Leaf area index (LAI) of a stand of adult black alder trees (Alnus glutinosa L., Gaertn.) was determined by means of three independent methods. (1) The seasonal course of LAI was directly obtained by counting leaves in situ and adding up their areas, estimated from harvested subsamples of leaves, (2) The seasonal variation of LAI in the stand was estimated using the Li-Cor LAI-2000 PCA in parallel and with this instrument a Vegetation Area Index (VAI, projected area of all phyto-elements) was actually measured. (3) Maximum LAI was calculated from leaf litter collections taking into account specific leaf area within different layers of the alder crown. Direct LAI estimates (1) and calculations from leaf litter (3) revealed the same figure of maximum LAI (4.8). This LAI was reached in August. The LAI-2000 PCA captured the seasonal variation and underestimated, by 11% on average, the LAI obtained directly, Compared with results gained with other broad-leaved tree species the LAI-2000 PCA values for alder were reliable. It is suggested that this is due to the horizontal homogeneous structure of the main leaf layer. This is in the periphery of the crown, where 90% of the light interception occurs. Taking the heterogeneity into account a satisfactory compatibility of the three methods applied to the alder stand was achieved.
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