Restoration of the Montane Longleaf Pine Forest Can Increase Understory Plant Species at Bankhead National Forest in Alabama, USA

Longleaf pine (Pinus palustris Mill.) forests are well known for their high-quality timber and biodiversity. However, it is unclear whether restoring montane longleaf pine forests from mixed pine–hardwood forests increases understory plant species richness. This study documented understory plant species in 16 stands across three stages (Early: ≤25 years, Late: 25–75 years, and Mature: >75 years) of restored montane longleaf pine forest and in nearby mixed pine–hardwood forests (Control) in Bankhead National Forest, Alabama, USA. Ground LiDAR was used to capture the forest structure, yielding 14 derived parameters. These data were statistically analyzed using ANOVA and Correlation. Results indicated higher understory plant species richness in the montane longleaf pine forest, with the highest richness observed at the Mature stage, particularly for shrub and grass species. While the average understory plant species richness did not differ significantly between the Early and Late stages across categories, their community similarity index was only 0.53. The percentage of shade-tolerant species was 71% at Control sites, 51% at Early sites, 60% at Late sites, and 62% at Mature sites. Understory plant species richness was negatively correlated with total crown area and basal area. These findings imply that forest management practices (e.g., harvesting, thinning, and prescribed burning) could create forest structures suitable for a great diversity of understory plant species. Forest structure can be quantitatively described by 3D structure indices derived from LiDAR and other tools.