Long-term effects of big sagebrush control on vegetation and soil water.

Herbaceous productivity of mountain big sagebrush (Artemisia tdnlataspp. vsyana)areasspnayed with 2,4-D(2,4-dichlorophenoxyacetic acid) was nearly twice that of untreated areas 10 years after spraying, while the number of sagebrush plants on treated areas was 4% of that before spraying. Soil at the Wyoming study site was a Youga loam (Argic Cryoboroll). On treated areas, soil water depletion from the surface 0.9 m of soil slightly exceeded that of untreated areas beginning the third year after spraying when herbaceous vegetation had fully responded to release from sagebrush competition. Water depletion in soil 0.9 m to 1.8 m deep was substantially less on sprayed areas compared to unsprayed areas. Seasonal water depletion in the surface 1.8 m of soil was reduced 31% the year of treatment, and about 7% between 5 and 11 years after treatment. Mathematical relationships were developed to predict the effect of sagebrush control on seasonal water depletion in the surface 1.8 m of soil, the surface 0.9 m of soil, and soil 0.9-1.8 m deep. Control of big sagebrush (Artemisia tridentata) with the herbicide, 2,4-D (2,4-dichlorophenoxyacetic acid) beginning in the late 1940's revolutionized management of sagebrush rangeland. Millions of acres have been sprayed to benefit livestock forage production. The response by vegetation to spraying has been investigated at numerous locations in years immediately after treatment, but information about reestablishment of big sagebrush on treated areas is less common. Both biotic and abiotic values are affected by spraying. Hydrologic impacts, for example, have received little attention compared to vegetative responses. The soil water regime may be altered when deeply rooting big sagebrush is replaced by shallower rooting herbaceous species and there is speculation that treatment can increase water yields in some locations. This paper focuses upon the responses by vegetation and soil water to big sagebrush control 6 to 11 years after treatment. Treatment responses in the first 5 years were previously reported (Sturges 1977). Vegetation Characteristics Spraying big sagebrush vegetation with 2,4-D commonly increases grass production 2 to 3 times above pretreatment levels, where adequate populations of herbaceous species are present (Hull et al. 1952, Hyder and Sneva 1956, Tabler 1968, Miller et al. 1980). Forb production is suppressed by spraying, but control of big sagebrush by burning does not greatly alter vegetative composition (Harniss and Murray 1973, Nimir and Payne 1978). Sagebrush control is not a permanent type conversion, but the time required for big sagebrush to return to pretreatment levels is quite variable. Harniss and Murray (1973) found that grass and forb production remained above preburn levels for 12 years after a mountain big sagebrush (A. tridentata spp. vaseyana) stand was burned, but was below preburn levels 30 years after treatment. The useful life of a spray project in Oregon exceeded 17 years (Sneva 1972). Sagebrush was a minor vegetation component in the first Author is research forester, Rocky Mountain Forest and Range Experiment Station, Laramie, Wyo. 82070. Station headquarters is in Fort Collins, in cooperation with Colorado State University. Portion's of the research were supported by the Bureau of Land Management, U.S. Department of the Interior. Manuscript received July 22, 1982. decade after treatment, but sagebrush density was similar to untreated vegetation in the fifteenth year. Big sagebrush reestablishment was investigated by Bartolome and Heady (1978) in another Oregon study. The age of sagebrush plants growing on sprayed areas indicated that treatment seldom killed all plants and that most reestablishment occurred in years immediately following treatment. Thilenius and Brown (1974) and Johnson (1969) investigated the return of big sagebrush to sprayed areas in Wyoming. Soil Water Withdrawal Characteristics Changes in the soil water regime that follow control of big sagebrush depend upon rooting depths of sagebrush and replacement herbaceous species and upon the depth of soil water recharge (Sturges 1977). Several studies indicate that water withdrawal in the surface meter of soil decreases slightly the first 2 years after sagebrush control. (Sonder and Alley 1961, Cook and Lewis 1963, Tabler 1968, Shown et al. 1972, Sturges 1977). Thereafter, water use by treated and untreated vegetation is similar. A substantial reduction in seasonal water withdrawal was detected at 2 Wyoming sites when mountain big sagebrush was sprayed. Tabler (1968) found that seasonal evapotranspiration decreased 14% the second year after spraying based on measurements to a 1.8 m soil depth, while Sturges (1977) detected differences of 19, 15, and 8% in the first, second, and fifth year after treatment, respectively. Treatment differences in seasonal water use were located almost entirely in soil 0.9 m to 1.8 m deep and accrued while vegetation was actively growing. No soil water study extended more than 5 years beyond the treatment year.