Long-Term Trends of Planted Loblolly Pine Diameter Distribution Characteristics

Diameter distributions are fundamental characteristics of stand structure. It is widely assumed that unthinned plantation loblolly pine (Pinus taeda L.) diameter distributions are unimodal and slightly skewed. In this work, the assumption of unimodality is formally tested and confirmed using 413 long-term permanent plots representing three generations of genetics and silviculture across the native range of loblolly pine in the southeastern United States. Approximately 96% of plot measurements had no significant evidence to reject the hypothesis of unimodality. While levels often significantly differed, similar developmental trends of skewness, kurtosis, and estimated Weibull parameters were observed despite the advances of genetics and silviculture. The results of the study indicate the continued need for a flexible distribution for characterizing diameter distributions in plantation loblolly pine. Study Implications: Knowledge of diameter distribution helps inform management activities. Further, assessing monetary or ecological value requires an understanding of a stand's diameter structure. Using three long-term research studies established across the native range of the loblolly pine, this investigation confirms the assumption of slightly skewed, unimodal distributions. Additionally, long-term trends in skewness, kurtosis, and fitted Weibull parameters across the three generations of genetics and silviculture represented in the studies are presented. The results of this work confirm the need for a flexible distribution model form and indicate that managers can expect similar trends in diameter distribution structure in both vintage and contemporary stands at least until first thinning.