Measuring fractional cover and leaf area index in arid ecosystems: Digital camera, radiation transmittance, and laser altimetry methods

Field measurement of shrubland ecological properties is important for both site monitoring and validation of remote sensing information. During the May 1997 NASA Earth Observing System Jornada Prototype Validation Exercise, we calculated plot-level plant area index, leaf area index, total fractional cover, and green fractional cover with data from four instruments: (1) a Dycam Agricultural Digital Camera (ADC), (2) a LI-COR LAI-2000 plant canopy analyzer, (3) a Decagon sunfleck Ceptometer, and (4) a laser altimeter. Estimates from the LAI-2000 and Ceptometer were very similar (plant area index 0.3, leaf area index 0.22, total fractional cover 0.19, green fractional cover 0.14), while the ADC produced values 5% to 10% higher. Laser altimeter values, depending on the height cutoff used to establish total fractional cover, were either higher or lower than the other instruments' values: a 10-cm cutoff produced values ~80% higher, while a 20-cm cutoff produced values ~30% lower. The LAI-2000 and Ceptometer are designed to operate in homogenous canopies, not the sparse and irregular vegetation found at Jornada. Thus, these instruments were primarily useful for relative within-site plant area index monitoring. Calculation of some parameters required destructive sampling, a relatively slow and labor-intensive activity that limits spatial and temporal applicability. Validation/monitoring campaigns therefore should be guided by consideration of the amount of time and resources required to obtain measurements of the desired variables. Our results suggest that the ADC is both efficient and accurate for long-term or large-scale monitoring of arid ecosystems. (C) Elsevier Science Inc., 2000.