Establishment and effects of establishment of creosotebush, Larrea tridentata, on a Chihuahuan Desert watershed

Creosotebush (Larrea tridentata) seedlings were planted in plots that were irrigated, plots that were irrigated and fertilized with ammonium nitrate, and plots that were not amended in three plant communities on a Chihuahuan Desert watershed: ungrazed black grama (Bouteloua eriopoda) grassland, creo sotebush shrubland, and overgrazed grassland. No seedlings were planted in one-half of the area of each plot. No seedlings survived in the black grama grassland or the creosotebush shrubland 2 years after planting. Growth of established creosotebush shrubs was highest in plots with the lowest grass cover. Fifteen years after the seedlings were planted in the overgrazed gra ssland, the area under the shrubs was nearly devoid of perennial grasses an d forbs. The aggregate stability of the soils under the established creosot ebush shrubs was significantly lower than the soils in the unplanted split- half of the plots. Electrical conductivity, calcium, and nitrate were signi ficantly lower in soils under shrubs than soils in the unplanted split-half of the plots. There were also significant reductions in densities of annua l plants growing under the shrubs than in the unplanted split-half of the p lots. Successful establishment of creosotebush in desert grasslands is depe ndent upon the presence of large patches of soil with no perennial plant co ver. Intense grazing by domestic livestock creates microsites and landscape characteristics favourable for seed dispersal, germination and establishme nt of creosotebush. Soil changes resulting from shrub establishment reduce the probability of re-establishing perennial grasses in creosotebush-domina ted shrubland. (C) 2001 Academic Press.