Seasonal variation of leaf chlorophyll content of a temperate forest. Inversion of the PROSPECT model

This paper presents part of a 7-month field and laboratory experiment over the deciduous forest of Fontainebleau. Leaf visible and near infrared optical properties of three tree species (oak, beech and hornbeam) were measured each month between April and October 1996. We distinguished the cases of sun and shade leaves, and also abaxial and adaxial leaf surfaces. Spectra were analyzed with reference to leaf chlorophyll content and leaf mass per area. As expected, we observed strong variations of leaf optical properties during the season, with differences between sun and shade leaves and abaxial and adaxial surfaces. We also investigated how leaf reflectance and transmittance can provide realistic information about the seasonal variation of leaf chlorophyll content. For that, we used a leaf optical properties model: the PROSPECT model. Inversion of this model with leaf spectra led to the seasonal variation of leaf chlorophyll concentration (mugcm2) we compared with ground measurements. The analysis of spectral data showed that leaf chlorophyll concentration increases strongly at the beginning of the growing season (from April to May), remains stable during several months (from June to August), and decreases strongly when leaves are senescent (from September to October/November). Chlorophyll concentration of sun leaves tends to be always larger than that of shade leaves. Moreover, chlorophyll concentration depends on leaf species, with oak leaves having the largest chlorophyll concentrations.