Modeling relative wind speed by optical stratification porosity within the canopy of a coastal protective forest at different stem densities

Abstract

Wind speed and optical stratification porosity (OSP) were measured at various heights inside a coastal protective forest thinned to different stem densities to assess whether any characteristics of the wind profile in the coastal protective forest could be predicted from OSP. OSP was defined as vertical distribution of the proportion of sky hemisphere not obscured by tree elements inside a forest stand, and was determined for various heights using hemispherical photographic silhouettes on a computer processing system. The distribution of OSP in the coastal forest follows the Lambert-Beer s law with an extinction coefficient (v). The relative wind speed within the canopy can be described using an exponential form with an attenuation coefficient (α). Variation in relative wind speed was very closely correlated with the distribution of OSP within the canopy. While below the canopy, i.e., in the trunk space, relative wind speed was little correlated with the distribution of OSP because the distribution of OSP was relatively constant there. Therefore, the linear relationships between relative wind speed and OSP and between the two coefficients v and α were established within the canopy. The results suggest that OSP can be used to predict the wind profile in case of the application within the canopy of the coastal forest.