Understand the grass stage adaptation in Pinus: A case study on longleaf pine (Pinus palustris)
Longleaf pine (LLP; Pinus palustris) is one of the seven Pinus species in the world that possess a grass stage (i.e., no height growth with the first 2 to 20 years). This unique adaptation ensures the regeneration success of LLP under a very frequent surface fire regime, which resulted in the historical dominance of LLP in the costal plains and sandhills of the southeast United States. However, mechanisms through which the grass stage adaptation helps the fire survival of LLP regeneration are not well studied. Our current understandings are largely speculation based on field observations, which needs to be either substantiated or refuted by data. In this study, I collect growth and fire survival data from naturally and artificially regenerated LLP seedlings and saplings. Comparative data were also collected for loblolly pine, a sympatric but fire susceptible co-genus. These data were analyzed to elucidate the mechanism of fire survival in LLP regeneration.
During the grass stage, fire survival was affected by seedling size. Regardless of seedling size, most seedlings (>80%) survived fire had their root collar buried in mineral soils. Almost 46% of the survived seedlings were due to sprouting. During the early height growth stage, LLP had a similar height growth pattern but a distinctive different diameter growth pattern when compared to loblolly pine. The bark thickness of LLP was 2-3 times thicker than that of loblolly pine when diameter was < 5 cm. These results indicate that the position of root collar combined with the ability to sprout contribute significantly to fire survival during the grass stage while developing a critical bark thickness is critical to fire survival during the early height growth stage.