MODIS seasonal and inter-annual responses of semiarid ecosystems to drought in the Southwest U.S.A

摘要

High temporal frequency observations with the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra Earth Observing System platform offer unique opportunities to study climate- and anthropogenic-induced land transformations in the temporal domain. Shifts in vegetation type and physiognomies alter biologic activity and responses to climate patterns in unpredictable ways. Increases in insect populations (e.g. bark beetle) and fire associated with a multi-year drought in the Southwest U.S.A have greatly impacted the health of its native ecosystems from pinyon-juniper, Ponderosa pine, and mixed conifer forests to the savanna, grassland, and desert shrub ecosystems. In this study, MODIS time series data combined with AVIRIS overflights were analyzed for detection and evaluation of the causes, severity, and extent of changes in ecosystem health. We used the 16-day MODIS enhanced vegetation index (EVI) product and a normalized difference water index (NDWI) to analyze the seasonal, inter-annual, and spatial patterns of vegetation activity over a wide range of land cover types across eco-climatic and elevational gradients and through the winter and variable monsoon rainfall 'pulses'. The temporal dynamics of vegetation were found to be highly sensitive to both anthropogenic and climatic forcings found in the semiarid and arid Southwest with seasonal and inter-annual profiles varying markedly with land cover type and land surface disturbance (e.g., drought, insects). All land cover types and eco-climatic gradients from desert shrub to montane forest were significantly affected by the drought, with grasslands most impacted. Tree mortality variability could also be assessed and we found that combined MODIS-AVIRIS data offer the potential of ecosystem health and risk assessment.