The Progression of Shrub Encroachment Into a Desert Grassland in Southern New Mexico from 1936-2003

Woody plant encroachment into arid and semiarid grass-dominated landscapes has been noted in many parts of the Southwestern United States. This shrub advance has been observed on both the USDA-ARS Jornada Experimental Range (JER) and the adjacent New Mexico State University Chihuahuan Desert Rangeland Research Center (CDRRC) in southern New Mexico using varying ground survey techniques extending back to 1858. These two experimental ranges have also been imaged numerous times using aerial photography (and recently with high resolution commercial satellites) starting with USDA flights in the 1930's. Since 1936, thirty-seven separate aerial photography missions have been flown over parts of JER/CDRRC. Because rangeland remediation treatments are common in the Jornada Basin, it is difficult to find areas where no treatments, especially herbicide applications, have been employed to arrest the shrub advance. Aerial photographs were analyzed over Pasture 2 on the CDRRC where no herbicides or other treatments had been applied. There are 22 separate overflight dates covering Pasture 2 from 1936 to 2003. The aerial photos were scanned and digitized to analyze the amount of grass cover and its change over 60+ years. Despite encountering numerous difficulties in analyzing a greatly varying aerial photo data set (including varying resolutions, spectral intervals, moisture conditions, time of year of observation, and appearance of grass cover), the aerial photos provide an invaluable historic record. The grass cover obtained from remote sensing data in Pasture 2 is compared with the less frequent conventional vegetation ground surveys. The changing amounts of grass cover with time are related to various outside factors such as persistent drought periods. The rate of shrub advance across Pasture 2 has a direct influence on changes in the water, carbon, and energy cycles of these arid lands.