Penumbral Effects on Sunlight Penetration in Plant Communities

When sunlight passes through openings in leaf or overstory plant canopies, strong penumbral effects (e.g., penumbral light—spreading) can result in major changes in the total sunlit area, the irradiance level, and exposure duration at the incident surface. These effects are based on well—known optical principles whereby light that originates from a non—point source (e.g., sunlight) and passes through a circular aperture will project an image composed of a bright center portion (numbra) circumscribed by a shadow—edge (penumbra). A hypothetical opening diameter (G) and distance to the incident surface (D) was expressed as the ratio G/D (= GDR) and used to quantify changes in the projected sun spot diameter (Gp) and area Gparea), and the relative irradiance levels and durations of the numbra and penumbra. When GDR was <0.01, full sunlight (numbra) lasted for less than ° 10% of the total exposure period and penumbral light—spreading resulted in an increase in Gparea/Garea of near sixfold. When GDR approached 0.05, Gparea/Gp was only ° 1.2 and full sun lasted for >80% of the exposure period. Beyond a GDR of 0.05, penumbral effects were negligible. Definitions of a sunfleck, sunpatch, gap, and clearing are proposed based on the specific sunlight regimes generated by the corresponding GDR values. These penumbral and latitudinal effects on sunlight penetration through canopy openings are discussed for a variety of leaf canopy and forest overstory types using representative gap sizes and vegetation heights.