|Knorr, C - NEW MEXICO STATE UNIV|
|Murray, L - NEW MEXICO STATE UNIV|
Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 4, 1999
Publication Date: N/A
Interpretive Summary: We have assumed for many years that rangeland vegetation would return to prior conditions (similar to conditions that were present in the 19th century) if they were properly managed. There is mounting evidence that this is not always true for our more arid rangelands. Once our desert grasslands have been invaded by shrub, even native species, we have observed that these shrub dominated states can persist for decades even when these lands are properly managed. A study at the Jornada Experimental Range in southern New Mexico's Chihuahuan Desert was established in 1938 to examine effects of removal of shrubs and grazing on vegetation. In an area of former grasslands, researchers removed shrubs by hand from several experimental plots and fenced out rabbits from others. The entire study area was excluded from any livestock grazing. These treatments have been maintained for nearly sixty years. Vegetation has been remeasured on these eplots several times since 1938, and the plots were remeasured again in 1995. Exclusion of grazing has had very little effect on vegetation. Species, both grasses and shrubs, that are preferred foods of rabbits were slightly more prevalent on exclusion plots. Shrub removal had a more pronounced effect, but it took 50 years before an increase in grass cover was measured. Shrub dominance in these arid rangelands has a tremendous effect that lasts even after the shrubs have been removed.
Technical Abstract: Cover of perennial species in long-term experimental plots in a creosotebush (Larrea tridentata Sess. & Moc. Ex DC.) dominated community in the Chihuahuan Desert was monitored for 56 years. Sixteen 21.3 x 21.3 m plots were established in 1938-39 to evaluate effects of lagomorph exclusion and shrub removal. Major dominant shrubs were individually severed at ground level and removed by hand in 1939, and this process was repeated after measuring plant cover in 1947, 1956, 1960, 1967, 1989, and 1995. Lagomorphs were excluded with poultry wire fencing. Shrub removal increased (Pó0.05) the basal cover of two major desert grass species, black grama (Bouteloua eriopoda Torr.) and spike dropseed (Sporobolus contractus A.S. Hitch.) between 1939 and 1995, but differences were not evident until 50 years post initial treatment. Temporal effects of lagomorph exclusion were less pronounced than for shrub removal. Clearly, shrub dominance has an extremely important and lasting role in determining vegetation community structure in this arid environment, even when aboveground shrub structures are periodically removed.