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

Canopy complexity can influence tree physiology and ecosystem processes. Here, we relate canopy complexity of old-growth longleaf pine to physiology and explore implications for ecosystem resilience. Mature longleaf grown in open woodlands with frequent prescribed fire exhibit either discrete canopies where branches extend down ~1/3 of tree height, or continuous canopies where branches extend down ~2/3 of tree height. Continuous canopies accounted for 76% and discrete canopies accounted for 24% of mature trees in the old-growth woodland we surveyed. Leaf area was 43% higher in continuous than discrete canopies. Similarly, cumulative whole-tree transpiration was 41% higher in continuous than discrete canopies, suggesting that differences in transpiration were proportional to differences in leaf area. Cumulative transpiration was similar between upper and lower strata of continuous canopies; however, proportional contributions to whole-tree transpiration varied seasonally. Specifically, upper strata of continuous canopies contributed a higher proportion of transpiration during the summer, whereas lower strata contributed a larger portion during the winter. Daily transpiration responded to environmental variables consistently in continuous and discrete canopies, and in upper and lower continuous canopy strata, with a few important exceptions which will be discussed. Our results suggest that canopy complexity in old-growth longleaf pine woodlands influences tree physiology and ecosystem processes, which are linked to ecosystem resilience. These results have implications on efforts to restore longleaf pine ecosystems that rely largely on establishing high-density plantations of seedlings that self-prune lower branches through stand development and result in relatively simple, discrete canopy trees when mature. We suggest that best practices for longleaf restoration should incorporate thinning at earlier stages to promote retention of lower branches, which will ultimately result in an old-growth longleaf stand reflective of natural regeneration that occurs in canopy gaps and provide greater resilience.