Hydraulic patterns and safety margins, from stem to stomata, in three eastern US tree species

Adequate water transport is necessary to prevent stomatal closure and allow for photosynthesis. Dysfunction in the water transport pathway can result in stomatal closure, and can be deleterious to overall plant health and survival. Although much is known about small branch hydraulics, little is known about the coordination of leaf and stem hydraulic function. Additionally, the daily variations in leaf hydraulic conductance (K-leaf), stomatal conductance and water potential (psi(L)) have only been measured for a few species. The objective of the current study was to characterize stem and leaf vulnerability to hydraulic dysfunction for three eastern US tree species (Acer rubrum, Liriodendron tulipifera and Pinus virginiana) and to measure in situ daily patterns of K-leaf, leaf and stem psi, and stomatal conductance in the field. Sap flow measurements were made on two of the three species to compare patterns of whole-plant water use with changes in K-leaf and stomatal conductance. Overall, stems were more resistant to hydraulic dysfunction than leaves. Stem P-50 (psi resulting in 50% loss in conductivity) ranged from -3.0 to -4.2 MPa, whereas leaf P50 ranged from -0.8 to -1.7 MPa. Field psi(L) declined over the course of the day, but only P. virginiana experienced reductions in K-leaf (nearly 100% loss). Stomatal conductance was greatest overall in P. virginiana, but peaked midmorning and then declined in all three species. Midday stem. in all three species remained well above the threshold for embolism formation. The daily course of sap flux in P. virginiana was bell-shaped, whereas in A. rubrum sap flux peaked early in the morning and then declined over the remainder of the day. An analysis of our data and data for 39 other species suggest that there may be at least three distinct trajectories of relationships between maximum K-leaf and the % K-leaf at psi(min). In one group of species, a trade-off between maximum K-leaf and % K-leaf at psi(min) appeared to exist, but no trade-off was evident in the other two trajectories.