Document: PAU-3-69-36

Effects of fire, grazing, and the presence of shrubs on desert grassland species of southern New Mexico.

DREWA, P.B.* and K.M.HAVSTAD

USDA, Las Cruces, NM 88003 USA. ¹

Abstract:
Changes in community structure of herbaceous species to single growing season fires, livestock grazing, and presence of shrubs were investigated experimentally in Chihuahuan desert grasslands in southern NM. Eight, 12m by 8m plots were established in each of two different blocks. Treatments were allocated to plots at two levels: Applied and not applied. Moreover, each of the eight treatment combinations of fire, grazing, and shrubs were assigned randomly to one of the eight plots within each block. Fires were applied during June 1995 after which plots received only 49% of precipitation expected for the rainy season of July; unfenced plots were subsequently exposed to dormant season grazing over the next four years. Plots were established to either include or exclude existing shrubs. Cover and relative abundance of herbaceous species were estimated immediately prior to and four years following treatments using a vertical line point intercept method along five, 8m transects within each plot. Perennial grass cover decreased 13% but increased 5% in burned and unburned plots respectively (P = 0.017). Also, relative abundance of perennial grasses decreased 30% more in burned than unburned areas (P = 0.004). These responses were attributed almost entirely to Bouteloua eriopoda, the dominant perennial grass species. Conversely, cover (P < 0.001) and relative abundance (P < 0.001) of perennial forbs were greater in burned than unburned plots. Similar to fire responses, cover and relative abundance of perennial grasses and forbs changed in much the same way between grazed and ungrazed plots. Annuals generally increased in cover and relative abundance regardless of fire and grazing. Herbaceous species were not impacted by the presence of shrubs. These results suggest that fires had similar effects as grazing on community structure. Treatments did little to eliminate species. Instead, fire and grazing were effective in decreasing dominant perennial grasses such as B. eriopoda that coincided with appreciable increases in coverage and relative abundance of mostly perennial forbs and some annuals. Growing season fires may alter communities similar to anthropogenic grazing if the timing of fires do not coincide with appropriate synoptic weather conditions.

Keywords: desert grasslands, fire, grazing

This abstract is being presented at: 12:00 PM in session:
Oral Session #22: Multiple Disturbance Effects, Including Fire.