ESTIMATION OF LEAF-AREA INDEX FOR COTTON CANOPIES USING THE LI-COR LAI-2000 PLANT CANOPY ANALYZER

Measurement of leaf area index (LAI) is useful for understanding cotto n (Gossypium hirsutum L.) growth, water use, and canopy light intercep tion. Destructive measurement is time consuming and labor intensive. O ur objective was to evaluate sampling procedures using the Li-Cor (Lin coln, NE) LAI 2000 plant canopy analyzer (PCA) for nondestructive esti mation of cotton LAI on the southern High Plains of Texas. We evaluate d shading as a way to allow PCA measurements in direct sunlight and th e influence of solar direction when using this procedure, We also eval uated a test of canopy homogeneity (information required for setting P CA field of view), determined the number of below-canopy measurements required, examined the influence of leaf wilting on PCA LAI determinat ions, and tested an alternative method (masking the sensor's two outer rings) for calculating LAI from PCA measurements. The best agreement between PCA and destructively measured LAI values was obtained when PC A observations were made either during uniformly overcast conditions o r around solar noon using the shading method. Heterogeneous canopies w ith large gaps between rows required both a restricted (45 degrees) az imuthal held of view and averaging the LAI values for two transects, m ade with the held of view parallel and then perpendicular to the row d irection. This method agreed well (r(2) = 0.84) with destructively mea sured LAI in the range of 0.5 to 3.5 and did not deviate from a 1:1 re lationship. The PCA underestimated LAI by greater than or equal to 20% when measurements were made on canopies wilted due to water stress. M asking the PCA sensor's outer rings did not improve the relationship b etween estimated and measured LAI in the range of LAI sampled.