The hydraulic architecture of Thuja occidentalis

Abstract

Leaf specific conductivities (LSC) were measured on stem segments excised from various points within the canopy of eastern white cedar tress, T. occidentalis L. LSC is defined as the water flow rate (kilograms per second) through a stem caused by a unit of pressure potential gradient (megapascals [MPa] per meter) per unit leaf surface area supplied by the stem (square meters). LSC were measured on stems of various diameters and varied over a factor of 30 in magnitude from 1 .times. 10-5 kg s-1 m-1 MPa-1 for stems 1 mm in diameter to 3 .times. 10-4 kg s-1 m-1 MPa-1 for stems 100 mm in diameter. LSC was related to stem diameter (D (millimeters)) by the following empirical formula: LSC = 9.58 .times. 10-6 .times. D0.727. LSC measured on stem segments including a node had significantly lower LSC than internodal stem segments of the same length. Various water relations parameters were measured on cedar trees on a diurnal basis including evaporative flux, leaf resistance to evaporation, shoot water potential, air temperature and air humidity. Water potential isotherms were also measured on excised green shoots. About 15% of the total water evaporated from green shoots comes from stored water while the shoot water potentials are growing progressively negative. The typical peak evaporative flux in midday was 1.5 .times. 10-5 kg s-1 m-2; using this value and our measured LSC, it was estimated that the pressure potential gradients in the stems must be 50 kPa m-1 in stems 100 mm in diameter and 1500 kPa m-1 in stems 1 mm in diameter. Pressure potential gradients were measured in stems 30 to 50 mm in diameter by the pressure bomb technique and were 69 kPa m-1 during a typical afternoon and this confirms the accuracy of the above estimates.