Spectroradiometric Measurements of Tree Species in the Caspian Forests of Iran

Forest type maps play a significant role in sustainable forest management. For many years aerial photos and satellite data were a primary data source supporting forest type mapping. Recent developments in remote sensing provide opportunities to further enhance forest type maps by introducing variations of spectral, biochemical and biophysical properties at various scales. A structural sampling and collection of the above variables will support an improved interfacing between spatially continuous data, forest type maps and finally will support forward and inverse modeling of advanced forest biochemical, -structural, and other relevant variables. The main objective of this study is to acquire, process and analyze spectral signatures of main forest tree species of the Caspian forest (namely Fagus orientalis, Quercus castaneifolia, Carpinus betulus, Alnus subcordata, and Parrotia persica) located in the Research Forest owned by the University of Tehran on the Northern slopes of the Elburz mountains, Iran. We have sampled 102 spectra each of the afore mentioned tree species using leaf ‘pile’ reflectance and branch pile reflectance. We build a comprehensive database of leaf optical properties, and other measures such as branch and twig reflectance. Field spectroradiometric measurements (350-2500nm) were carried out in the course of summer 2007. Spectral measurements were acquired in altitude gradients between 400-2100m (low, mid and high elevation) of the Elburz mountains. All spectral signatures after preprocessing were analyzed physically and statistically. We select a set of vegetation indices related to optical properties of the leaves and exploit changes of vegetation reflectance signature dependent on illumination conditions (shaded vs. non-shaded leaves) and chlorophyll content. We conclude that the Vogelmann index (R740/R720) is more sensitive to chlorophyll content in comparison with the other indices. It shows that hornbeam (Carpinus betulus) is significantly different in spectral signatures compared to beech (Fagus orientalis), oak, (Quercus castaneifolia) and alder (Alnus subcordata) as well as ironwood (Parrotia persica) with alder and oak being statistically different (p<0.0001, α= 0.01). Variability in spectral properties related to tree age and exposition is also assessed. We conclude by presenting a comprehensive spectral database of leaf optical properties of the main dominant tree species for further use in the determination of photosynthetic and non-photosynthetic fractions, remote determination of dominant species, radiative transfer based on forest modeling, and ecosystem change analysis (invasive species, etc.).