LI-COR LAI-2000 Research Articles

Indirect measurement of leaf area index in Pima cotton ( Gossypium barbadense L.) using a commercial gap inversion method

Leaf area index (LAI) is an important indicator of canopy development and function, but is time consuming to measure directly. In this paper the authors investigate an indirect and non-destructive method of determining LAI in field-grown Pima cotton ( Gossypium barbadense L.) planted in rows, using a commercially available instrument (LiCor LAI-2000, Plant Canopy Analyzer (PCA); Lincoln, NE, USA). The PCA uses a circular sensor head, with a full 360° field of view, consisting of five sensor rings, each viewing a narrow range of zenith angles. Excellent agreement was obtained between destructively determined LAI and indirectly measured LAI, over a range of LAI induced by irrigation treatments. However, the near 1:1 relationship between the two methods required specific measurement and calculation protocols. All measurements were obtained at dusk. Only a 90° sector (25%) of all five sensor rings was exposed, directed along the rows and, when possible, away from the setting sun. Measurements were obtained at four positions across the row/inter-row, which were averaged to characterize the canopy. Underestimation of LAI was observed when data from all five sensor rings were used in the calculation. This bias was removed when data from only the central three, most vertical, rings were used. This protocol empirically compensated for significant violations of the theory underlying the indirect measurement technique. As most row crops similarly violate the theory, this protocol could have widespread utility.

Effects of clumping on estimates of stand leaf area index using the LI-COR LAI-2000

Eighteen natural second-growth stands of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) from a wide range of stand densities and stocking levels were measured for stand leaf area index (LAI) from allometric relationships (direct LAI) and the LI-COR LAI-2000 plant canopy analyzer (indirect LAI). LAI was based on one-half total leaf area (m2) per square metre of ground surface area, normal to any slope. The LAI-2000 underestimated direct LAI by 62%. The underestimate of LAI was due to the confinement of needles within branches (74%) and nonrandom branch distribution (26%). In effect, the branches behaved like big leaves. The LAI-2000 can be used to estimate LAI across a variety of stand density and stocking levels by using a single stand clumping factor of 2.63.

Spatial Variability of Canopy Foliage in an Oak Forest Estimated with Fisheye Sensors

Abstract The vertical distribution of forest canopy elements is an important factor in canopy-atmosphere exchange processes and knowledge of the shape of the average vertical profile of leaf area density is a critical input to models of these processes. This study was conducted to determine the spatial variability of Leaf Area Index (LAI) in an oak forest when estimated with indirect fisheye techniques. A 35 mm camera with a 180° hemispherical lens and a LI-COR LAI-2000 Plant Canopy Analyzer (wavelengths < 490 nm) were used simultaneously to estimate vertical leaf area profiles and the horizontal variability in an oak forest in central Pennsylvania. Measurements were made at eight different heights in the canopy and repeated at nine different locations. The two sensors were in reasonable agreement. The photographic technique estimated the total leaf area index (LAI) to be 3.58, and the light sensor estimates resulted in an estimate of 3.40. The average precision was 8% for the LAI-2000 and 17% for the photographic technique. The point-to-point spatial variability of the LAI estimates was low when the whole canopy was measured from the ground and quite high when measuring the upper canopy. Ground level estimates of LAI using these fisheye techniques spatially average the canopy to the point where only four replications were necessary to sample the total LAI with 90% confidence. For. Sci 38(4):854-865.

Evaluation of hemispherical photography for determining plant area index and geometry of a forest stand

Hemispherical photographs were taken beneath an 80-years-old Douglas-fir ( Pseudotsuga menzie sii) forest stand under clear and overeast sky conditions, and under clear sky with the canopy partially covered with snow. The photographs were digitized using an optical scanner. The ‘effective plant area index’ ( L e ) and the distribution of the inclination angle of plant elements were obtained with Norman and Campbell's linear least-square inversion technique. Photographic exposure was determined by matching the L e values obtained from the photographs with those measured using the Plant Canopy Analyzer (PCA) (Li-Cor LAI-2000). Agreement between the inverted and measured values was found when the photographs were underexposed by 4–5 stops compared with readings of a light meter facing vertically upward under the canopy. These values of L e were about half the plant area index calculated from the measurements of tree diameter at breast height ( D bh ) using published relationships for Douglas-fir trees. The distribution of sky radiance under clear and overcast conditions had a considerable effect on the determination of the angular distribution of plant elements. Pronounced differences were found between inclination angle distributions calculated from the PCA gap fraction measurements and the digitized photographs taken with and without a blue filter (Wratten 80B). These differences may have resulted from the effect of the scattering and possibly diffraction of the visible radiation by the foliage. The scattering and diffraction caused more leaves to blend into the sky at small than at large zenith angles.