Longleaf pine-grassland are characterized by a diverse surface plant community of graminoids and forbs that co-exist with patches of even-aged cohorts of longleaf pine (Pinus palustris) growing in initially dense clusters. Little attention has been given to how differing ages of longleaf pine clusters during their multi-century cycle affect the surface plant community. This study used one of the few remaining old-growth longleaf pine savannas (Wade Tract in southern Georgia) to attempt to test the hypothesis that physical processes related to the cycle of longleaf pine’s regeneration clusters predictably affect plant species composition and contribute to maintenance of diversity and spatial heterogeneity of the community. We categorized clusters into five age classes spanning 250 years based on increment cores of selected trees associated with a 40-year census of tree individuals covering 60 hectares. We measured soil characteristics, tree population structure, fuel loads and composition, plant productivity, woody plant abundance, and plant species cover in ten 10m2 plots per cluster age class.
Results/Conclusions
We found that environmental characteristics such as total soil carbon and organic matter occurred in relatively distinct gradient pattern, beginning in low quantities in younger clusters and increasing somewhat with age before returning to low quantities in the gaps. Total basal area initially increased with cluster age, then decreased as individuals thinned out over time. Correspondingly, pine needle litter and total heat released during fires peaked in 51-90 year old clusters and gradually declined over time. Although highly prevalent species muted any overall plant community pattern, plants in certain taxonomic groups, including families Fabaceae, Asteraceae and Euphorbiacea and genera Dichanthelium and Rubus were strongly associated with particular cluster age classes, presumably responding to changes in physical conditions. Results suggest a conceptual model in which the life cycle of longleaf pine clusters drive environmental conditions, fire regimes, and plant species composition that contribute to plant biodiversity and spatial heterogeneity.