**3.1 Requirements for forest data**

The canopy grain approach must be calibrated at the forest plot scale i.e. by conducting forest inventories from which above ground biomass will be estimated. Areas of about one hectare are necessary to take into account structural diversity within the forest plot. This area of inventory can possibly be reduced for simpler forest stands and plantations, but this is basically dependent on the size of the canopy trees since the computation of FOTO indices should be meaningful at plot scale (Couteron et al. 2005). AGB estimation for each plot will be taken as the AGB of reference to correlate with FOTO indices. Since very labor-intensive destructive measures are necessary to acquire biomass values, reference field AGB values are generally computed indirectly using pre-established allometric functions predicting tree AGB from the measure of the tree diameter at breast height (DBH) as explained, for example, by Chave et al. (2005). On this basis prediction of stand AGB in reference field plots can be computed by measuring DBH>5cm in young forest and DBH>10cm in adult forest. Allometric equation between DBH and tree biomass are established from few cut trees that are weighed on site (e.g. Fromard et al. 1998 for mangroves and Brown et al, 1989 for tropical moist forest). Due to the extreme difficulty of achieving this kind of field work, relationships are often limited to trees with DBH<40cm whereas DBH histograms in tropical forest show values above 150 cm.

Biomass Prediction in Tropical Forests: The Canopy Grain Approach 71

sufficient number of studies in diversified locations and contexts are still needed before general conclusions can be reached about the robustness of such correlations. Independent ongoing studies suggest that the correlation with density is highly contextspecific while the correlation with the mean quadratic diameter may be a more robust

Fig. 9. Radial spectra and associated 100 x 100 m images of different mangrove growth stages using a 0.5 m panchromatic Geoeye image acquired in 2009. Forest inventories dated of 2010 and 2011. Note the r-spectra of the open canopy decaying stage. A photograph of

this plot is available in Fig. 11.

feature.

Additionally, for a given species varying tree heights and crowns dimensions may yield important mass differences that the parsimonious relationships cannot take into account. Selecting an appropriate allometric model is then crucial and the sampling uncertainty relative to the size of the study plot should also be addressed carefully (e.g. Chave et al. 2004).

Tree location, crown shape, tree height and wood specific gravity also constitute useful information that will contribute to the characterization of the forest structure typology. Although it remains unrealistic in heterogeneous forests without the help of skilled botanists, identification of tree species is advisable in low-diversified situations, since the inclusion of a specific wood gravity parameter into allometric equations proved to improve significantly the model (Chave et al. 2005). Such additional data will also be valuable for initializing 3D forest templates. It is important to note that, in tropical forest, tree height measurements from the ground are problematic and cumbersome explaining the enthusiasm aroused by Lidar data (e.g. Gillespie et al. 2004). Another important point to improve AGB prediction would be to conduct forest inventories simultaneously to image acquisitions.
