An assessment of vegetation response to different moisture conditions at multiple resolutions

Extreme variations in climatological inputs often have the most rapid and significant impacts on vegetation state. Better understanding of these problems requires monitoring of the land surface, quantification of surface characteristics and assessment of vegetation response to different moisture regimes. Therefore, this research was aimed at studying the impact of climatic conditions on vegetation, and the landscape structure using both coarse and fine spatial, spectral and temporal resolution remotely sensed data (i.e., SPOT-XS, AVHRR and SPOT-VGT) for the Black Hills region, South Dakota. The study focused on two objectives: including (a) investigation of the potential use of fine spatial resolution SPOT XS data for vegetation state detection and as mapping and monitoring tools for relatively heterogeneous land areas; (b) characterization and quantification of the effects of sub-pixel vegetation heterogeneity on the AVERR and SPOT-VGT NDVI response to different moisture conditions using high-resolution SPOT imagery. The following results were obtained from this study. Vegetation is more responsive to dry conditions than wet conditions. Variations in NDVI values were generally associated with several landscape parameters such as vegetation type, canopy structure and density, health of vegetation, and heterogeneity or homogeneity of landscape. Examination of vegetation state over heterogeneous areas is scale dependent. Mixed pixels with a significant component of different land cover types were more responsive to moisture variations than homogenous pixels. Detecting the alfalfa and mixed areas are significantly affected by the properties of sensor systems such as spectral bandwidths. 1 x 1 km coarse NDVI pixel values were over/or underestimated based on dominant land cover by AVHRR and SPOT-VGT sensors. The geometrically and radiometrically improved SPOT-VGT sensor, which was specifically designed for vegetation studies with appropriate bandwidths, provided information content comparable with the fine resolution SPOT data. The most important contribution of this research was the indication that the coarse resolution (i.e., AVHRR, SPOT-VGT) NDVI response to different moisture conditions can be associated directly with the nature of the land cover encompassed by single pixel.