No abstract

Multiple fires conducted in spring (March-April) and summer (June-July) on blue grama (Bouteha gmdis [H. B. K. Lag.]) grassland near Corona, N.M. were used to relate broom snakeweed (Gutierreziu sarohrue [Pursb] Britt & Rusby) control to pre-burn vegetation, weather, and fire conditions. Spring fws moved faster and burned cooler than summer fires as indicated by measurements from thermocouples giving the fire's rate of spread, temperature, and beat. In spring, broom snakeweed was in the bud stage with little green foliage and fves provided less average crown destruction (8 %) and shrub mortality (65 %) compared to summer fws (66% crown destruction, 92% mortality) when the shrub was growing actively. Air temperature and total fuel biomass positively influenced fire temperatures, and duration of beat above 6O'C resulting in high broom snakeweed mortality. Conversely, as relative humidity, wind speed, and fuel moisture increased, fue beat decreased, resulting iu less broom snakeweed mortality. Attempts to conduct spring or summer fires over a 6-year period in central New Mexico were complicated aud often unsuccessful because of unsuitable weather and fuel conditions. We concluded ideal weather conditions must converge before, during, and after a prescribed burning event in order to maximii broom snakeweed control and forage growth on these grasslands.

Vegetation development bordering the Middle Rio Grande, as with most major southwestern U.S. tributaries, has historically undergone rapid and dynamic change. The introduction of saltcedar (or Tamarisk, genus Tamarix) and other exotic species into this environment within the 20th century has contributed to this process. These plants are now an integral component of the riparian vegetation mix. Manpower, logistics, and financial resources constrain the degree to which a desired riparian habitat can be restored from saltcedar thickets on the Bosque del Apache National Wildlife Refuge near Socorro, NM. Saltcedar clearing is accomplished using a combination of herbicide, burning, and mechanical control techniques costing from $750 to $1,300/ha. Soil salinity and depth to water are the principal physical features limiting revegetation efforts. Cottonwood and black willow plantings and natural regeneration after timed irrigations have produced diverse habitats that support a wide array of faunal species in areas previously occupied by homogeneous saltcedar.

Complete removal of broom snakeweed resulted in perennial grass production 833% of that on untreated rangeland after one growing season, and 712% and 300% the second and third year, on a pasture heavily grazed and in poor range condition. On a moderately grazed pasture in good range condition, grass standing crop increased 42% the first year, 81% the second, and 25% the third compared to untreated rangeland. Perennial grass production on the heavily grazed pasture was far below that on the moderately grazed pasture at the start of the study (40 vs 454 kg/ha). After 3 years, with complete broom snakeweed removal and no grazing, perennial grass production was comparable on the pastures once heavily and moderately grazed (1014 vs 939 kg/ha, respectively).Broom snakeweed (Xanthocephalum sarothrae) is a serious perennial weed problem on rangeland in New Mexico. The problem is two-pronged in that, under some conditions, broom snakeweed is poisonous, causing abortion in cattle (Sperry and Robinson 1963), and the weed competes with more valuable forage plants (Ueckert 1979). Platt (1959) estimated that spe'cies of the genus Gutierreria (since treated as Xanthocephalum, Correll and Johnson 1970) occurred on more than 350 million ha of rangeland in the United States. Broom snakeweed is native on about 60% of New Mexico rangelands.Broom snakeweed populations have increased and subsequently decreased in a cyclic pattern at the Fort Stanton Experimental Ranch in New Mexico following droughts in 1970-71, 1974, and 1976 (Pieper and Donart 1973. Above-average fall, winter, and spring precipitation following drought years appears to be related to the establishment of broom snakeweed on the blue grama (Bouteloua gracilis) range.Since some studies (Jameson 1970, Vallentine 1971 and observations indicate that broom snakeweed populations are cyclic, it seems logical to assume that these plants are fairly short lived. Dittberner (197 1) analyzed permanent quadrat records from the Jornada Experimental Range in southern New Mexico collected over a 53-year period and found the average life span of all age classes of snakeweed to be about 2.5 years. Dittberner determined that nearly 70% of the broom snakeweed seedlings die in the first year. Plants living beyond the first year were found to have an average life span of about 4 years. Longevity of the oldest plants ranged up to 15 years.Mature broom snakeweed begins its seasonal growth in late winter to early spring in the Southwest (Ragsdale 1969). Dormant buds, developed in a band above and below the root crown, initiate growth earlier than associated grasses. Because of the competitive advantage from this early growth, there is evidence that production of perennial grasses is decreased where broom snakeweed densities are high. Ueckert (1979) reported herbage production on short grass range to be severely reduced under a dense stand of broom snakeweed. When Ueckert reduced broom snakeweed by 25 and

Field data collected over a 20-year period at 8 sites in northwestern New Mexico was used to determine Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis Beetle and Young) recovery following control with tebuthiuron (N-[5-(1,1dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-N9-dimethylurea) and to relate understory perennial grass yield to overstory canopy cover. Tebuthiuron killed between 80% and 95% of mature Wyoming big sagebrush plants within 18 months of chemical treatment, but through recruitment plant numbers equaled or exceeded pretreatment density (plants/m 2) at 3 of the 8 sites and were increasing at other locations near the study's end. Wyoming big sagebrush canopy cover averaged ,2% the first 10 years after herbicide treatment but had returned to near pretreatment levels (.15%) at 2 sites, to between 5% and 10% at 4 sites, and to less than 3% at the remaining 2 sites. Treatment life was projected to exceed 35 years for 6 of the 8 study sites. Higher rates of tebuthiuron generally extended treatment life. Annual average perennial grass yield increased on treated areas relative to untreated rangeland at all study sites over the 20-year study period. Grass yield was highly variable between years, with pronounced increases when weather and environmental conditions were favorable for grass growth. A nonlinear S-shaped curve best described overstory-understory relationships and also defined the time path of Wyoming big sagebrush recovery, which differed by study site. Resumen Datos obtenidos durante un periodo de 20 añ os en ocho sitios en el Noroeste de Nuevo México fueron usados para determinar la recuperació n de ''Wyoming big sagebrush'' (Artemisia tridentata ssp. wyomingensis Beetle and Young) después de control con ''tebuthiuron'' (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N-N9-dimethylurea), y para relacionar la ganancia del estrato vegetal de pastos perennes con la cobertura del estrato arbustivo. El ''tebuthiuron'' eliminó entre el 80 y el 95% de las plantas maduras de ''Wyoming big sagebrush'' dentro de 18 meses después del tratamiento químico, pero debido a la recuperació n el nú mero de plantas igualó o excedió la densidad de pre-tratamiento (plantas/m 2) en 3 de los 8 sitios y se incrementó en los otros sitios cerca del término del estudio. La cobertura promedio de ''Wyoming big sagebrush'' fue ,2% los primeros 10 añ os después del tratamiento con el herbicida, pero regresó pró ximo a los niveles de pre-tratamiento (.15%) en 2 sitios, entre 5% y 10% en 4 sitios, y a menos de 3% en los 2 sitios restantes. La vida proyectada del tratamiento excedió los 35 añ os en 6 de los 8 sitios de estudio. Altos niveles de ''tebuthiuron'' generalmente extendieron la vida del tratamiento. El promedio anual de ganancia en producció n de pastos perennes en á reas tratadas comparadas con pastizales no tratados se incrementó en todos los sitios a lo largo del periodo de estudio de 20 añ os. La ganancia de pasto fue altamente variable entre añ os, con pronunciados incrementos cuando las condiciones climá t...