Long-term stand dynamics of old-growth mountain longleaf pine (Pinus palustris) woodlands

Understanding the mechanisms and patterns of tree establishment, growth, and mortality are important to management and restoration of savannas, woodlands, and forests. Reference old-growth woodlands and forests can be used as benchmarks for emulating historic structure and spatial and temporal patterns of tree establishment and death. Southeastern USA longleaf pine (Pinus palustris Mill.) woodlands and savannas, formerly extensive ecosystems now considered among the most imperiled in the USA, lack long-term data on growth and mortality, particularly in the mountain provinces. We used permanent plots established in 1999 in two old-growth stands (Caffey Hill and Red-Tail Ridge) in northern Alabama to evaluate changes in stand structure and examine spatial patterns and drivers of growth and mortality over a 15-year period. Both stands experienced substantial tree mortality (12% and 26%) and initial decreases in stand basal area in response to periodic fires; however by 2014 both stands had recovered and exceeded 1999 basal area. Average annual rates of mortality ranged between 1.0% and 1.8% across the two stands. Pine ingrowth over the 15 year period differed significantly by site, with 182.0 and 6.7 new trees ha−1 at Caffey Hill and Red-Tail Ridge, respectively. Stem density (live longleaf pines ha−1) increased by 32% at Caffey Hill and decreased by 16% at Red-Tail Ridge. The ingrowth and mortality of longleaf pine in both stands was significantly spatially clustered. Density-dependent mortality affected smaller pines (2.5-10 cm DBH), accelerating self-thinning of dense patches of young pines. These long-term data illustrate the dynamic nature of two old-growth mountain longleaf pine stands and provide insight into longleaf pine stand dynamics and more broadly to the understanding of dynamics within fire-prone woodlands.