Leaf transpiration decreases similarly among five pine species as height increases over stand development

With increasing tree height, leaf transpiration (EL) is increasingly restricted by path-length resistance and gravity's discount of the driving force of xylem water flow. The effect of height on leaf transpiration is nearly always assessed using chronosequence data; however, in this long-term, dynamic study, we assessed increasing height's effects on EL using continuous monitoring of sap-flux for five Pinus species growing in a common-garden and experiencing a wide range of environmental conditions. We assessed how three drivers of EL-path-length (h), water-potential gradient (ΔΨ) and sapwood-to-leaf area ratio (AS:AL)-affect transpiration of the five Pinus species ranging five-fold in needle length by performing gas-exchange and water potential measurements and monitoring tree biometrics, sap-flux and soil and atmospheric conditions over 5 years at the Duke Forest, NC. With our methods controlling for all but the effect of tree hydraulics on transpiration, we found that EL, derived early in the study based on gas-exchange and later based on sap-flux measurements, were similar among species under both wet and dry soil moisture conditions. When soil moisture was not limiting, ΔΨ decreased across species with increasing needle length while whole-plant conductance (kplant) increased, leading to similar EL among species. Under soil drought, the trends with needle length of both variables became weaker as shorter-needle species showed a greater decrease in ΔΨ, while longer-needle species had a greater decline in kplant, again resulting in similar EL among species. Increasing h over time reduced EL similarly in all species, in part owing to similar annual minima of AS:AL among species and its invariance over a four-fold range in h. Controlling for non-hydraulic sources of variation showed that EL decreased with h similarly in five Pinus spp. of a wide range in leaf and crown characteristics.