Detection of Canopy Opening in Undisturbed and Selectively Logging Tropical Forests by Gap Fraction Data

Tree mortality in tropical forests often leads to canopy openings or gaps that are critical to regeneration and the dynamics of tropical forests. If measures of canopy gaps could be collected using remote sensing, then tropical forest dynamics could be investigated at the regional scale. We ask whether it is possible to detect canopy gaps by using of remotely sensing data gathered on the ground and whether there is a relation between forest disturbance age and canopy opening? To answer these questions, we collected an intensive set of gap fraction measurements in the Tapajos National Forest, Pará. The data were measured using the Licor LAI-2000 instrument in 10 plots with dimensions of 50 m × 200 m distributed in areas with different forest disturbance ages (forests logged in 1997, 1999, 2000, 2001, 2002 and 2003) and undisturbed forest (control plots). For each one of these treatments, two plots were installed. All plots were subdivided in 10 m × 10 m grids where pipes marked corner points. Gap fraction data were collected for each of these grid points (126 per plot). A geostatistical analysis showed that gaps < 200 m2 could be detected and differentiated from medium (200 400 m2) and large (400 m2) gaps. A preliminary analysis indicated that after at least three years of forest regeneration, there was no detectable difference in gap fraction between the logged areas and the undisturbed forest.