A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

ESTIMATION OF LEAF AREA INDEX USING THE GAP FRACTIONMETHOD: AN ALGORITHM USING THRESHOLD'S DEFINITION FORCANOPIES OF TROPICAL FOREST, PASTURELAND AND SAVANNAHRobinson I. Negrón-Juárez and Humberto da RochaIAG-USPThe Leaf Area Index (LAI) estimated using hemispherical photographs can use thegap fraction method. We used the CID-110 digital canopy imager under three differentcanopies of tropical forest (Santarém km 83) and pastureland (Santarém km 77) and awoodland savannah (Cerrado sensu strictu) in São Paulo during May to June 2001. It usesthe non-linear estimation method (Norman and Campbell, 1989), which has howeverappeared to underestimate the observations (destructive mesurements or tipical literature’svalues) under heterogeneous canopies. The algorithm varies with the threshold’s selection,which in turn depends on sky’s brightness and the local canopy’s architecture. Weintroduced on the calculations the entropy crossover method (Sahoo et al. 1997) and anmutually exclusive hypothesis to select the optimal threshold. The optimal threshold isinitially based on the minium histogram entropy’s difference, and the final decision toaccept/neglect the threshold is taken by the mutually exclusive hypothesis. The clumpingfactor was assumed as equal to 1. The mutually exclusive hypothesis computes the meansquare error (MSE) between the transmitted light fraction (predicted by Norman andCampbell method) and that one absolutely accounted over the image’s pixels. Thresholdsassociated to values of MSE greater than 1 are disregarded. We have calculated averageLAI values equal to 4.7 and 1.1 for the tropical forest (wet season) and the cerrado (earlydry season), respectively. At the pasture areas, LAI was estimated at three 1m 2 plots on adestructive basis, and the calculated LAI using the above method showed errors lower than5%.