Vertical gradients of water potential and tissue water relations in Sitka Spruce trees measured with the pressure chamber.
Reports a study in 1970-71 on 23-year-old trees of Picea sitchensis growing on a NE-facing slope at Fetteresso Forest, Kincardineshire, Scotland, in which water, solute and pressure potentials were measured with a pressure chamber. Water potentials of -15 bar or lower frequently occurred in the upper parts of the canopy on warm sunny days of low vapour pressure deficit, although there was never a shortage of water in the soil. Root water potentials were in the range -0.5 to -4.5 bar. Gradients of water potential of up to 2 bar/m occurred in the stem, and larger gradients occurred in the primary and secondary branches. On overcast or wet days the water potentials in the canopy were higher and the gradients smaller. It was concluded that the resistance to flow in the stem was largely responsible for the marked drop in potential between roots and leaves. The resistance per unit length was estimated as 107 bar.s.m-4 in the stem, which is considerably higher than in the angiosperms or Pines for which comparable data are available. The solute potential of shoots in the canopy varied between -13 and -24 bar depending on the time of year. There was a tendency for solute potentials to be lower higher up in the canopy. The change in pressure potential (turgor pressure) with water content was determined from typical Hofler diagrams constructed from the water and solute potentials, and a pressure-dependent bulk modulus of elasticity was derived. Empirical equations relating solute, pressure and water potential to water content were determined.