Vol. 5, No. 2, 161-172, 2006

Abstract

Total chlorophyll (C_ab) content of a forest canopy is used as indicator for the current state of a forest stand, and also as an input for various physiological vegetation models (i.e. models of photosynthesis, evapo-transpiration, etc.). Recent hyperspectral remote sensing allows retrieving the C_ab concentration of vegetation using the appropriate optical indices, and/or by means of biochemical information, scaled up from leaf to canopy level within radiative transfer (RT) models. Plenty of chlorophyll optical indices can be found in the literature for the leaf level, nevertheless, only some of them were proposed for a complex vegetation canopy like a forest stand. A robust chlorophyll optical index at the canopy level should be driven by the C_ab concentration without negative influence of other factors represented by soil background or understory, canopy closure, canopy structure (e.g. leaf area index (LAI), clumping of leaves), etc.

A new optical index named Area under curve Normalised to Maximal Band depth between 650-725 nm (ANMB_650-725) is proposed to estimate the chlorophyll content of a Norway spruce (Picea abies, /L./ Karst.) crown. This index was designed to exploit modifications of a vegetation reflectance signature invoked within the red-edge wavelengths mainly by the changes in leaf chlorophyll content. ANMB_650-725 is based on the reflectance continuum removal of the chlorophyll absorption feature between wavelengths of 650-725 nm. Suitability of the index and sensitivity on disturbing factors was tested using a 3D Discrete Anisotropic Radiative Transfer (DART) model coupled with a leaf radiative transfer model PROSPECT adjusted for spruce needles. The results of the ANMB_650-725 abilities within a coniferous forest canopy were compared with the performance of the chlorophyll indices ratio TCARI/OSAVI.

Test results, carried out with the DART model simulating hyperspectral data with 0.9 m pixel size, showed a strong linear regression of the ANMB_650-725 on spruce crown C_ab concentration (R²=0.9798) and its quite strong resistance against varying canopy structural features such as LAI and canopy closure. The root mean square error (RMSE) between real and the ANMB_650-725 estimated C_ab concentrations was only 9.53 mg/cm² while the RMSE generated from prediction of the TCARI/OSAVI was two times higher (18.83 mg/cm²). Chlorophyll retrieval using the ANMB_650-725 index remained stable also after introduction of two reflectance signal disturbing features: a) 20% of the spectral information of epiphytic lichen (Pseudevernia sp.) regularly distributed within the spruce canopy, and b) simulation of the sensor noise (computed for a signal to noise ratio equal to 5). RMSE of predicted C_ab concentration after the introduction of lichens appeared to be 10.51 mg/cm² and the combined influence of lichen presence and sensor noise in the image caused an increase of the RMSE to 12.13 mg/cm².

ISSN 1729-3782