© 2007 Heron Publishing—Victoria, Canada
Nocturnal transpiration causing disequilibrium between soil and stem predawn water potential in mixed conifer forests of Idaho
Kathleen L. Kavanagh (1, 2), Robert Pangle (1) and Alisa D. Schotzko (1)
1. Department of Forest Resources, University of Idaho, P.O. Box 441133, Moscow, ID 83844-1133, USA / 2. Corresponding author (katyk@uidaho.edu) / Received March 16, 2006; accepted August 31, 2006; published online January 2, 2007
Summary
Soil water potential (Ψs) is often estimated by measuring leaf water potential before dawn (Ψpd), based on the assumption that the plant water status has come into equilibrium with that of the soil. However, it has been
documented for a number of plant species that stomata do not close completely at night, allowing for nocturnal transpiration
and thus preventing nocturnal soil–plant water potential equilibration. The potential for nighttime transpiration necessitates
testing the assumption of nocturnal equilibration before accepting Ψpd as a valid estimate of Ψs. We determined the magnitude of disequilibrium between Ψpd and Ψs in four temperate conifer species across three height classes through a replicated study in northern Idaho. Based on both
stomatal conductance and sap flux measurements, we confirmed that the combination of open stomata and high nocturnal atmospheric
vapor pressure deficit (D) resulted in nocturnal transpiration in all four species. Nocturnal stomatal conductance (gs-noc) averaged about 33% of mid-morning conductance values. We used species-specific estimates of gs-noc and leaf specific conductance to correct Ψpd values for nocturnal transpiration at the time the samples were collected. Compared with the unadjusted values, corrected
values reflected a significantly higher Ψpd (when D > 0.12 kPa). These results demonstrate that comparisons of Ψpd among species, canopy height classes and sites, and across growing seasons can be influenced by differential amounts of nocturnal
transpiration, leading to flawed results. Consequently, it is important to account for the presence of nocturnal transpiration,
either through a properly parameterized model or by making Ψpd measurements when D is sufficiently low that it cannot drive nocturnal transpiration. Violating these conditions will likely result in underestimation
of Ψs.
Keywords:
Larix occidentalis, nocturnal transpiration, Pseudotsuga menzesii, sapflux, soil water, Thuja plicata, Tsuga heterophylla
.
