Relationship Between Foliar δ13C and Hydraulic Pathway Length in Pinus palustris

Because of the importance of hydraulic architecture in understanding physiological differences between small and large trees, we tested the hypothesis that foliar δ13C increased with aboveground flow-path length in longleaf pine (Pinus palustris Mill.) trees of varying age and size at two sites in the lower Coastal Plain. This species offered a unique opportunity to examine variability in foliar δ13C in relation to pathway length because of the open, well-mixed canopy in savanna-like stands and concentration of foliage in splayed tufts on branch ends. At both sites, models with total flow-path length (bole + branch) explained 78– 81% of the variation in foliar δ13C, and a full model with separate slopes and intercepts for each tree was required. Branch flow-path length was highly correlated with foliar δ13C and the models accounted for 76–88% of the variation in foliar δ13C, and a reduced model with one slope was suitable for each site. Flow-path length was not as well correlated to foliar N concentration, specific leaf area or tuft leaf area. Greater δ13C enrichment with increasing flowpath length indicates the need for mechanistic studies to better understand the role of hydraulic resistance in controlling leaf-level physiology within the canopy and among different size longleaf pine trees. FOR. SCI. 49(5):790–798.