Evaluation of ADAR imagery for mapping moisture stress in oaks with advanced symptoms of Sudden Oak Death
Desheng Liu1 and Maggi Kelly2
We investigated the ability of high spatial-resolution 4-band imagery (Airborne Digital Acquisition and Registration - ADAR) to discern moisture stress in trees affected by Sudden Oak Death. The disease progression in oaks includes a stage where the canopy fades quickly as a result of advanced cancer growth on the main stem. Immediately prior to this color change, we assume the leaves experience moisture stress. It has been well documented that moisture stress in vegetation is correlated with a decrease in reflectance in the near and middle infrared portions of the electromagnetic spectrum. We wanted to test ADAR imagery could be used to distinguish between green oak trees with advanced SOD trunk symptoms, and green oaks with no SOD trunk symptoms.
ADAR imagery of China Camp State Park in Marin Co. was flown in spring 2000. Training samples from the field consisting of the locations of 25 green healthy oaks and 25 green stressed oaks were used to locate individual tree crowns in the imagery, from which spectral signatures were defined for the two classes. Both hierarchical unsupervised classification (HUC) and maximum likelihood classification (MLC) were used to classify the two groups. Accuracy assessment and other spectral measurements were performed to analyze the separability of the two signatures. Poor overall accuracy 55.17% was obtained by the HUC method. A better overall accuracy 74.19% was obtained by MLC method, but the low transformed divergence (1448) indicated poor separability of the training samples. The poor accuracy results can be explained by the fact that ADAR image has relatively broad spectral bands that combine narrow stress-sensitive regions with broader stress-insensitive regions; such combination could decrease the capability of ADAR to detect moisture stress. In addition, healthy oaks in the area display a marked variability in canopy condition, making it impossible to separate healthy trees from those experiencing some stress. In conclusion, the classified results indicated the limitation of using ADAR image to map moisture stress in oaks.
1Graduate Student. Ecosystem Science Division, Department of Environmental Sciences, Policy and Management, UC Berkeley, Berkeley, CA 94720-3110; (510) 642-6678; dsliu@nature.berkeley.edu
2Cooperative Extension Specialist and Adjunct Assistant Professor, Ecosystem Science Division, Department of Environmental Sciences, Policy and Management, UC Berkeley, Berkeley, CA 94720-3110; (510) 642-7272; mkelly@nature.berkeley.edu
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