Crown Complexity Influences the Physiological Activity of Old-Growth Longleaf Pine Trees

Crown complexity can influence tree physiology and ecosystem processes. We quantified crown complexity of an old-growth longleaf pine woodland, related it to tree physiology and ecosystem process, and considered restoration implications. Trees with continuous crowns (live crown ratio ~66%) accounted for 75% and trees with discrete crowns (live crown ratio ~33%) accounted for 23% of mature trees. Leaf area was 1.69 times higher in trees with continuous compared to discrete crowns; however, sapwood area, sapwood area-to-leaf area ratio, and cumulative tree transpiration were similar, indicating different hydraulic architectures. Transpiration was 1.59 times higher in lower compared to upper portions of continuous crowns, presumably because higher solar zenith angle supported photosynthesis in lower crown portions during winter. We estimated 467.2 mm of annual mature longleaf transpiration with continuous and discrete crowns transpiring 378.6 and 88.6 mm, respectively. We observed diel oscillations in soil moisture during extended drying periods consistent with hydraulic redistribution that maintained soil moisture and transpiration for > 8 days. Best practices for longleaf restoration where the primary goal is to reproduce crown complexity should incorporate wider spacing and/or earlier thinning to promote retention of lower branches, which will ultimately result in an old-growth stand reflective of natural regeneration.