Vegetation indices (VIs) are among the oldest tools in remote sensing studies. Although many variations exist, most of them ratio the reflection of light in the red and NIR sections of the spectrum to separate the landscape into water, soil, and vegetation. Theoretical analyses and field studies have shown that VIs are near-linearly related to photosynthetically active radiation absorbed by a plant canopy, and therefore to light-dependent physiological processes, such as photosynthesis, occurring in the upper canopy. Practical studies have used time-series VIs to measure primary production and evapotranspiration, but these are limited in accuracy to that of the data used in ground truthing or calibrating the models used. VIs are also used to estimate a wide variety of other canopy attributes that are used in Soil-Vegetation-Atmosphere Transfer (SVAT), Surface Energy Balance (SEB), and Global Climate Models (GCM). These attributes include fractional vegetation cover, leaf area index, roughness lengths for turbulent transfer, emissivity and albedo. However, VIs often exhibit only moderate, non-linear relationships to these canopy attributes, compromising the accuracy of the models. We use case studies to illustrate the use and misuse of VIs, and argue for using VIs most simply as a measurement of canopy light absorption rather than as a surrogate for detailed features of canopy architecture. Used this way, VIs are compatible with “Big Leaf” SVAT and GCMs that assume that canopy carbon and moisture fluxes have the same relative response to the environment as any single leaf, simplifying the task of modeling complex landscapes.

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[1] The effect of elevated pCO 2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m 3 coral reef mesocosm at the Biosphere-2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present-day and a doubled CO 2 future condition. Each experiment consisted of a 1-2 month preconditioning period followed by a 7-9 day observational period. The pCO 2 was 404 ± 63 matm during the present-day pCO 2 experiment and 658 ± 59 matm during the elevated pCO 2 experiment. Nutrient levels were low and typical of natural reefs waters (NO 3 À 0.5-0.9 mM, NH 4 + 0.4 mM, PO 4 3À 0.07-0.09 mM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present-day experiment and 7.4 ± 0.5 mol photons m À2 d À1 during the elevated pCO 2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla. Algal biomass was 10.4 mol C m À2 during the present-day and 8.7 mol C m À2 and during the elevated pCO 2 experiments. As previously observed, the increase in pCO 2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO 3 m À2 d À1. Net community production (NCP) and dark respiration did not change in response to elevated pCO 2 . Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8-fold increase with increasing pCO 2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m À2 d À 1 ). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41-100% and completely missed the increased rate of carbon cycling due to elevated pCO 2 . We conclude that under natural, undisturbed, nutrient-limited conditions elevated CO 2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an increase pCO 2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.

Evapotranspiration (ET) is the largest term after precipitation in terrestrial water budgets. Accurate estimates of ET are needed for numerous agricultural and natural resource management tasks and to project changes in hydrological cycles due to potential climate change. We explore recent methods that combine vegetation indices (VI) from satellites with ground measurements of actual ET (ET a ) and meteorological data to project ET a over a wide range of biome types and scales of measurement, from local to global estimates. The majority of these use time-series imagery from the Moderate Resolution Imaging Spectrometer on the Terra satellite to project ET over seasons and years. The review explores the theoretical basis for the methods, the types of ancillary data needed, and their accuracy and limitations. Coefficients of determination between modeled ET a and measured ET a are in the range of 0.45-0.95, and root mean square errors are in the range of 10-30% of mean ET a values across biomes, similar to methods that use thermal infrared bands to estimate ET a and within the range of accuracy of the ground measurements by which they are calibrated or validated. The advent of frequent-return satellites such as Terra and planed replacement platforms, and the increasing number of moisture and carbon flux tower sites over the globe, have made these methods feasible. Examples of operational