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66 R. M. Gazal et a1.lAgricultural and Forest Meteorology 137 (2006) 56-67
<br />2001). Decline in E while D remains high may indicate
<br />increasing stomata] closure (Unsworth et al., 2004). In
<br />June and July, maximum stomatal resistance of
<br />cottonwood trees at both intermittent and perennial
<br />steam sites increased which may indicate that soil
<br />moisture stress was starting to influence maximum
<br />stomatal opening. Stomatal resistance at the intermit-
<br />tent stream site was consistently higher than the
<br />perennial stream site throughout the growing season
<br />(Fig. 10).
<br />Our study provides full growing season estimates of
<br />riparian cottonwood E at two contrasting sites that differ
<br />in groundwater depth. Determining sources of spatial
<br />and temporal heterogeneity of cottonwood E is crucial
<br />in characterizing actual consumptive water loss from
<br />this vegetation and its role in the overall riparian water
<br />budget (Cleverly et al., 2002). For example, occurrence
<br />of floods during the monsoon season, fluctuation in
<br />ground water depth and the surface flow volume
<br />contribute to the spatial and temporal variations in
<br />transpiration of cottonwood forests (Stromberg, 1993).
<br />The extent and timing of structural and morphological
<br />changes (i.e., seasonal variation in LAI and AL:As) and
<br />growth responses of cottonwoods to the rate, depth and
<br />duration of water table decline interact with water
<br />demand (i.e., temperature, humidity and wind speed)
<br />influence the intensity and duration of water stress in
<br />this riparian vegetation (Scott et al., 1999). The loss of
<br />young age classes and death of mature cottonwood trees
<br />as a result of water table decline (Stromberg et al., 1996)
<br />and the replacement of native species (Populus
<br />fremontii and Salix gooddingii) by more drought
<br />tolerant T. chinensis (Schaeffer et al., 2000; Horton
<br />et al., 2001b) pose a great threat to the existence of
<br />riparian cottonwood forests. Recognizing these vari-
<br />abilities as they relate to the riparian hydrology and
<br />functions will contribute substantially to a better
<br />estimate of water budget of this riparian system (Scott
<br />et al., in press).
<br />In summary, riparian cottonwood trees are often
<br />exposed to large fluctuations in environmental forcing
<br />throughout the growing season; their accessibility to
<br />shallow groundwater sources dictates their structural
<br />and physiological responses to drought. A cottonwood
<br />stand located at the perennial stream site had a higher
<br />ratio of leaf area to sapwood area, leaf area and
<br />shallower depth to groundwater than the cottonwood
<br />stand at an intermittent stream site. Correspondingly,
<br />total daily sap flow and total annual water use were
<br />higher at the perennial stream site than the intermittent
<br />stream site. Variations in transpiration at the inter-
<br />mittent stream site were related to fluctuations in the
<br />depth to groundwater. A significant linear relationship
<br />between transpiration and vapor pressure deficit
<br />indicates a high hydraulic conductance along the
<br />root —shoot pathway of cottonwood trees at the perennial
<br />stream site. During the peak dry period, the cottonwood
<br />stand at the intermittent stream site exhibited daytime
<br />stomatal depression in response to high D. However,
<br />transpiration increased with no apparent daytime
<br />stomatal closure after significant monsoonal rain and
<br />runoff events that recharged groundwater at both sites.
<br />Riparian cottonwood trees are very much dependent on
<br />shallow groundwater sources and hence relatively small
<br />changes in water table depth through time make them
<br />more susceptible to drought stress. Understanding the
<br />different unique attributes and behaviors of cottonwood
<br />forests in different parts of this riparian area is important
<br />to accurately estimate the water budget of the whole
<br />riparian corridor.
<br />Acknowledgements
<br />This work is supported by financial support provided
<br />to USDA -ARS from the Upper San Pedro Partnership.
<br />We would like to thank the US Bureau of Land
<br />Management, and especially all the rest of the staff from
<br />the USDA -ARS located in Tucson and Tombstone,
<br />Arizona, for their invaluable support of this work. Also,
<br />we would like to thank Stan Wullschleger for his
<br />technical advice on sap flow calculations.
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