Measuring Marsh Vegetation Structure Using Multi-angular Remote Sensing

Kevin R. Turpie, University of Maryland (Presenting)

Remote sensing techniques offer an efficient approach to monitor and quantify changes in marsh vegetation. In particular, off-nadir radiometry, can provide useful information about ground cover conditions, including vegetation structure parameters. It is hypothesized that remote sensing from multiple viewing angles, especially at near-infrared wavelengths, could provide information about vegetation structure for coastal marshes. Of particular interest is leaf area index (LAI), which can be used to tie vegetation structure to photosynthesis and radiation absorption. This study focuses on possible methods of retrieving LAI using space-based instruments or field measurements to assess vegetation structure over a widespread area. To that end, this study has three main components: 1) satellite data, 2) ground data, and 3) retrieval methods. For the satellite component of the study, multi-angular imagery were taken over the Blackwater National Wildlife Refuge using spaceborne instruments with relatively high spatial resolution (15-20 m at nadir). This imagery is being evaluated for applicability to LAI retrieval. This currently includes one stereographic scene from early October 2007 collected from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) aboard the NASA spacecraft Terra and four late Summer / early Fall scenes that were acquired from the Compact High Resolution Imaging Spectometer (CHRIS) aboard the European Space Agency’s Project for On-Board Autonomy (Proba) spacecraft. The ground data component of this study involves at least one field campaign, the first of which was completed late in the 2007 growing cycle. Several thousand high-resolution reflectance spectra (450-900 nm) were collected at multiple viewing angles along a 1.6 km segment of Maple Dam Road from a moving vehicle with an instrument boom situated over the marsh. LAI measurements also were taken every 1/10 of a mile along this transect, once in September and once in early October. Finally, a method to retrieve marsh LAI is being developed from the multi-angular radiometry and LAI data. The work will begin with a statistical analysis to look for any functional relationship between the sampled multi-angular radiometry (satellite and ground measurements) and LAI measured on the ground. However, a more promising approach would be to develop a canopy reflectance model that is dependent primarily on LAI.