Leaf area, visual obstruction, and standing crop relationships on Sandhills rangeland

Abstract

The objective of this study was to determine if leaf area index (LAI) or visual obstruction (VO) could be used in an efficient double-sampling format for estimating total above-ground standing crop on upland range sites in the Nebraska Sandhills. Sampling was conducted in pastures used for summer grazing research in which treatments consisting of stocking at 16, 32, or 48 animal-unit-days (AUD) ha-1 in June or July and an ungrazed control that were replicated 3 times. During trial 1, LAI, VO, and yield of standing crop were measured in 1995and 1996 at 12 random sampling sites in each of twenty-one,1.0-ha pastures. Trial 2 compared modified LAI and VO sampling procedures against those used in trial 1. Modifications included the use of a circular 0.25 m2 sample plot area rather than a rectangular one and increasing both the number of LAI and visual obstruction readings that were used in the calculation of the mean value at each sampling site. During trial 2, data were collected from 12 sampling sites within each of 14 pastures that comprised 2 blocks of grazing treatments. There was a significant (P<0.01) linear relationship between LAI and yield of standing crop during trial 1, but only 33% of the variation in standing crop was attributable to LAI. The modified LAI sampling procedure increased R2 to 0.59. Similarly, the relationship between visual obstruction and standing crop was significant (P<0.01), but R2 values were only 0.31 and 0.41 during the first and second years of trial 1, respectively. The modified visual obstruction (VO) sampling procedure resulted in only minimal R2improvement compared to the trial 1 method. Pooling LAI or VO data for individual sample sites into stocking rate means resulted in the detection of significant (P<0.01) quadratic relationships between fall LAI or VO and summer stocking rate. Based on the sampling procedures used in this study, neither LAI nor VO would be useful as direct predictors of total standing crop at individual sample locations on upland range sites in the Nebraska Sandhills. However, with pastures as experimental units, these methods can detect the relative effects of stocking rate with replicated treatments.