Identification of the Best Spectral Indices to Remotely Trace the Diurnal Course of Water Use Efficiency of Tamarix ramosissima in the Gurbantunggut Desert

Water availability is one of the most important factors limiting photosynthetic assimilation of carbon dioxide and growth of individual plants in terrestrial ecosystems. In particular, water availability acts as primary factor limiting plant transpiration and carbon gain, especially in arid ecosystems. Leaf water availability of desert shrubs is of particular interest to ecologists, because shrubs are extensively distributed and frequently dominant in arid ecosystems. Water use efficiency (WUE) is an optimal indicator of leaf dynamic and diurnal water availability in arid ecosystems. However, approaches for remotely sensing WUE are still lacking, particularly in arid ecosystems. It is necessary to explore the significant and/or consistent relationships between spectral index and WUE.

Tamarix ramosissima is a native dominant desert shrub in central Asia, Mediterranean Sea, and African desert habitats, including gravel desert, heavy-textured desert soil, and sandy desert. The Fukang Station of Desert Ecology, Chinese Academy of Sciences is 8 km from the southern edge of the Gurbantunggut Desert and 72 km north of the highest peak of the eastern Tianshan Mountains. The plains in the vicinity of the station are typically temperate desert with varying soil salinity. In areas of high salinity and high groundwater table (5 m), native vegetation is usually dominated by T. ramosissima.

WUE was estimated using both a field method to determine Pn and Tr and by spectral indices of T. ramosissima in this area. Based on diurnal measurements of spectral reflectance, photosynthesis, and micrometeorological variables, simple and useful spectral indices for estimating diurnal WUE at the assimilative organ scale were explored. From six types of spectral indices, ranging from simple to sophisticated, the best wavelength domains for a given type of index were determined by screening all combinations using correlation analysis. The coefficient of determination (R2), ranging from 0.19 to 0.60, for WUE was calculated for all indices derived from spectra taken from the assimilative organs. With only two wavelengths and a significant correlation coefficient (R2 = 0.60, P＜0.001), the simple ratio (SR) type index was the most sensitive to WUE among all of the indices. Furthermore, SR is a useful indicator to determine the dynamic and diurnal processes of photosynthesis and transpiration of T. ramosissima. Although it has a few weaknesses, SR serves as a simple and robust indicator of WUE in arid ecosystems.

The main finding has been published on Environmental Earth Sciences, 2012, 65(1): 11-20. The paper is also archived at http://www.springerlink.com/content/481j324rp03748q4/.