PARENT SESSION
Contributed Oral Session 16: Photosynthesis and Water Relations: Conductivity; Stress
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 524 A, Level 5, Palais des congrès de Montréal

Disequilibrium between soil and foliar predawn water potential due to nighttime canopy water loss in temperate conifers exposed to nocturnal vapor pressure deficits.

Kavanagh, Kathleen^*,1, Pangle, Robert¹, Schotzko, Alisa¹, ¹ Forest Resources Dept., Moscow, ID, USA

ABSTRACT- Measurement of predawn foliar water potential (pd) is commonly used to determine the water potential (s) of the soil surrounding tree roots. However, recent studies have documented instances where plants do not reach equilibrium with the soil due to nighttime transpiration. During June 2004, we observed predawn disequilibrium in four Inland Northwest conifer species exposed to nighttime increases in vapor pressure deficit (VPD). In addition, we observed unstable baseline temperature differences (Tm) in several measured trees that were also equipped with sap-flux sensors. pd averaged -0.78 MPa (SE = 0.015) during June 18 measurements in a mixed species 20-year-old stand, which experienced a nocturnal AM spike in mean VPD and wind speed of 0.81 kPa and 1.75 m s-1. This contrasts with July 14 pd measurements in the same stand where foliar water potential averaged -0.58 MPa (SE = 0.032) following a nocturnal VPD minimum of 0.04 kPa and negligible wind. In a mixed species 120-year-old stand, June pd averaged -1.00 MPa (SE = 0.070) following early morning spikes in mean VPD and wind to 1.1 kPa and 4.65 m s-1. In comparison, July predawn values averaged -0.75 MPa (SE = 0.044) in the older stand following a nocturnal minimum VPD of -0.38 kPa and low wind. We attribute the lower June predawn values to nighttime water loss due to the fact that July measurements followed a period of steadily decreasing soil water content and minimal precipitation. Soil water content (10 to 30 cm depth) in the young stand exhibited a decline from 31.0 to 18.3 % during the period between measurements. In the older stand, soil water declined from 27.5 to 15.0 % during this period. Precipitation totaled 2.49 cm between measurement periods, however only a percentage of this total reached the soil due to canopy interception. These observations suggest that measurements of pd should be performed following nocturnal periods when VPD is approaching zero to minimize canopy water loss due to nocturnal conductance and thus resulting disequilibrium between pd and s.

Key words: nighttime transpiration, nocturnal conductance, soil water potential, vapor pressure deficit