Evaluation of Four Estimators of Herbicide Treatment Efficacy for Woody Competition Control Studies
Four estimators of herbicide treatment efficacy based on hardwood crown measurements were evaluated. Using data from untreated plots, expected post-treatment size of treated hardwood rootstocks was obtained by accounting for growth between pre- and post-treatment evaluations with an unadjusted mean, a ratio-of-means estimator, and a regression estimator. Formulas were developed to compare the estimates of hardwood control based on change in crown size relative to pretreatment size, unadjusted mean post-treatment size, and ratio- and regression-adjusted post-treatment size. Conditions for optimum application of the estimators were also discussed. The effects of all four estimation procedures on the estimated means and treatment contrasts were demonstrated for flowering dogwood (Cornus florida L.) and sweetgum (Liquidambar stryaciflua L.) in a site-preparation study located in the Georgia piedmont. The lowest mean control was obtained using change in crown size, while control based on regression-adjusted crown size resulted in the highest mean control. Treatment contrasts for control based on change in crown size and control based on ratio-adjusted crown size had the same level of significance for both species. For dogwood, all four estimators of efficacy resulted in the same conclusion (acceptance or rejection of the null hypothesis) for 8 of 9 contrasts, although regression-based control tended to be the most conservative. For sweetgum, the null hypothesis of no differences among treatments was rejected more frequently using efficacy based on the unadjusted mean post-treatment crown size or regression-adjusted post-treatment crown size. A regression estimator of post-treatment crown size appears to be more flexible in its application than ratio-adjusted estimates. For. Sci. 36(2):201-211.
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Document Type: Journal Article
Affiliations: Assistant Professor, School of Forest Resources, University of Georgia, Athens, GA 30602
Publication date: 1990-06-01
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