Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/10/1997
Publication Date: N/A
Citation: Interpretive Summary: In the middle 1800's, much of the northern Chihuahuan Desert was dominated by grassland. Since that time land degradation in this region, identified as the invasion of grasslands by desert scrub species, has been widespread and well documented. Our limited understanding of the processes involved in land degradation, however, inhibits our ability to understand, monitor, and model ecosystem health and vulnerability to degradation. This researc increased our understanding by characterizing temporal landscape processes and evaluating regional landscape parameters derived from high temporal resolution satellite data and determining their utility in mapping, monitoring and modeling landscape health and degradational processes. Temporal growth characteristic curves were derived from satellite data for 1987 through 1993. Differences in growth characteristics were evaluated for plant communities at various stages in the degradation process. Satellite-derived measures of vegetation growth, spatial variability, surface brightness, temporal patterns, and magnitudes were compared with field cover and composition measures for twenty sites at varying levels of degradation. Comparisons with precipitation measures were also conducted. It was concluded that with carefully calibrated, high temporal resolution satellite data, it was possible to detect plant community composition shift from grasses to shrubs that is indicative of land degradation in the northern Chihuahuan Desert. Additionally, because of the relationship between precipitation and plant growth, the satellite-derived measures were also useful for monitoring moisture stress and moisture status across the landscape.
Technical Abstract: The need for relatively inexpensive and rapid means of assessing ecosystem health is widely recognized. If much of the sampling can be done by analysis of satellite imagery, some of the cost and sampling problems may be tractable. The health of rangeland ecosystems can very from virtually dead to robustly healthy. A triage assessment that can accurately identify ythe irreversibly degraded (virtually dead) ecosystems can provide information that is critical in assessing the health of a region and in designing monitoring programs for the future. We obtained imagery from the National Oceanic and Atmospheric Administration's (NOAA) operational polar orbiting satellites' Advanced Very High Resolution Radiometer (AVHRR). For the northern Chihuahuan Desert rangelands, the most accurate triage assessments can be made using imagery from a single year with the optimal rainfall - vegetation response characteristics. In order to clearly distinguish the early spring peak in greeness of the mesquite in coppice dunes and the creosotebush shrublands, it is obviously advantageous to have imagery where that signal is not compromised by the addition of a signal from herbaceous spring annuals. In shrub environments, the densities of spring herbaceous annuals is greatest under the canopies of shrubs and this adds considerably to the NDVI. Winter-spring herbaceous annuals require relatively high rainfall beginning in November and continuing through February. By selecting imagery for years when the November through February rainfall is below average, it is possible to get clear resolution of the boundaries of irreversibly degraded sites from a single year's imagery.