. Our results support the use of a combination of vegetation and gravel admixtures for erosion control. Vegetation seasonally depleted rootzone water storage to about 6.5 volume % regardless of precipitation amount or the presence of gravel admixture amendments. In contrast, yearly increases in soil water storage as deep as 225 cm in plots without vegetation may be a leading indicator of recharge. The composition and abundance of vegetation changed over time and in response to precipitation amount, but was not influenced by gravel amendments. Seeded wheatgrasses [Agropyron sibericum Wilde and Agropyron dasystachyum (Hook.) Scribn.] established only when irrigated with twice average precipitation (32 cm/yr), but persisted after the irrigation ceased. Cheatgrass (Bromus tectorum L.) and Russian thistle (Salsola kali L.) colonized areas that received both irrigation and ambient precipitation. Stands with wheatgrasses extracted water more rapidly and depleted soil water to lower levels than cheatgrass‐dominated stands. Increases in gravel cover and near‐surface gravel concentrations after 5 yr were evidence of the formation of a protective gravel veneer.

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