Structural diversity of the longleaf pine ecosystem

Structural diversity is an important attribute of forest ecosystems and is related to ecosystem stability, adaptability and resilience as well as biodiversity and productivity. Structural diversity in the longleaf pine (Pinus palustris Mill.) ecosystem, the most diverse ecosystem of North America, has not been well documented, especially across the longleaf pine's range of occurrence. We utilized data from 919 Forest Inventory and Analysis (FIA) plots of longleaf pine distributed across 9 states of the southeastern United States and classified these plots on the bases of stand origin (natural or artificial), ownership (public or private), burn condition (burned or not burned in the past 5 years), site conditions (xeric, mesic, or hydric), and number of age classes (one or two). For each plot under a classification category, we calculated Shannon diversity index based on 5-cm diameter classes. The structural diversity estimates, based on Shannon diversity indices, were then analyzed for the entire range of the longleaf pine ecosystem. Our findings indicate that the structural diversity varies between 0.00 and 2.20 across the longleaf pine range, with mean and median structural diversity of 1.35 and 1.42, respectively. Stand origin, site condition, ownership, and number of age classes significantly affected mean structural diversity (alpha = 0.05). Plots with natural origin, mesic or hydric site conditions, public ownership, and two age classes had higher structural diversity. Using the geographic coordinates of each FIA plot and the corresponding Shannon index value, we created a structural diversity distribution map and a hot spot map of the longleaf pine ecosystem. The maps showed that the longleaf pine ecosystem exhibited variable and heterogeneous distribution of structural diversity in the southeastern United States, with southeastern Mississippi and central Alabama areas as the hotspots. Southcentral Georgia exhibited least structural diversity in longleaf pine forests in the southeastern United States.