A retrospective comparison of carrying capacity of two generations of loblolly pine plantations

Abstract Permanent sample plots were installed in two generations of operationally-established loblolly pine plantations across the southeastern US. The first population, established with non-genetically improved seedlings, did not receive mid-rotation fertilizer or competition control treatments. Approximately 20 years later, using consistent protocols for plot locations and tree measurements, permanent plots were installed in operationally-established plantings with genetically improved seedlings and receiving mid-rotation fertilizer and release treatments. These two studies, both of which reflect contemporary plantation management at the time they were initiated, were analyzed to examine the impact of changing management intensity and environmental influences on carrying capacity. Results showed that the second generation of plantings generally exhibited substantial increases in site index, as well as in maximum stand density. Assuming a constant rotation age of 25 years and computing mean annual increment using an empirical yield function fitted to the overall data, gave an estimated increase in volume production of 47% for the region. The estimated productivity gains were disparate by physiographic regions with the Piedmont showing an estimated increase of 83%, Atlantic Coastal Plain 64%, and Gulf Coastal Plain 34%. All physiographic regions can be assumed to have benefitted from increased levels of atmospheric CO2. The extraordinary increase in the Piedmont from one generation to the next likely resulted from a combination of factors, including maintaining more trees at maximum stand density and disproportionate gains in improved soil quality and climate change, as compared to the two coastal plain regions.