Spatial Distribution of Canopy Gaps in Lodgepole Pine Forest

Forest canopy, through its influence on optical, infrared, and microwave signatures observed from remote sensing platforms, impacts the retrieval of information on snow cover properties and extent. The number and size of gaps within and between tree crowns determines the scale and quality of information that can be obtained remotely. This paper explores ground based descriptions of the spatial distribution of canopy gaps using hemispherical photography and summarizes how the statistics may be useful in validating 1) a synthetic 3-D canopy to be used in an infrared canopy model, and 2) a terrain capable, viewable gap fraction model. Hemispherical photographs were taken with a Nikkor 8mm/f2 lens at 20-m grid spacing in the Fraser Experimental Forest, an area of predominantly lodgepole pine (Pinus contorta), and were analyzed with Gap Light Analyzer software. The study site is part of the ongoing interagency Cold Lands Processes Experiment (CLPX) that seeks to advance techniques for large scale remote observation of hydrologic properties (Cline et al 2002). Gap fraction distributions, gap fraction variograms, and subcanopy radiation environment were characterized for the site. Gap fraction distributions for discontinuous canopy had broader peaks than for continuous canopy, reflecting the greater heterogeneity in discontinuous settings. Mean correlation lengths in continuous settings increased from 10o to 20o in azimuth between 30o and 60o zeniths and the maximum variances also consistently increased. In comparison, discontinuous setting correlation lengths were longer but the trend with zenith angle was less consistent. Total daily solar radiation transmittance varied sensitively with location within the site, providing addition characterizations for comparisons between actual and modeled canopy.