Winterfat-blue grama community ecotones under defoliation and removal of domestic herbivory

This study examined ecotones between winterfat (Ceratoides lanata) shrublands and adjacent blue grama (Bouteloua gracilis) grasslands in Colfax County, near Raton, New Mexico on N. R. A. Whittington Center and adjacent rangelands. The study area lies on foothill slopes and alluvial fans below pinyon-juniper vegetation. Vegetation response to defoliation was compared for interior and peripheral winterfat shrubland community positions on two locations having different grazing histories. Domestic grazing was removed from Whittington locations in 1973, while adjacent lands continued to sustain season-long grazing.

Canopy volume of mature winterfat decreased at interior community positions from 1992 to 1994; winterfat in peripheral positions did not decrease in canopy volume during the three years. Results depended upon method of observation. Basal area of winterfat was greater at Whittington locations on per unit area basis; basal area of individual winterfat plants was greater at adjacent locations. Winterfat recruitment was affected by all treatments; at Whittington locations, winterfat seedlings emerged on all sampling dates, primarily on interior positions, but did not emerge on adjacent locations by the first sampling date. At adjacent locations, seedlings of other shrub species had lower emergence on dates of increased winterfat seedling emergence. Basal cover of grasses was not affected by community position. Defoliation of both browse and herbaceous plants increased basal cover of grasses and bare ground cover and decreased litter cover.

Interior and peripheral community positions differed in density and species composition of emerged and surviving seedlings, basal and canopy dimensions of mature winterfat, and standing crop of forbs. Plants in ecotone positions may diverge from their conspecifics in interior positions in response to unique competitive constraints in the two positions.