EASTERN COTTONWOOD CLONAL MIXING STUDY - PREDICTED DIAMETER DISTRIBUTIONS

A parameter recovery procedure for the Weibull distribution function w as modified to incorporate monocultures and mixtures of eastern cotton wood (Populus deltoides Bartr.) clones planted in Mississippi and Kent ucky. Components of the system included functions to predict stand-lev el basal area and four percentiles (0th, 25th, 50th, and 95th) of the cumulative diameter distribution. Basal area was predicted as a functi on of surviving number of trees, dominant height, age, planting locati on, and the proportion of each clone planted. Clonal proportions, whic h accounted for 3.6% of the variation in observed basal area, were mor e important than differences in planting locations, which accounted fo r 3.0% of the variation. Interactions between clones in mixtures were not significant (p = 0.5676), but some cases of both over- and under-c ompensation appeared to be developing. Percentiles of the cumulative d iameter distribution were predicted as functions of quadratic mean dia meter, and therefore included indirect effects of both genetic and pla nting site differences. Only the minimum diameter (D-0) was directly a ffected by porportions of clones planted. Most of the monocultures and mixtures of clones had smaller minimum diameters than expected for a given value of quadratic mean diameter. The predicted quadratic mean d iameter and percentiles were used to recover parameters of the Weibull distribution such that the predicted diameter distribution has the sa me quadratic mean diameter as obtained from the stand basal area model . The predicted distributions indicated that a common stand-level mode l was not sufficient for accounting for variations in diameter distrib utions of eastern cottonwood clones. As a result of the differences in diameter distributions, monocultures and mixtures of the Texas clones appeared to have less volume and greater stand variance than the Miss issippi clones.