Effects of chemical creosotebush (Larrea tridentata (D.C.) Cov.) control on vegetation composition, edaphic properties, and hydrologic processes as influenced by successional stage

Creosotebush (Larrea tridentata (D.C.) Cov.) encroachment has converted many southwestern New Mexico grasslands to creosotebush-dominated shrublands during the past 150 years. In an effort to control creosotebush, the Bureau of Land Management initiated a brush control program in 1981 that involved application of the chemical herbicide tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N ′-dimethylurea) to creosotebush-dominated shrublands. The objectives of this study were: (1) assess vegetation changes following chemical herbicide treatments as a function of successional stage, (2) quantify edaphic properties as a function of creosotebush proximity and identify changes in soil properties during succession following creosotebush control, and (3) quantify differences in hydrologic processes between the canopy zone and interspace, and identify changes in hydrologic processes as a function of successional stage. Successional stages included early (sites treated 1993–1997), mid (sites treated 1984–1987), and late (untreated areas contiguous with treated sites). Vegetation composition was assessed with a reconnaissance survey at 62 sites, and intensive sampling of vegetation parameters at a subset of 12 sites. Soil properties and hydrologic processes were studied at the same subset of 12 sites. Results indicate that chemical herbicide treatments reduced creosotebush density approximately 75% and creosotebush cover approximately 95%. Herbaceous cover increased following creosotebush control, with shortgrasses dominating early successional areas and midgrasses dominating mid-successional areas. Effects of chemical brush control on vegetation composition persist at least 15–20 years, and possibly much longer. Soil properties were measured at the canopy center, canopy edge, and interspace. Soil samples were taken by dead and live creosotebush in treated areas. In untreated areas, soil samples were only taken by live creosotebush. Organic matter, nitrogen, phosphorus, potassium, aggregate stability, and sand content increased with creosotebush proximity. Magnesium, calcium, pH, cation exchange capacity, clay content, and silt content increased with distance from creosotebush. Most soil properties were not significantly different among successional stages. A portable rainfall simulator was used to examine hydrologic processes. Infiltration rates and sediment concentration were greater and the wetting front deeper in the canopy zone than interspace. Infiltration rates were highest in early successional areas and lowest in mid-successional areas. Sedimentation was significantly influenced by creosotebush proximity and successional stage.