Spatial heterogeneity of a Scots pine canopy: An assessment by hemispherical photographs

To characterize the spatial heterogeneity of forest canopies at various scales, leaf area index (LAI) and transmission of photosynthetically active radiation (PAR) were measured. Six Scots pine (Pinus sylvestris L.) stands, representing typical growth stages, from regeneration to mature forest, were selected for study, in Alsace, France. In each stand, hemispherical photographs were taken near the ground at regular intervals along transects, digitized, and analyzed. Gap fractions were inverted to provide estimates of LAI. Light transmission through the canopy was calculated by means of a numerical method. In a typical mature stand, the variability of LAI and PAR transmittance was spatially assessed by means of semi-variograms; spatial autocorrelation occurred up to about 10 m. This result provided a guide to optimize the sampling 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 sky and from 0.01 to 0.48 under clear sky (June). Maximum variability was found in the youngest stand, owing to a row effect, and in the two oldest stands, which include Norway spruce (Picea abies (L.) Karst.) patches in the understory. Intermediate variability characterized maturing stands. Methodological problems concerning LAI and PAR transmission estimated by indirect approaches are discussed, as well as the spatial structure of these variables.