Mapping Indicators of Land Degradation Induced by Bush Invasion in Jornada Southern New Mexico Using Landsat TM Imagery

Abstract

This research aims to determine the effectiveness of satellite remote sensing for mapping the vegetation change in the form of bush invasion into grasslands caused by land degradation at Jornada in southern New Mexico using mixture modelling (MM), Spectral Vegetation Indices (SVIs) (i.e., simple ratio, NDVI, SAVI, SSI, and PD54), and regression analysis. Bush invasion is one of the most important land degradation factors at Jornada. Historical records pointed out that during the last century, perennial grasslands have been replaced by invasive xerophytic shrub communities. Linear mxture modelling has been used to estimate the proportions of green vegetation, dry vegetation, shade, soil, and rock. The results indicate that mixture modelling is a reasonably accurate measure of these materials. In contrast, many of the correlations between SVIs and the various vegetation parameters are not significant or resulted in low R^2 values. Phenological differences among species were used in estimating grass cover and shrub cover. In dry years such as 1996, in June the shrubs are green but grasses are not. Under these conditions there is a reasonably strong relationship (R^2=0.93) between green vegetation cover and green shrub cover. Dry vegetation cover was also correlated with grass cover (R^2=0.68). Thus, it is possible to obtain the shrub to grass ratio map, which is one of the critical parameters related to bush invasion and associated land degradation in the study area.