VARIATION IN POTENTIAL VOLUME YIELD OF LOBLOLLY-PINE PLANTATIONS

Variation in the potential volume at a given site index, which is a result of differences in the maximum basal area, was investigated using data from permanent plots in loblolly pine plantations across the southern United States. The data were stratified into three physiographic regions-Atlantic Coastal Plain, Piedmont, and Gulf Coastal Plain-and the maximum stand density index (SDI(max)) value (Reineke 1933) was estimated for each stand by applying Sterba's (1975) extension of the competition density rule of Kira et al. (1953). The Gulf Coastal Plain exhibited the lowest density potential and differs significantly from each of the other two regions. Between the Atlantic Coastal Plain and the Piedmont, no significant differences in the density potential exist. Significant differences mean different maximum basal areas and therefore differences in the total yield within the same site index; these differences in yield cannot be detected using the site index method only. Data from the control plots, which represent the stand development without any thinning influences, showed that the variation between the regions was mainly due to differences in the potential of supporting trees/ha. This result was confirmed by calculating the mean relative space (Hart 1928) for each region. The relationship between top height and volume/ha for the unthinned control plots showed different volume production potential under comparable conditions at a given top height among the regions. The volume development figures follow the same order by region as the SDI(max) values. The highest potential (volume/ha at a given top height) is represented by the Atlantic Coastal Plain, followed by the Piedmont, followed by the Gulf Coastal Plain.