Regeneration dynamics in longleaf pine: Challenging conventional wisdom to improve management
Longleaf pine (Pinus palustris) has unique characteristics relative to other Southeastern US pine species. Some features are related to regeneration phenology, most specifically the prolonged “grass stage” during which the seedlings do not grow appreciably in height. Grass-stage longleaf pine seedlings are extremely fire tolerant, though not fireproof. This fire adaptation gives longleaf pine a competitive advantage in the fire-maintained forest, yet the slow aboveground growth makes them vulnerable to competition from other plant species, and natural regeneration cohorts in longleaf pine forests are observed to be located primarily within canopy gaps. These two factors led to the classification of longleaf pine as one of the most shade-intolerant tree species in North America, and silviculture for the species to be oriented toward even-aged regeneration systems. However, natural longleaf pine stands, whether old-or second-growth, are often multi-aged, and many forests managed over decades for ecological objectives were able to maintain this multi-aged structure. This apparent dichotomy between putative shade tolerance and observed longleaf pine regeneration dynamics was the subject addressed in a systematic manner by Dr. Bob Mitchell and associates, with results that expanded our understanding of longleaf pine and how to best manage the species for multiple objectives.
Competition effects on longleaf pine seedlings were shown to be from both above- and belowground sources from several series of plot-based experiments. Aboveground competition for light did have a significant effect on seedlings grown under different levels of canopy coverage, but trenching plots showed that belowground competition for water and nutrients also had significant effects. In addition, even seedlings grown in overstory canopy gaps with abundant light availability were often not free to grow due to intense woody plant competition caused by interactions between fuels and fire behavior in the gaps. The complex influences of above- and belowground competition were summarized in a 3-stage model for longleaf pine seedling survival and growth with different levels of canopy cover and over time. This model showed that, contrary to conventional wisdom, longleaf pine actually has characteristics typical of both shade intolerant and tolerant species. These results significantly changed our understanding of appropriate management and restoration practices for longleaf pine: multi-aged silvicultural systems are feasible with attention to canopy manipulations and stocking; advance longleaf pine regeneration is necessary before opening the canopy; and underplanting longleaf pine below the canopy of other pine species is a potential tool for restoration.