Foliar morphology and canopy nitrogen as predictors of light-use efficiency in terrestrial vegetation

Two methods for determining the leaf area of trees growing in rows using an LAI-2000 Plant Canopy Analyser were tested against destructive measurements for Croton megalocarpus and Melia volkensii, species with differing canopy characteristics. The trees ranged between 4.0 and 8.0 m in height and formed part of an agroforestry experiment in semi-arid Kenya where rapid fluctuations in canopy cover rendered allometric approaches inappropriate for determining leaf area. The first method used unmodified theory for determining leaf area in continuous canopies which has proved suitable for isolated bushes. In the second method, path lengths through the canopy were calculated from simple measurements of canopy dimensions and the importance of subsidiary assumptions concerning leaf angle distribution was tested. Leaf angle distribution, which is required for canopy simulation models, was also determined using both direct and indirect approaches and the effect of using assumed leaf angle distributions when calculating leaf area was assessed. The canopy analyser proved unsuitable for measuring leaf angle distributions in isolated canopies, and it was necessary to make direct canopy measurements for this instrument to be used for smaller canopies. It was also shown that, even when path lengths are measured, calibration may be necessary to avoid bias; uncalibrated leaf area density values were, on average, underestimated by 16% for M. volkensii and overestimated by 8% for C. megalocarpus with respect to destructively determined values.