Seasonal course of the spectral properties of alder and birch leaves

To interpret time series of hyperspectral measurements of vegetation canopies, the basic characteristics of the scattering elements forming these canopies-shoots, leaves, or needles-must be known. Unfortunately, data on the seasonal variation of leaf reflectance and transmittance are very scarce. To obtain a ground truth dataset applicable to modeling the phenological development of European (hemi)boreal forests, we measured the spectral properties of green leaves of two species common to this biome, gray alder (Alnus incana), and silver birch (Betula pendula). Measurements covered the full growing season of 2008 in Tõravere, Estonia. Leaves were sampled from sunlit locations and measured in a laboratory using an integrating sphere and a VNIR spectroradiometer. We measured four different optical parameters: directional-hemispherical reflectance and transmittance factors for leaf adaxial and abaxial surfaces. Leaf reflectance was used to calculate four leaf-level indices which, according to literature, are highly correlated with leaf chlorophyll content. Additionally, we calculated the derivative spectra of leaf reflectance in the red edge (RE) spectral region and fitted it with two Gaussian curves. Our analysis indicated continuous changes of leaf optical properties almost until the end of the growing season. The changes in the weights of the two Gaussian curves led to an effect known as peak jump, an abrupt shift in the RE inflection point. In near infrared (NIR), leaf absorption was negligible in the beginning of the growing period. However, we noted a slow but steady increase in the leaf NIR absorption with time.