Longleaf Pine Patch Dynamics Influence Ground-Layer Vegetation in Old-Growth Pine Savanna

Old-growth longleaf pine savannas are characterized by diverse ground-layer plant communities comprised of graminoids, forbs, and woody plants. These communities co-exist with variable-aged patches containing similar-aged trees of longleaf pine (Pinus palustris Mill.). We tested the conceptual model that physical conditions related to the cycle of longleaf pine regeneration (stand structure, soil attributes, fire effects, and light) influence plant species' composition and spatial heterogeneity of ground-layer vegetation. We used a chrono-sequence approach in which local patches represented six stages of the regeneration cycle, from open areas without trees (gaps) to trees several centuries old, based on a 40-year population study and increment cores of trees. We measured soil characteristics, patch stand structure, fuel loads and consumption during fires, plant productivity, and ground-layer plant species composition. Patch characteristics (e.g., tree density, basal diameter, soil carbon, and fire heat release) indicated a cyclical pattern that corresponded to the establishment, growth, and mortality of trees over a period of approximately three centuries. We found that plants in the families Fabaceae and Asteraceae and certain genera were significantly associated with a particular patch stage or ranges of patch stages, presumably responding to changes in physical conditions of patches over time. However, whole-community-level analyses did not indicate associations between the patch stage and distinct plant communities. Our study indicates that changes in composition and the structure of pine patches contribute to patterns in spatial and temporal heterogeneity in physical characteristics, fire regimes, and species composition of the ground-layer vegetation in old-growth pine savanna.