Comparison of Techniques Used for Adjusting Biomass Estimates by Double Sampling

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Abstract
This paper compares the ratio and regression estimator procedures for adjusting ocularly estimated plant species biomass in different sizes and shapes of plots. The study was conducted in northeastern Colorado [USA] on shortgrass rangeland dominated by blue grama (Bouteloua gracilis). No significant differences were found in clipped plant biomass in 4 quadrat sizes between 0.18 and 0.50 m2 and 2 shapes, circular and angular quadrats. For double sampling, the scatter plots of data strongly indicated a linear relationship through the origin for estimation and clipping. There were no significant differences between the adjusted mean weights by use of regression with and without intercept. The intercept was not significantly different from zero. Interpretation of correlation coefficient and variance of regression estimate with no intercept becomes difficult because the regression is forced through zero. It is helpful to use regression with intercept. Estimates of both green and dry weights by ratio and regression estimation were comparable. Regression estimation is a minimum variance estimation comparable to ratio estimation even when the assumption of homoscedasticity is not true.

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