Field estimates of green leaf area index using laser-induced chlorophyll fluorescence

Leaf area index (LAI) is an important determinant of canopy photosynthesis, evapotranspiration and competition among crop plants and weeds. Direct methods of measuring LAI are labor-intensive. Previous indirect methods can be inaccurate under some sky conditions and fail to distinguish between green and senescent leaves. Most indirect methods are also somewhat invasive, in that the instrument must he placed below the crop canopy. We have modified the inclined-point quadrat method to estimate LAI, by substituting a laser beam for the traditional metal probe. Laser-induced chlorophyll fluorescence is used to detect living leaves (or other organs containing chlorophyll), allowing the crop canopy to be scanned from above. Field tests with sudangrass, wheat, and maize found reasonably good correlations (r² = 0.76 to 0.98) between LAI estimates from our prototype instrument and from direct harvest of leaves. None of the crops tested had significant dead leaf area. This new method can also provide greater detail on the spatial distribution of LAI (e.g., across crop rows) than previous methods. Because of the limited range at which laser-induced chlorophyll fluorescence can be detected, this approach probably would not be useful for research on forests or tree crops.