Comparison of indirect optical methods for estimating effective leaf area index in young Norway spruce stands

Forest canopy structure is crucial for regulating light interception, carbon exchange, and water balance, making accurate estimation of leaf area index (LAI) essential for forest ecology and management. This study compared three indirect optical methods (LAI-2200 PCA, LaiPen LP 110, and digital hemispherical photography - DHP) for estimating effective leaf area index (LAIe) in young Norway spruce [Picea abies (L.) Karst.] stands. It evaluated the influence of the field of view (FOV) and seasonal timing on inter-method agreement. The research was conducted in young, even-aged Norway spruce stands at the Křivina experimental site in the Czech Republic, using three research plots with varying stand densities. Results showed that LAIe estimates were influenced by the selected FOV, with mean values generally decreasing as FOV increased, particularly for the LAI-2200 PCA. Narrow FOV configurations produced greater local variability among sampling positions, whereas broader FOVs resulted in more homogeneous spatial canopy patterns. The strongest agreement among methods was observed between LaiPen LP 110 and LAI-2200 PCA, with correlation coefficients ranging from 0.71 to 0.84 across temporally matched measurements. In contrast, DHP exhibited weaker and more variable relationships with LaiPen LP 110. Differences among research plots were most pronounced between the dense control stand and the heavily thinned stand, indicating that canopy density and structural heterogeneity substantially affected LAIe estimation. The study demonstrates that indirect optical methods are sensitive to both canopy structure and measurement configuration; therefore, careful instrument selection, FOV standardisation, and synchronised measurements are essential for obtaining reliable and comparable LAIe estimates.