Long-term Term Effect of Bedding and Vegetation Control on Dominant Height of Slash Pine Plantations in the Southeastern United States

The long-term effect of bedding and vegetation control on dominant height in slash pine (Pinus elliottii Engelm) was evaluated using data from a site preparation study established in 1979 by the Plantation Management Research Cooperative (PMRC) at the University of Georgia in the southeastern United States. The experimental design corresponded to a 2 x 2 factorial with replications over 16 different locations, distributed over the natural range of slash pine. Our results show sustained gains in dominant height, reaching a peak increment around age 11, with values of 1.0, 2.2, and 2.9 m of average gain for the bedding, vegetation control, and combined (Bed + Veg.) treatments, respectively. At age 31, an approximate rotation age, these gains were no longer present for the bedding treatment, whose dominant height trajectory converged to the values of the untreated control and decreased to 1.9 m for both treatments involving competing vegetation control. These results are similar to previously reported results in the literature for these two treatments in slash pine. We proposed a modified Chapman-Richards type model to describe these trends. In this modeling approach, the base equation was modified using a set of dummy variables in the form of power functions to reflect the treatment effect. Both treated and untreated plots were simultaneously fitted in this model, and contrarily to the most common approach of adding an independent factor to a base model to account for the treatment response, our model does not assume the control plot to be error free. The flexibility of the proposed model allows practitioners to include observed gains in dominant height from these treatments. A slash pine site index model using the algebraic difference approach (ADA) was also derived.