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 optic al properties of three tree species (oak, beech and hornbeam) were measured each month between April and October 1996. We distinguished the cases of s un and shade leaves, and also abaxial and adaxial leaf surfaces. Spectra we re analyzed with reference to leaf chlorophyll content and leaf mass per ar ea. As expected, we observed strong variations of leaf optical properties d uring the season, with differences between sun and shade leaves and abaxial and adaxial surfaces. We also investigated how leaf reflectance and transm ittance can provide realistic information about the seasonal variation of l eaf chlorophyll content. For that, we used a leaf optical properties model: the PROSPECT model. Inversion of this model with leaf spectra led to the s easonal variation of leaf chlorophyll concentration (mu g cm(-2)) we compar ed with ground measurements. The analysis of spectral data showed that leaf chlorophyll concentration increases strongly at the beginning of the growi ng season (from April to May), remains stable during several months (from J une to August), and decreases strongly when leaves are senescent (from Sept ember to October/November). Chlorophyll concentration of sun leaves lends t o be always larger than that of shade leaves. Moreover, chlorophyll concent ration depends on leaf species, with oak leaves having the largest chloroph yll concentrations.