Dynamics of size-variable distribution parameters in juvenile loblolly pine ( Pinus taeda L.) stands

Abstract

Spacing trials were established at four locations in the Piedmont and Coastal Plain regions of Virginia and North Carolina, USA, in 1983. The groundline diameter, later diameter at breast height, total height and crown height of each tree were measured annually for 8 years after establishment. Based on statistical analysis of the data, parameters of tree-size distribution such as the coefficient of variation (CV), the Gini coefficient (GC) and skewness were found to be significantly correlated with stand age and with number of trees per unit area. Some subtle differences were found among the distributions. The CV and GC of crown height decreased with both age and density ( P-value less than 0.0001). However the CV and GC of diameter measurements and total height decreased with age, but increased with density. The negative relationship between the CV and stand age is partially due to the fact that the competition effects on variability were largely compensated for by tree growth (i.e. an increase in the value of the mean) in juvenile stands. Competition resulted in increased negative skewness. Kurtosis values were not found to be significantly different among densities, but kurtosis generally increased with stand age, ranging from slightly negative or zero for very young stands to positive for older stands. Normality can generally be assumed for diameter measurements, but not for total height, whose distributions deviate from normality with intensified competition. Increasing intertree competition resulted in distributions of crown height becoming more normal and those of crown width becoming less normal, indicating that trees tend to have a larger photosynthetic capacity when they are subject to competition. The dynamics of distribution parameters were related to the dominant tree heights and mean stand crown ratio.