Modeling the change of stem form in plantations of young loblolly pine (Pinus teada)

When predicting future stand volumes using present mensurational methods, stem form is often considered as a static variable and thus ignored. This may lead to incorrect estimates since stem form may change during the time period considered. These changes can be further influenced by natural occurrences or by management practices such as thinning. The objectives of this study were to: (1) develop a model to describe the changes in stem form with age in a plantation, and (2) correlate the effects of age at which a loblolly pine (Pinus taeda L.) plantation was thinned with changes in form. The data used were obtained by stem analysis on randomly selected trees in a young loblolly pine plantation which had four thinning treatments: a control (with no thinning), and thinnings at ages 10, 15, and 20 years. Three types of models were used to describe the change in stem form. They consisted of past growth models, taper equations, and the use of the primary units of volume, surface, and length as predictors. The accuracy desired for the past growth models and the taper equations was the capacity to predict the radius inside bark at known heights within 0.25 of an inch 95 percent of the time. In the models using the primary units of volume, surface and length the desired accuracy was predicting these units in 1975 within 5 percent of the means 95 percent of the time. None of the models tested were able to describe the form of the whole length of the stem within the desired accuracy. However the model using the radius at breast height inside bark growth (1970 to 1975) was able to satisfactorily predict the form of the stem up to 50 percent of the total height of the tree. Evaluations of the taper equations showed that for the years 1970 and 1975 there were significant differences in the populations. For the year 1965 no significant differences were found, and evaluation of the regression equations showed no significant differences in stem form between the thinning regimes for that year. Thus it was concluded that thinning at age 10 had no significant effect on the change in stem form, but thinnings at ages 15 and 20 had a significant effect on the change in stem form. Models using the primary units of volume, surface, and length showed that surface and length could be predicted from past values of the primary units within reasonable limits. This was shown for both total (from stump height to the tip) and sectional (based on the tree being composed of bolts of a fixed length) values of the primary units. Predicting volume within reasonable limits was not possible for either total or sectional values. Evaluation of curves showing the change in Girard form class for ages 10 to 22, revealed that the older the stand when thinned the less pronounced is the reduction in the rate of natural increase of Girard form class with young trees overtime.