Sapwood hydraulic characteristics of lodgepole pine, defined using Darcy's law, in relation to competitive position, height repression, and tree water use following thinning

In this dissertation, I investigated the relationship between the water conducting properties of lodgepole pine (Pinus contorta Dougl. Ex Loud.) stems and the phenomenon of height repression in high-density fire-origin stands. The literature dealing with the physical limitations on water movement through trees uses inconsistent terminology, symbols and units to describe the water conducting properties of wood, due to application of either an Ohm's law analogy and/or Darcy's law. Explanations of Ohm's law and Darcy's law are provided to clarify this confusion. A proposal for a unified nomenclature based on Darcy's law to define hydraulic conductivity (K Psi), permeability (k), hydraulic capacity ( Qh) and leaf specific hydraulic capacity (Q L) is presented. Hydraulic properties of excised dominant (D), co-dominant (CD), and suppressed (SP) tree stem segments from low (height repressed) and medium sites were measured. Leaf area, crown projection area and growth over the last five years were measured to assess growth efficiency (GE ) and crown efficiency (CE). Leaf area index (LAI) was also measured. Faster growing trees from medium sites were more permeable to water, as were D (k = 1.97 x 10-12 m2) and CD (k = 1.79 x 10 -12 m2) trees compared to SP (k = 0.71 x 10-12 m2) due, in part, to a greater proportion of early-wood. The leaf/sapwood area ratio ( S) varied with crown dominance position (D > CD > SP), but not by site type. Volume growth was closely associated with both hydraulic capacity (Qh) and leaf area, but LAI and Q L did not differ between sites. Height repression was not associated with lower GE, but was associated with lower CE. SP tree needles had the highest concentration of N and P in their needles, allowing them to maintain higher GE for a given QL. Height repression on low sites may be a result of total leaf area being distributed among too many small trees, capable of supplying sufficient water to small crowns. Finally, water use in thinned and un-thinned trees was examined using thermal dissipation sap flow sensors. Declines in k of thinned trees did not result in declines in tree water use, suggesting that soil water availability is more important than sapwood hydraulic properties in determining stomatal behavior.