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Western N(wth American Naturalist 65(2), C 2005. pp, 175 -18-5a
<br />NONRESPONSE OF NATIVE COTTONWOOD TREES TO
<br />WATER ADDITIONS DURING SUMMER DROUGHT
<br />Greg Cox 1-2, Dylan Fischerl,3,4,5, Stephen. C. Hartt•3, and T.G. Whitharn"
<br />ABSTRAM—StUdiCS have demonstrated that some riparian trees may switch their reliance on surface soil water
<br />(unsaturated or vadose zone) to groundwater (saturated zone) sources during the growing season in association with
<br />changes in moisture availability. A closely related question i& How do these trees respond to pulse increases in water
<br />availability in previously dry zones? the tested the whole-tree physiological response of 6 natural Poptdta genotypes to
<br />water additions during the peak of summer drought in northern Utah, USA. We found clear evidence that trees were
<br />insensitive to water additions to the sorfasm soil that were twice the magnitude of whole-tree transpiration rates. Our
<br />results suggest that some cott(mwoods may have little immediate transpiration or leaftonductatice response to pulse soil
<br />moisture increases, This lack of response may be related to a water-use strategy associated with regional climate pat-
<br />terns (i.e., genetic or environmental programming), cavitation recovery, or other physical determinants of water use such
<br />as depth to groundwater. Our data suggest that it is important to consider potential nonresponsiveness to changes in soil
<br />water availability when evaluating the impact ofelimate change on these important and productive ecosystems.
<br />Key words. sap floe, cottonwood, draught, water addition, conductance, tearer potential, Populus.
<br />Studies that examine cottonwood (Populus
<br />spp.) response to increasing soil moisture are
<br />important for several reasons, First, cotton-
<br />woods are dominant trees of many western
<br />intermountain river ecosystems of the United
<br />States. Poptdus angusti
<br />folia (narrow leaf cotton-
<br />wood), Efi-emontii (Fremont cottonwood), and
<br />their natural hybrids are often described as
<br />facultative phreatophytes (Snyder and Williams
<br />2000, Horton et al. 2001a, 2001b, 2003; but
<br />see Busch et al. 1992). They are generally re-
<br />stricted to riparian areas where they are the
<br />dominant plant species and play a major role in
<br />ecosystem processes (Driebe and Whitham
<br />2(x1O, Schweit-7:er et al. 2004, Fischer et al. 2004).
<br />Second, it is important to know how riparian
<br />species may respond to altered hydrological
<br />patterns induced by global change. For exam-
<br />ple, many modeling efforts predict increased
<br />pulse-event summer rainfall in the southwest-
<br />ern U.S. (National Assessment Synthesis Team
<br />2002), but knowledge of intermountain and
<br />southwestern riparian species responses to these
<br />rainfall events is incomplete. Finally, many stud-
<br />ies on cottonwood responses to water additions
<br />have been conducted in plantations. Results
<br />from these studies have been interpreted in
<br />the context of implications for silviculture
<br />(Marron et al. 2002) rather than in terms of the
<br />functioning of native forests (Horton et al- 2001a,
<br />2001b). Understanding how cottonwoods re-
<br />spond to changing water availability is impor-
<br />tant to conservation and restoration for this
<br />threatened habitat (cottonwood riparian forests).
<br />Cottonwoods may alternate water source
<br />use between groundwater and surface soil
<br />moisture (Smith et al. 1991, 1998, Rood et al.
<br />2003) or -act as obligate phreatophytes (Busch
<br />et al. 1992) by depending entirely on ground-
<br />water. For instance, in the spring, cottonwoods
<br />may derive water mostly from near-surface
<br />sources and in the summer mostly from deeper
<br />groundwater sources {Mang et al. 1999). Cotton-
<br />wood response to surface moisture may also
<br />be dependent on life history and adaptation to
<br />local weather patterns. For example, isotope
<br />studies in regions where summer precipitation
<br />and soil surface moisture are historically unre-
<br />liable have found evidence that cottonwoods
<br />do not use vadose zone water (i.e., Busch et al.
<br />1992, Horton et al. 2003). Thus, it is unclear
<br />whether cottonwoods are able to use water
<br />sources when water becomes suddenly avail-
<br />able where it was previously scarce.
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