Shrubland encroachment in southern New Mexico, U.S.A.: An analysis of desertification processes in the American southwest

Abstract

More and more the conviction grows that we are not merely at one stage of a cycle repeating itself, but that the past eighty years have witnessed the evolution of a significant new vegetation put together out of the floristic remains of the old. There is no sign that the evolution has seen its full development yet, or is slackening its pace. (Hastings and Turner, 1965) Abstract. The area dominated by the shrubs creosotebush (Larrea tridentata) and mesquite (Prosopis glandulosa) in the American southwest has increased severalfold over the last century, with a corresponding decrease in areal coverage of productive grasslands and increased surface soil erosion throughout the region. The factors thought to be responsible for this regional shift in vegetation are: (1) overgrazing by domestic livestock; (2) fire suppression; and (3) historical changes in climate. We examine the evidence concerning each of these factors and develop a synthetic model outlining the principles affecting shrubland encroachment, which focuses on life history characteristics of the dominant shrubs and a number of positive biotic and edaphic feedback mechanisms contributing to their establishment and persistence. We conclude that the expansion of shrub dominance that has occurred over the last century may have been triggered by extreme livestock overgrazing at the end of the nineteenth century, which coincided with rainfall regimes that were unfavorable for perennial grass growth. Hence, the landscape we observe today may be a product of positive feedback mechanisms triggered over a century ago by management practices that were uninformed with regard to the importance of historical climate patterns and the life history characteristics of important rangeland species. Our consideration of these issues also addresses potential land surface -climate interactions that could occur as a result of regional alterations in vegetation dominance and physiognomy.

Humans as Agents in the Termination of the African Humid Period

“…Cattle and caprines are selective grazers and browsers, and their feeding habits can significantly enhance the range of shrub vegetation (Grover and Musick, 1990;Moleele et al, 2002). Following a fire, grasses will be the first pioneers on the landscape, which will be consumed by grazing animals.…”

Short-Term Fire Effects on Small Mammal Populations and Vegetation of the Northern Chihuahuan Desert

“…However, on our study sites many creosote plants which were almost entirely burned off at the soil surface had resprouted one year postburn, indicating that June fires will not cause total mortality of creosote bush in the NCD. Density may be affected for a longer period of time because creosote bush seedling establishment is unpredictable, dependant on fall soil temperatures, and heavy fall precipitation [26]. Likewise, asexual reproduction following fires was not documented during our study or by Brown and Minnich [24].…”

Long Term Effect of Major Disturbances on the Northern Mixed Grassland Ecosystem—A Review

“…topography, soil, fauna (insects, herbivores), together with herbaceous plants [57] [60], can restrict the encroachment of woody plant (trees, shrubs), release nutrients bound up in organic matter, accelerating the rate of decomposition in the soil, so as to maintain the establishment and the stability of grasslands [6] [57] [61]. Descriptive studies [62] [63] show that fire occurrences decrease herbaceous production for one to three years.…”