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2001 CorrONNVOOD WATER RESPONSE DvRtNc Ditt3t caff high vulnerability to cavitation (Blake et al. 1996). On day 245 of the study, T,.jd of study trees averaged -2.0 ti1Fa (Fig. 1); this value corresponds to approximately a 70% loss of hydraulic conductivity in stems due to xylem embolismn according to one P, angtutifolia vul- nerability curve (Tyree et al. 1994), and poten- tially IOM loss in some tissues according to Blake et al, (1996). because we did not assess the origin of the water transpired by trees in our current study, we cannot conclude un- equivocally that they did not take up surface soil moisture. However, since T ..ia values indicate high levels of stem xylem embolism (but see Blake et al. 1996) and since roots are more susceptible to xylem embolism than stems (Sperry and Saliendra 1994, H:acke and Sauter 1996, Sperry and Ikeda 1997), it is probable that xylem dysfunction occurred dur- ing our study period. If significant xylem dys- function did occur in surface roots, and if embolism repair in cavitatod roots was too slow to reestablish function, then this might account for the lack of physiological response in cottonwoods to the irrigation treatment in our study Finally, it is possible that our irrigation treatments, which lasted 3 weeks, may not have been long enough to elicit a physiological response. for instance, Devitt et al. (1991) found a physiological response to surface irri- gation in trees from and environments after 4 weeks. However, given that sporadic pulse precipitation events are common and short - lived in the region J oik et al. 2004), it is unlikely that a 4 -week response time to increased sur- face moisture is ecologically meaningful unless early precipitation events are harbingers of prolonged wet periods. Thus, a Iaek of response within the time frame used in our study may be ecologically equivalent to a lack of response. This lack of responsiveness in cottonwoods to surface water additions may reflect an evo- lutionary constraint on soil water uptake due to a long regional history of low summer precip- itation in northern Utah. For example, lack of significant response to soil moisture may be. a successful carbon allocation strategy of cotton- woods. The tinting of our irrigation treatments corresponded with a period of seasonally dry soils, when infrequent rains only temporarily elevate soil moisture. A long -term evolution- ary response, maximizing carbon allocation and Iimiting unnecessary growth (i.e., easily cavi- 110 tated root tissues; Sperry and Saliendra 1994, Backe and Sauter 1996, Sperry and Ike(l, 1997), may be avoidance of embolism repair in such tissues during seasonal drought periods. This level of genetic specificity would not be sur- prising given the extensive documented genetic variation in cottonwood water- stress tolerance, root growth, and water use (Tschapiinski and Blake 1984 IWb, Blake et al. 1996, Pregitxer and Friend 19%, Fischer et al. 2004). When surface soil moisture levels are more consis- tently high, whole -tree response to soil mois- ture increases may be more common. We found a siguifrcant relationship between average daily Et and Tpre, and average daily G and �T' . However, in both cases the rela- tionship was opposite of our hypothesized re- lationship; both El and GG decrease=d rather than increased with increasing IF Me values (14g. 2). This pattern may be partially due to the high transpiration rates of some cotton- wood trees and their poor stomatal regulation (Stettler et al. 1996, Fischer et al. 2004); the relatively high transpiration rates may have led to progressively poor whole-plant water status (as measured by lower ''4 pre values), reflecting that some cottonwood trees seem to operate with a small margin of safety from cavitation events (Blake et al. 1996). Furthermore, these statistically significant negative correlations may be somewhat spurious given that they were fairly weak (r2 = 0.35 and 0.44, respec- tively) and occurred over a fairly narrow range of Tpre values (-0.45 to -0.65 MPa). Our results suggest that, while some species may show strong physiologic responses to pulse increases in soil moisture (Donovan and Ehler finger 1994, Cui and Caldwell 1997; also see Ogle and Reynolds. 2004, Schwinning and Sala 2004), some cottonwood trees may exhibit little immediate physiological response to in- creases in soil moisture from precipitation. events. This lack of response may be related to a water -use strategy associated with regional climate patterns, cavitation recovery, or other physical determinants of water use such as depth to groundwater. Cottonwood riparian forests represent some of the most biologically productive ecosystems in the West, and our data suggest that it is important to consider potential nonresponsiveness to changes in soil water availability when evaluating the impact of climate change on these important and pro- ductive ecosystems.