Comparison of direct and indirect methods for assessing leaf area index across a tropical rain forest landscape

Many functional properties of forests depend on the leaf area; however, measuring leaf area is not trivial in tall evergreen vegetation. As a result, leaf area is generally estimated indirectly by light absorption methods. These indirect methods are widely used, but have never been calibrated against direct measurements in tropical rain forests, either at point or landscape scales. Here, we compare direct harvest leaf area index (LAI) measurements taken across an old-growth tropical rain forest landscape with data from two indirect methods, digital hemispherical photography and the LI-COR LAI-2000 Plant Canopy Analyzer. Direct measurements of leaf area were done by collecting all leaf material within an area of 4.6 m2, extending from the forest floor to the top of the canopy using a portable aluminum scaffolding tower. The tower was erected at 45 locations following a stratified random design. Mean direct-harvest LAI above 1 m was 5.5 ± 0.3 SE. Plant area index (PAI, leaves + wood) was 5.1 ± 0.2 for the LAI-2000, and for the hemispherical photographs was 3.9 ± 0.2, analyzed using Gap Light Analyzer (GLA), and 4.9–6.0 ± 0.2 using WinSCANOPY software. Correction for leaf clumping (non-random distribution of leaves) generally improved LAI estimates of the hemispherical photographs. At the local scale, direct-harvest LAI was not significantly correlated with LAI estimates for either indirect method. However, correlations between direct-harvest LAI and both indirect methods along vertical canopy transects from forest floor to the canopy top were significant. Relationships between harvest LAI and canopy closure (from which indirect LAI values are derived) showed very small changes in closure with large changes in LAI at LAI values > 6, indicating that the estimations of LAI using canopy closure values were reaching an asymptote. As a result, at high canopy closure indirect LAI is underestimated. Overall, the LAI-2000 performed better than hemispherical photography for estimating direct-harvest LAI at landscapes scales. However, with corrections for leaf clumping, hemispherical photography can be effective for estimating and characterizing landscape level LAI of tropical rain forest.