Abstract
The hyper-arid Atacama Desert is one of the most extreme environments for life and only few species have evolved to survive its aridness. One such species is the tree Prosopis tamarugo Phil. Because Tamarugo completely depends on groundwater, it is being threatened by the high water demand from the Chilean mining industry and the human consumption. In this paper, we identified the most important biophysical variables to assess the water status of Tamarugo trees and tested the potential of WorldView2 satellite images to retrieve these variables. We propose green canopy fraction (GCF) and green drip line leaf area index (DLLAIgreen) as best variables and a value of 0.25 GCF as a critical threshold for Tamarugo survival. Using the WorldView2 spectral bands and an object-based image analysis, we showed that the NDVI and the Red-edge Chlorophyll Index (CIRed-edge) have good potential to retrieve GCF and DLLAIgreen. The NDVI performed best for DLLAIgreen (RMSE = 0.4) while the CIRed-edge was best for GCF (RMSE = 0.1). However, both indices were affected by Tamarugo leaf movements (leaves avoid facing direct solar radiation at the hottest time of the day). Thus, monitoring systems based on these indices should consider the time of the day and the season of the year at which the satellite images are acquired.
Highlights
The conservation of arid vegetation is difficult to address by managers and policy makers since vegetation dots or patches are frequently immersed into a large matrix of bare land
We showed that the structural variables green canopy fraction (GCF), drip line leaf area index (DLLAI), and DLLAIgreen were good field indicators of the Tamarugo water status at tree level
GCF, and second, we studied the relationship between vegetation indices (NDVI and CIRed-edge) and GCF and DLLAIgreen, considering the spectral bands of the WorldView2 sensor
Summary
The new generation of very high spatial resolution (VHSR) satellites, starting with IKONOS in 1999 and followed by QuickBird in 2001, GeoEye in 2008 and WorldView in 2009, has become a real option to approach the problem of assessing the water condition of desert vegetation Satellite images from these sensors allow identification of small dots such as individual trees, shrubs or small grass patches [2]. The estimation of biophysical properties such as canopy water content, chlorophyll content or leaf area index for small vegetation objects has not been sufficiently studied yet due to the limited spectral information of VHSR satellites [6,7] These sensors typically have only three bands in the visible region and one band in the near infra-red region (IKONOS, QuickBird and Geo-Eye). In the near future (2014), the WorldView will provide images with the same eight bands of WorldView plus eight additional bands in the short-wave-infra-red region, opening new opportunities for the retrieval of biophysical properties of small desert features [8]
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