Overstory individuals in old-growth longleaf pine (Pinus palustris) stands display differences in canopy complexity, with some trees having discrete canopies while others retain more continuous canopies comprised of lower branches. Canopy complexity could prove to be an important component of longleaf pine ecosystem resilience and thereby an important restoration consideration, if it infers higher tolerance to drought by increasing the ability to maintain physiological functions with limited water availability. Here we investigate the influence of structural canopy complexity on ecosystem resilience in the longleaf pine ecosystem by quantifying leaf area and physiological processes along canopy height gradients. Vertical leaf area distribution was determined using terrestrial LIDAR scans from three different old-growth longleaf pine stands. Water potential (ψ) and sap flow (Fd) were determined as a function of vertical canopy position across periods of high and low soil moisture availability.
Results/Conclusions
We found that a large amount of leaf area is contained in the lower strata of complex old-growth longleaf pine canopies. By measuring tree physiological functions above and below branches in the lower strata through wet and dry periods, we calculated the difference this additional leaf area makes regarding resilience to drought stress. Preliminary results show that lower branches exhibited higher water potential than upper branches within the same continuous canopies, even when accounting for the constant gravitational component (-0.01MPa m-1); however, water potential was higher in discrete canopies compared to continuous canopies when measured at the same height. Whole-tree water use was higher in continuous canopies than discrete canopies. The majority of water use occurred in upper portions of complex canopies, but relative proportions and absolute volumes changed with soil moisture availability and plant water stress. Together, these data suggest that lower strata of complex canopies are under less water stress than upper portions of continuous and discrete canopies. Our data also suggest canopy complexity and leaf area may influence the relationship between water potential and height. Therefore, additional lower leaf area found in more structurally complex canopies should maintain a greater capacity for physiological functions under drought stress.