vegetation in 2000s corresponds to areas of reported top dying. As
explained above, the lack of multiple images for each era, the
different seasons of acquisition for images of different eras, and
variation in the degree of tidal inundation in the various images
prevents comparison of absolute values derived from each of the
canopy closure layers. While the absolute values for canopy closure
that the model is designed to generate are not reliable, patterns of
relative canopy closure are confirmed as generally valid. Visual
confirmation of the validity of the canopy closure layer comes from
two sources: the 1985 (1983 data) Chaffey et al. inventory maps and
QuickBird high-resolution remote-sensing images from 2002. The
Chaffey et al. (1985) maps from 1983 aerial photography, while
compiled approximately 6 years later, support the validity of the
1970s-era canopy closure layer. The 1983 maps show roughly twothirds
of this area as having canopy closure above 70% and little or
none of this area to be below 30% canopy coverage. These areas
correspond well to the high and low canopy closure areas in the
1970s-era canopy closure layer. The largest change in the pattern of
canopy closure is between the TM and ETMþ eras, when a large
corridor of reduced canopy closure appears between the Bal and
Sibsa Rivers. This corresponds to forest compartments that have
high rates of top dying (Canonizado and Hossain,1998; Iftekhar and
Islam, 2004).