Leaf gas exchange characteristics differ among Sonoran Desert riparian tree species

We investigated leaf gas exchange responses to lear temperature, leaf-to-ai r vapor pressure deficit (VPD), and predawn and midday shoot water potentia l (Psi (pd) and Psi (md), respectively) of two native Sonoran Desert ripari an tree species, Fremont cottonwood (Populus fremontii S. Wats.) and Gooddi ng willow (Salix gooddingii Ball), and one exotic riparian tree species, sa ltcedar (Tamarix chinensis Lour. and related species). Measurements were ma de at two sites over 2 years that differed climatically. Because multiple l inear regression models explained less than 29% of the variation in stomata l conductance (g(S)) and less than 48% of the variation in net photosynthet ic rate (P-n) of all species, we used boundary-line analysis to compare gas exchange responses among species. Gas exchange rates were high in all spec ies. The hyperbolic relationship between P-n and g(S) suggested that initia l reductions in g(S) at high g(S) did not inhibit P-n. Reductions in g(S) o f cottonwood and willow occurred at Psi (md) values at or below previously reported xylem cavitation thresholds (-1.6 and -1.4 MPa, respectively), ind icating tight stomatal regulation of water loss and a narrow cavitation saf ety margin. In contrast, reductions in g(S) of saltcedar occurred at Psi md values well above the cavitation threshold (-7.0 MPa), but at much lower P si (md) values than in cottonwood and willow, suggesting a wider cavitation safety margin and less tight regulation of water loss in saltcedar. High V PD had a smaller effect on leaf gas exchange in willow than in cottonwood. In contrast, willow had a less negative Psi (pd) threshold for stomatal clo sure than cottonwood. Compared with cottonwood and willow, leaf gas exchang e of saltcedar was more tolerant of high VPD and low Psi (pd). These physio logical characteristics of saltcedar explain its widespread success as an i nvader of riparian ecosystems containing native Fremont cottonwood and Good ding willow in the Sonoran Desert.