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Abstract
Slash pine (Pinus elliotii Englem.) and loblolly pine (Pinus taeda L.) are important commercial species in the Southeastern United States. With intensive management of these pine plantations gaining popularity, growth and yield models that incorporate cultural treatments such as thinning, fertilization and weed control are required. This study developed such models to predict stand structure of thinned slash and loblolly pine stands. Preliminary results showed that after first thinning selective thinning from below applied at 33%, 40% and 50% thinning intensities shifted the diameter distributions towards the right increasing significantly the 0th and 25th percentiles. Row thinning irrespective of thinning intensity did not change the percentiles. 0th, 25th, 50th and 95th percentile prediction equations were developed to account both for these effects and information that is possibly available after thinning. Weibull distributions recovered from predicted percentiles were not significantly different from observed diameter distributions. This indicates that Weibull that has been well documented for unthinned stands can be used to describe thinned pine diameter distributions. Height-dbh prediction equations to be used with diameter distribution prediction models to predict stand volume were developed. Effects of thinning on average total tree heights were such that when compared to row thinning and unthinned stands, selective thinning from below, depending on intensity, increased the average height of trees towards the left of the diameter distribution. An empirical evaluation of generalized stand table projection method (GSTP) of Pienaar and Harrison (1988) against relative size relationship model (CRS) showed that both models performed reasonably well in projecting current stand tables. Results also indicated absence of consistency in the estimate for the rate parameter in GSTP model and when thinning was accounted for in this model, it implied conflicting trends in relative size for row and select thinned stands. On the other hand plots of observed data showed no significant change in relative size over time. This indicates it is reasonable to assume that relative tree size for these data remains constant over time and with respect to type and timing of thinning.