ESTIMATING LEAF MASS FROM LIGHT INTERCEPTION MEASUREMENTS ON ISOLATED PLANTS OF ERYTHROXYLUM SPECIES

To determine the extent of coca (Eryfhroxylum spp.) production in the world, some method for estimating yield (leaf mass) in a variety of habitats is required. A computer model that simulates the growth of coca plants has the versatitity for making these estimates, but there must be some reliable method to validate the model. The objective of this research was to validate the use of measurements of light interception and specific leaf area (SLA, m(2) kg(-1)) for estimating yield. Leaf area was calculated from light interception measurements made on plants grown as hedgerow or as isolated plants, then converted to leaf mass using measurements of SLA. Light interception was measured with the Decagon Sunfleck Ceptometer (SC) and the LI-COR Plant Canopy Analyzer (PCA). Leaf disks of known area from Erythroxylum coca var. coca and Erythroxylum novogranatense var. novogranatense were cut, dried, and weighed to determine SLA values. Leaf fresh weights were measured and corrected to dry weight using the fresh to dry weight ratio obtained from a sample at harvest. Estimated leaf mass regressed on observed leaf mass demonstrated that PCA was superior to SC in estimating leaf mass on isolated plants of various sizes using the techniques described in this paper. Leaf area was consistently underestimated by the SC. The estimates of leaf mass from the PCA gave the lower mean, absolute mean, and root mean square error terms compared with SC, and the slope of the regression line of observed leaf mass vs. predicted leaf mass was closest to a value of 1.0.