Ljaboratory for Applications of Remote Sensing

Leaf area index (LAI) is an important biophysical descriptor of crop canopies. Accurate measurements of LAI are laborious and time-consuming. Many methods of measuring LAI of corn (Zea mays L.) have been reported and vary greatly in their accuracy, precision, bias, and ease of measurement. We examined the magnitude of plant-to-plant variability of leaf area of corn plants selected from uniform plots and evaluated four representative methods for measuring LAI. The number of plants required and the relative costs for each sampling method were calculated to detect 10, 20, and 50% differences in LAI using 0.05 and 0.01 tests of significance and a 90$ probability of success (3 = 0.1). The natural variability of leaf area per corn plant was nearly 10$. Additional variability or experimental error may be introduced by the measurement technique employed and by nonuniformity within the plot. Direct measurement of leaf area with an electronic area meter had the lowest CV, required that the fewest plants be sampled, but required approximately the same amount of time as the leaf area/weight ratio method to detect comparable differences. Indirect methods based on measurements of length and width of leaves required more plants but less total time than the direct method. Unless the coefficients for converting length and width to area are verified frequently, the indirect methods may be biased. When true differences in LAI among treatments exceed 50$ of mean, all four methods are equal. The method of choice depends on the resources available, the differences to be detected, and what additional information, such as leaf weight or stalk weight, is also desired.