System GVI Global Vegetation Index GV Green Vegetation GPP Gross Primary Productivity HRVIR High-Resolution Visible Infrared kNN k-Nearest Neighbor Imputation LSP Land Surface Phenology LAI Leaf Area Index LUE Light Use E¶ciency MIR Middle Infrared MODIS Moderate-Resolution Imaging Spectroradiometer MNDVI Modi¤ed NDVI MSR Modi¤ed Simple Ratio MISR Multiangle Imaging Spectroradiometer MSS Multispectral Scanner Sensors NBAR Nadir BRDF-Adjusted Re¨ectance NASAEOS NASA Earth Observing System NOAA National Oceanic and Atmospheric

Administration NIR Near-infrared NEE Net Ecosystem Exchange NPP Net Primary Production NAOMI New AstroSat Optical Modular Instrument NLI Nonlinear Index NPV Nonphotosynthetic Vegetation NDMI Normalized Di¦erence Moisture Index NDVI Normalized Di¦erence Vegetation Index OLI Operational Land Imager PNW Paci¤c Northwest PRI Photochemical Re¨ectance index POES Polar-orbiting Operational Environmental

Satellites RF Random Forest RSR Reduced SR RDVI Renormalized Di¦erence Vegetation Index RBV Return Beam Vidicon RMSE Root Mean Squared Error SPOT Satellite Pourl’Observation de la Terre SWIR Shortwave-infrared SR Simple ratio SARVI Soil and Atmosphere-resistant Vegetation Index SARVI2 Soil and Atmosphere-resistant Vegetation

Index 2 SAVI Soil-adjusted Vegetation Index SAVI1 Soil-adjusted Vegetation Index 1 SOS Start of Season Suomi NPP Suomi National Polar-orbiting Partnership TOPS Terrestrial Observation and Prediction System

TM Ÿematic Mapper TIRS Ÿermal Infrared Sensors NDVI3g Ÿird-generation GIMMS NDVI TRAC Tracing Radiation and Architecture of

Canopies LandTrendr Trends in Disturbance and Recovery VI Vegetation Index VIIRS Visible Infrared Imaging Radiometer Suite WDVI Weighted Di¦erence Vegetation Index

Vegetation is the primary producer in the terrestrial ecosystem. Vegetation absorbs the energy of electromagnetic radiation from the Sun and converts it to the energy that consumers in the ecosystem can use. As a result, vegetation is the foundation for nearly all the goods and services that terrestrial ecosystems provide to humanity. Ÿe advent of optical remote sensing revolutionized our ability to map the characteristics of vegetation wall-to-wall in space and to do so repeatedly, in a cost-e¶cient manner. Many of these vegetation parameters serve as key inputs to ecological models aiming to understand terrestrial ecosystem functions, at regional to global scales. Ÿis chapter summarizes the progress made in characterizing vegetation structure and its ecological functions with optical remote sensing. We ¤rst provide a brief review of the development of optical sensors designed primarily for vegetation monitoring. Second, we synthesize the progress made in mapping the physical structure of vegetation with optical sensors, including vegetation cover, vegetation successional stages, biomass, leaf area index (LAI), and its spatial organization, that is, leaf clumping. Ÿird, we review the achievements made in understanding vegetation function with optical remote sensing, particularly vegetation primary productivity and related ecologically important functions. Primary production provides the energy that drives all subsequent ecosystem processes. Optical remote sensing has made it possible to estimate the primary productivity of vegetation over the entire Earth land surface (Running et al. 1994; Zhao et al. 2005).