SPATIAL HETEROGENEITY OF A SCOTS PINE CANOPY - AN ASSESSMENT BY HEMISPHERICAL PHOTOGRAPHS

To characterize the spatial heterogeneity of forest canopies at variou s scales, leaf area index (LAI) and transmission of photosynthetically active radiation (PAR) were measured. Six Scots pine (Pinus sylvestri s L.) stands, representing typical growth stages, from regeneration to mature forest, were selected for study, in Alsace, France. In each st and, hemispherical photographs were taken near the ground at regular i ntervals along transects, digitized, and analyzed. Gap fractions were inverted to provide estimates of LAI. Light transmission through the c anopy was calculated by means of a numerical method. In a typical matu re stand, the variability of LAI and PAR transmittance was spatially a ssessed by means of semi-variograms; spatial autocorrelation occurred up to about 10 m. This result provided a guide to optimize the samplin g at the forest mosaic level. For the whole forest, LAI ranged from 1. 6 to 7.8; PAR transmittance ranged from 0.02 to 0.59 under overcast sk y and from 0.01 to 0.48 under clear sky (June). Maximum variability wa s found in the youngest stand, owing to a row effect, and in the two o ldest stands, which include Norway spruce (Picea abies (L.) Karst.) pa tches in the understory. Intermediate variability characterized maturi ng stands. Methodological problems concerning LAI and PAR transmission estimated by indirect approaches are discussed, as well as the spatia l structure of these variables.