Distinguishing mangrove species with laboratory measurements of hyperspectral leaf reflectance

Abstract

As a first step in developing classification procedures for remotely acquired hyperspectral mapping of mangrove canopies, we conducted a laboratory study of mangrove leaf spectral reflectance at a study site on the Caribbean coast of Panama, where the mangrove forest canopy is dominated by Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. Using a high‐resolution spectrometer, we measured the reflectance of leaves collected from replicate trees of three mangrove species growing in productive and physiologically stressful habitats. The reflectance data were analysed in the following ways. First, a one‐way ANOVA was performed to identify bands that exhibited significant differences (P value<0.01) in the mean reflectance across tree species. The selected bands then formed the basis for a linear discriminant analysis (LDA) that classified the three types of mangrove leaves. The contribution of each narrow band to the classification was assessed by the absolute value of standardised coefficients associated with each discriminant function. Finally, to investigate the capability of hyperspectral data to diagnose the stress condition across the three mangrove species, four narrow band ratios (R ₆₉₅/R ₄₂₀, R ₆₀₅/R ₇₆₀, R ₆₉₅/R ₇₆₀, and R ₇₁₀/R ₇₆₀ where R ₆₉₅ represents reflectance at wavelength of 695nm, and so on) were calculated and compared between stressed and non‐stressed tree leaves using ANOVA.

Results indicate a good discrimination was achieved with an average kappa value of 0.9. Wavebands at 780, 790, 800, 1480, 1530, and 1550 nm were identified as the most useful bands for mangrove species classification. At least one of the four reflectance ratio indices proved useful in detecting stress associated with any of the three mangrove species. Overall, hyperspectral data appear to have great potential for discriminating mangrove canopies of differing species composition and for detecting stress in mangrove vegetation.

Acknowledgements

The study was supported by grants to Le Wang from the National Science Foundation (DEB‐0614040, DEB‐0810933 and BCS‐0822489), to W.P. Sousa from the National Science Foundation (DEB‐0108146) and the U.C. Berkeley Committee for Research. We are grateful to C.J. Hayden, H. Laederich, S. Rudolph, and R. Schneider for assisting with the collection of leaf samples. J. Endler gave us valuable advice on multivariate approaches to the analysis of spectral data.

We thank the Smithsonian Tropical Research Institute for excellent logistical support and for allowing us to use the Galeta Marine Laboratory. The field portion of the project was conducted under research permit SE/AP‐1‐04 from Panama's Autoridad Nacional del Ambiente. We especially thank the Republic of Panama for preserving their forests and making them available for study. Lastly, we want to thank the insightful suggestions made by all the anonymous reviewers.