GLA Version 2.0, Users Manual and Program Documentation 1
1.0 INTRODUCTION
The number, size, and location of gaps in a forest canopy have a direct influence on the
availability and distribution of understory light. The quantity and spectral quality of this incident
solar energy, in turn, plays a significant role in determining the abundance and diversity of
understory plants, the growth and mortality of seedlings, and the development, structure, and
species composition of the canopy trees (Canham et al. 1994, Gray and Spies 1996, Wright et al.
1998, Nicotra et al. in press ). Species, site, and age-related differences in the architecture of
canopies create a patterning of gaps that is highly variable across space and through time, leaving
a complex mosaic of forest structure and light environments at many scales across the landscape
(Lertzman et al. 1996, Frazer et al. 1998, Trichon et al. 1998).
Interest in documenting the relationships between forest structure and the understory light regime
has converged from two distinct lines of research. On the one hand, community and population
ecologists studying successional processes associated with canopy gaps formed by patchy tree-
mortality needed to quantify the environmental conditions associated with those gaps (Rich et al.
1993, Canham et al. 1994, Easter and Spies 1994). On the other, microclimatologists and
production ecologists required easily replicated and non-destructive methods for quantifying the
leaf area borne by forest stands (Chen et al. 1997). In both cases, the challenges and constraints
of direct measurement of the variables of interest (for instance, multiple light sensors running
over several seasons or direct destructive sampling of tree crowns) led to the development of
faster, less direct methods, which lend themselves more easily to spatial and temporal replication
(Welles and Cohen 1996).
Hemispherical canopy photography is one indirect optical technique that has been widely used in
studies of canopy structure and forest light transmission. Photographs taken skyward from the
forest floor with a 180
o
hemispherical (fisheye) lens produce circular images that record the size,
shape, and location of gaps in the forest overstory. Digital scanners or cameras convert these
hemispherical images into bitmaps, which are then analyzed using specialized image analysis
software. Image processing involves the transformation of image pixel positions into angular
coordinates, the division of pixel intensities into sky and non-sky classes, and the computation of
sky-brightness distributions. These data are subsequently combined to produce estimates of
growing-season light transmission, as well as other measures more directly related to canopy
structure, such as openness, leaf area, and sunfleck frequency (Chazdon and Field 1987, Becker et
al. 1989, Rich 1990, ter Steege 1993, Canham 1995).
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