Restoration of Longleaf Pine in Slash Pine Plantations

Upland forests in the southeastern United States (U.S.) were once dominated by the pyrogenic longleaf pine (Pinus palustris)-bunchgrass ecosystem that extended south from Virginia to Florida and west to East Texas. Historical land management has reduced the dominance of this ecosystem and today it occupies less than 3% of its original range. Restoration of the longleaf pine-bunchgrass ecosystem in the mesic flatwoods along the Gulf Coastal Plain of the southeastern U.S. has become a common habitat-recovery goal of public land managers, including those responsible for the Apalachicola National Forest (ANF). This research project explores potential techniques to accomplish this goal. There are three objectives: (1) to document changes in the extent of the longleaf pine-bunchgrass ecosystem in transition zones between the pine uplands and the hardwood wetlands of the ANF; (2) to test the importance of canopy retention in the transmission of fire across the landscape during conversion from slash pine (Pinus elliottii) plantations to longleaf pine forest; and (3) to test whether slash pine plantations can be successfully interplanted with longleaf pine seedlings. Overall, the study involved interactions between longleaf pine and fire in the ANF of northern Florida. Without frequent fire, the longleaf ecosystem can enter into what Lindenmayer et al. (2011) call, a "landscape trap," where ecological processes can no longer maintain the original forest type. A comparison of recent and old (1937) aerial photographs reveal that 32,000 hectares [ha] (80,000 acres [ac]) of the wet, grassy flats on the edges of the pine flatwoods have been invaded by shrubs, to become a shrub-dominated community. These shrubs, which are mostly titi (Cyrilla racemiflora and Cliftonia monophylla), have expanded primarily in sites with a greater than three-year fire-return interval. Restoration of the longleaf pine-bunchgrass ecosystem appears to need a <3-year fire-return interval if the processes favoring bunch grasses in the ground cover are to be maintained. For decades, forest managers in the southeastern U.S. have replaced natural longleaf pine communities with high-production slash pine plantations. I contend that restoration of longleaf pine in slash pine plantations will require retention of a fraction of the current slash pine canopy to produce enough needle cast to carry ground fire through the stand. In cooperation with the U.S. Forest Service, I monitored the results of 3 timber treatments in slash pine plantations. These experiments found that removal of the canopy reduces fire transmission. When all canopy trees were experimentally removed, only 12% of the landscape burned; whereas, 80% burned with only 33% canopy reduction. On the forest floor, partial retention of the slash pine canopy contributed ecological attributes similar to a longleaf canopy by promoting frequent low-intensity fires and creating ground cover conditions favoring herbaceous species. Where ground cover was primarily herbaceous, mortality of longleaf seedlings from fire averaged 8%, while in areas dominated by woody-vegetation, the average mortality was 30%. Canopy gaps also affected fire intensity and seedling mortality. In the 33% canopy reduction treatment, 23% of the seedlings were killed by fire, while in the large gaps only 12% were killed. Frequent low-intensity fires thus allow survival of the longleaf seedlings and promote herbaceous ground cover. Land managers generally consider longleaf pine to be a shade-intolerant species; therefore, they design restoration programs to maximize the growth rate of seedlings. In this experiment however, growth occurred in all canopy conditions. In the large gaps, with three years of growth, the seedlings increased their root-collar diameter by 190% to a mean diameter of 26.3 mm, and 96% under the 33% canopy reduction to a mean diameter of 18 mm in the same three years. In sum, while planting under the canopy does not maximize longleaf pine growth, it keeps a canopy, avoids clear-cutting, promotes fire, and allows ground cover restoration. Interplanting in combination with burning is an effective way to restore longleaf pine and avoid the shrubland landscape trap.