© 2001 Heron Publishing—Victoria, Canada
A method for estimating light interception by a conifer shoot
Sampo Smolander (1) and Pauline Stenberg (2)
1. Rolf Nevanlinna Institute, P.O. Box 4, FI-00014 University of Helsinki, Finland / 2. Department of Forest Ecology, P.O. Box 27, FI-00014 University of Helsinki, Finland / Received August 18, 2000
Summary
We present an operational method for estimating the amount of PAR intercepted by a coniferous shoot. Interception of PAR by
a shoot is divided into three components: the amount of radiation coming from the sky, the transmission of radiation through
the surrounding vegetation, and the shoot's silhouette area facing the direction of the incoming radiation. All three components
usually vary with direction. Radiation incident from the sky consists of direct and diffuse radiation. The well-known equation
of motion for the sun and Beer's Law for atmospheric transmittance are used to simulate the directional distribution of direct
sunlight for any given period of time. The diffuse component is assumed to be uniform. Meteorological field measurements are
used to calibrate the absolute amounts of the direct and diffuse components. The gap fraction (proportion of visible sky)
in different directions around a shoot is measured by analyzing a hemispherical fish-eye photograph, taken at the location
of the shoot, with an image processing program. Similarly, the shoot silhouette area (SSA) is measured by photographing the
shoot from many different directions. The measurements of SSA are interpolated by a method called trigonometric interpolation
to obtain the directional distribution of SSA over the entire hemisphere. This distribution is then rotated according to the
shoot's position in the canopy. Multiplying incoming PAR, canopy gap fraction and SSA in different directions, and summing
over all directions, gives an estimate of PAR intercepted by the shoot during the chosen period of time. The method is described
step by step, and applied, as an example, to a shoot from a Scots pine (Pinus sylvestris L.) stand in central Finland. Differences in radiation interception properties between sun and shade shoots and their relevance
to canopy-scale models are discussed.
Keywords:
gap fraction, light interception, shoot silhouette area, SPAR, STAR, trigonometric interpolation.
