Effects of stand structure on wind speed reduction in a Metasequoia glyptostroboides shelterbelt
Wind speed and optical stratifi cation 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 profi le in the coastal protective forest could be predicted from OSP. OSP was defi ned 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 coeffi cient (ν). The relative wind speed within the canopy can be described using an exponential form with an attenuation coeffi cient (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 coeffi cients ν and α were established within the canopy. The results suggest that OSP can be used to predict the wind profi le in case of the application within the canopy of the coastal forest.