LAI can be assessed directly by harvesting the vegetation leaf and measuring its area in the field or in the laboratory. This assessment can be done using either destructive sampling or litter traps. Comparatively, the destructive sampling method is more appropriate for short-stature ecosystems, for example, grasslands, agriculture crops, and tundra, while the litter traps are more appropriate for forests.
Leaf area meter (e.g., Li-3000, Licor, Nebraska, USA; see Figure 1) is one of the common instruments for this measurement. Alternatively, leaf area can be calculated through the specific leaf area (SLA: square centimeters of fresh leaf area per gram of dry foliage mass) in the laboratory. To do so, the SLA is determined first and the projected leaf area is measured with an image analyzer. The SLA and total dry mass of each foliage age class are multiplied and totaled to calculate the LAI for the canopy.
The direct sampling method is usually labor intensive when done for an area of sufficient size to adequately characterize the spatial heterogeneity. The litterfall method is suitable for the summer green vegetation but not for evergreens. Nevertheless, LAI determined in this manner is often considered the most accurate measurement, and therefore the direct methods are often implemented as calibration tools for indirect measurement techniques.
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