Assessing seasonal leaf area dynamics and vertical leaf area distribution in eastern white pine (Pinus strobus L.) with a portable light meter.

We evaluated the ability of a portable light meter (Sunfleck Ceptometer, Decagon Devices, Pullman, WA, USA) to quantify seasonal photosynthetically active radiation (PAR, 400-700 nm) interception, projected stand leaf area index (LAI), and vertical LAI distribution in a 32-year-old eastern white pine (Pinus strobus L.) plantation. Canopy PAR transmittance measured with the ceptometer was converted to LAI with the Beer-Lambert Equation. The ceptometer was sensitive to changes in PAR transmittance resulting from foliage growth. Predicted stand LAI ranged from 3.5 in the dormant season to a maximum of 5.3 in late July. Predicted LAI values were within 9% of values determined from destructive sampling. Published canopy extinction coefficients (k) were inadequate for converting PAR transmittance data to stand LAI because a significant amount of PAR was intercepted by dead branches and stems below the forest canopy. Because of interception by dead branches and stems, we estimated k = 0.84, which is substantially higher than previously reported values. The ceptometer was also sensitive to seasonal changes in PAR transmittance within the canopy. However, in contrast to predictions based on the Beer-Lambert Law, the relationship between proportional PAR transmittance (Q(i)/Q(o)) and cumulative LAI within the canopy was linear. Thus, vertical LAI distribution was best estimated with a linear model, as opposed to the non-linear model assumed in the Beer-Lambert Equation. We hypothesize that the linear relationship was a result of a gap in the canopy which was not represented by the cumulative leaf area distribution estimation procedure.