Whole-tree water relations of co-occurring mature Pinus palustris and Pinus elliottii var. elliottii

The natural range of longleaf pine (Pinus palustris P. Mill.) and slash pine (Pinus elliottii var. elliottii Engelm.) includes most of the southeastern US Coastal Plain, and there is now considerable interest in using these species for ecological forestry, restoration, and carbon sequestration. It is therefore surprising that little information is currently available concerning differences in their ecological water relations in natural stands. In this study, we compared water use, stomatal conductance at the crown scale (G(crown)), and whole-tree hydraulic conductance of mature pine trees growing in a naturally regenerated mixed stand on a flatwoods site in north-central Florida. We found remarkable similarities between longleaf and slash pine in stored water use, nocturnal transpiration, and whole-tree hydraulic conductance. Mean daily transpiration rate was higher for slash than for longleaf pine, averaging 39 and 26 L. tree(-1), respectively. This difference was determined by variations in tree leaf area. Slash pine had 60% more leaf area per unit basal sapwood area than longleaf pine, but the larger plasticity of longleaf pine stomatal regulation partially compensated for leaf area differences: longleaf pine had higher G(crown) on days with high volumetric water content (theta(v)) but this was reduced to similar or even lower values than for slash pine on days with low theta(v). There was no species difference in the sensitivity of G(crown) to increasing vapor pressure deficit.