PS 34-252 - Influences on longleaf pine regeneration in open woodland on deep-sand soils

Abstract: The most extensive remaining woodlands of longleaf pine, the iconic tree species of the American south, are on deep sand soils of the southeastern coastal plain. Fire exclusion through the early and mid-20^th century has resulted in poor regeneration of longleaf pine, but active management at Eglin Air Force Base in northwest Florida in the late 20^th century included reintroduction of fire, cutting of the invasive tree species sand pine, and killing competing broadleaved trees with herbicide. We analyzed plots monitored from years 2000 to 2015 to determine the factors governing transitions between life stages of longleaf pine. We addressed fire frequency; dominance of overstory longleaf pine and other major tree species; and abundance of midstory trees. Restoration activities resulted in >50% reduction of stem density of midstory trees, mostly mesic hardwoods (e.g., laurel oak and water oak) and sand pine. Mean stand basal area of overstory trees [classified as conifers with diameter at breast height (dbh) ≥ 10 cm or broadleaved trees with dbh ≥ 16 cm in the drought-prone and infertile system] remained at ~9 m^-2 ha^-1. Establishment was associated with a three-year masting cycle and the transition from germination to the onset of rapid height growth was slow, taking ca. 14 yr. Fire frequency had a positive effect on density of juveniles and no effect on transition from sapling (dbh >0 to 10 cm) to adult (i.e., ingrowth). Intraspecific competition from overstory trees became stronger at later life stages of longleaf pine: maximum seedling (dbh ≤ 0) numbers occurred at basal area 10 m² ha^-1, whereas the greatest rate of ingrowth occurred at 5 m² ha^-1. There was no evidence for facilitation and influence of interspecific competition was modest: there were strong negative effects of sand pines on seedling numbers and weak negative effects overstory xeric oaks on sapling density.

Judged by longleaf pine success the study provides varying support for elements of the restoration program. They provide strong support for sand pine removal, moderate support for increasing fire frequency, and no support for herbiciding midstory broadleaved trees. The results furthermore imply that density reductions to < 10 m² ha^-1 would not hurt longleaf pine regeneration. But changes in the management of this low productivity, high ecological-value system must be made with appreciation of the positive feedback of longleaf pine litter, and the negative feedback of most broadleaved trees’s litter, in sustaining the keystone process of fire.