Response of Four Shrub Species to Different Water Source Components in an Arid Environment

Shrubland species in the Great Basin (USA) depend on soil water recharged from precipitation and/or groundwater for survival and growth. Climate warming and possible basin water diversion could alter the amount and timing of water availability to these plants. The objective of this study was to quantify the extent to which each of four co-occurring shrub species, big sage [Artemisia tridentata], rabbitbrush [Ericameria nauseosus], greasewood [Sarcobatus vermiculatus] and shadscale [Atriplex confertifolia)) acquired water from different sources (precipitation, soil vadose zone and/or groundwater) during a growing season. Soil salinity increased linearly with depth over the upper 1.5 m of soil, with salinity ranging from 0.84 to 31.70 dSm-1 in saturation extracts (R2=0.78, p<0.001). Changes in soil water both with depth and time during the growing period indicated that all species accessed soil water from precipitation recharge. Evapotranspiration totals for the growing period exceeded total precipitation by 137 mm, indicating that plants also used water stored deeper within the vadose zone and/or from groundwater (particularly) by the phreatophyte greasewood. Delta18O in the soil solution declined linearly with depth over the upper 100 cm (R2=0.80, p<0.001). Delta18O values in greasewood corresponded closely to Delta18O values measured deeper in the vadose zone and groundwater. Output from a mixing model indicated a decrease in groundwater reliance for greasewood from 30% in July to 2% in September, with a major shift to deeper soil water in the vadose zone (180 cm depth) (38% in July to 97% in September). Our data suggested that the four shrub species at our site were able to coexist because of their different spatial, temporal, and physiological uses of available soil water, reflecting possible water resource partitioning based on differences in response to precipitation, ability to extract water at deeper depths and variable tolerance to elevated levels of soil salinity to access groundwater.

Black Greasewood (Sarcobatus vermiculatus), Gray Rabbitbrush (Ericameria nauseosa), and Perennial Grass Response to Chlorsulfuron and Metsulfuron

Black greasewood (greasewood) and gray rabbitbrush are important shrub components of several plant communities throughout western North America. Land managers may view these species differently depending on their goals. Greasewood and gray rabbitbrush shrub communities may be invaded by several invasive plants including Russian knapweed, perennial pepperweed, hoary cress, halogeton, and several annual mustards. Metsulfuron and chlorsulfuron are commonly used for controlling these and other invasive plants, but little is known regarding their impacts on greasewood and gray rabbitbrush. Our objective was to quantify the impact of these herbicides on greasewood and gray rabbitbrush communities from both an efficacy and nontarget impact perspective. Field studies were established in the spring of 2004 and repeated twice in 2005 near Laramie, WY, in a pasture with mixed stands of greasewood and gray rabbitbrush. Treatments included metsulfuron applied at 21, 42, 63, 84, 126, and 168 g ai/ha (0.3, 0.6, 0.9, 1.2, 1.8 and 2.4 oz ai/A), chlorsulfuron applied at 52, 105, and 157 g ai/ha (0.75, 1.5 and 2.25 oz ai/A), and an untreated control. All treatments contained methylated seed oil at 2% v/v. Treatments were applied in mid-June to 3.3 by 9-m (10 by 30 ft) plots with a handheld broadcast sprayer delivering 187 L/ha (20 gal/ac) in a randomized complete block, with three blocks per study. Plots were sampled 12 and 24 mo after treatment (MAT), utilizing visual control estimates and point frame sampling for vegetative cover of greasewood, gray rabbitbrush, perennial grasses, and bare ground. Metsulfuron at 42 g/ha and chlorsulfuron at 105 g/ha provided > 75% visual control of greasewood 24 MAT. For gray rabbitbrush, metsulfuron at 63 g/ha provided approximately 60% control 24 MAT, while chlorsulfuron provided negligible control at any rate. These results suggest differential impacts of these herbicides on greasewood and gray rabbitbrush, and provide land managers with a decision tool for noxious and invasive weed control when managing for or against greasewood and gray rabbitbrush.