© 2001 Heron Publishing—Victoria, Canada
Leaf gas exchange characteristics differ among Sonoran Desert riparian tree species
Jonathan L. Horton (1, 2), Thomas E. Kolb (1) and Stephen C. Hart (1)
1. School of Forestry, College of Ecosystem Science and Management, Northern Arizona University, Flagstaff, AZ 86011-5018, USA / 2. Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060-0406, USA / Received April 3, 2000
Summary
We investigated leaf gas exchange responses to leaf temperature, leaf-to-air vapor pressure deficit (VPD), and predawn and
midday shoot water potential (Ψpd and Ψmd, respectively) of two native Sonoran Desert riparian tree species, Fremont cottonwood (Populus fremontii S. Wats.) and Goodding willow (Salix gooddingii Ball), and one exotic riparian tree species, saltcedar (Tamarix chinensis Lour. and related species). Measurements were made at two sites over 2 years that differed climatically. Because multiple
linear regression models explained less than 29% of the variation in stomatal conductance (gs) and less than 48% of the variation in net photosynthetic rate (Pn) of all species, we used boundary-line analysis to compare gas exchange responses among species. Gas exchange rates were
high in all species. The hyperbolic relationship between Pn and gs suggested that initial reductions in gs at high gs did not inhibit Pn. Reductions in gs of cottonwood and willow occurred at Ψmd values at or below previously reported xylem cavitation thresholds (–1.6 and –1.4 MPa, respectively), indicating tight stomatal
regulation of water loss and a narrow cavitation safety margin. In contrast, reductions in gs of saltcedar occurred at Ψmd values well above the cavitation threshold (–7.0 MPa), but at much lower Ψmd values than in cottonwood and willow, suggesting a wider cavitation safety margin and less tight regulation of water loss
in saltcedar. High VPD had a smaller effect on leaf gas exchange in willow than in cottonwood. In contrast, willow had a less
negative Ψpd threshold for stomatal closure than cottonwood. Compared with cottonwood and willow, leaf gas exchange of saltcedar was more
tolerant of high VPD and low Ψpd. These physiological characteristics of saltcedar explain its widespread success as an invader of riparian ecosystems containing
native Fremont cottonwood and Goodding willow in the Sonoran Desert.
Keywords:
boundary-line analysis, Fremont cottonwood, Goodding willow, net photosynthetic rate, Populus fremontii, Salix gooddingii, saltcedar, stomatal conductance, Tamarix chinensis, water stress.
