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20051 CorroNNVOOD XVtwit RFsPONsr-, Du R[Nc DRou(;HT <br />0.22 <br />0.20 <br />0.18 <br />0.16 <br />0.14 <br />o 0.12 <br />X 0.10 <br />0.08 <br />Ur 0.06 <br />-1 26 <br />§ 20 <br />15 <br />10 <br />5 <br />0 <br />A-0 W&WW Et <br />Ar D <br />0 <br />'rill <br />B♦ W-9 <br />Mftraddftm <br />U-0 <br />210 <br />215 220 225 230 235 240 245 <br />Day of Year <br />• W -` , <br />0 u. T" <br />♦ W-W'. <br />& U-T� <br />C <br />4 m <br />a. <br />.W <br />3 <br />2 <br />140 <br />120 <br />100 <br />80 <br />so <br />40 2M <br />20 <br />0 <br />250 <br />240 245 250 <br />Day of Year <br />-2.2 <br />-2.0 <br />-1.8 <br />-1.6 <br />-14 <br />0- <br />-1.2 <br />-1.0 <br />-0,8 <br />-0.6 <br />-0.4 <br />-0.2 <br />177 <br />Fig, 1. Physiological responses of watered (%V) and unwatered (U) cottonwood trees over time in a common wrden <br />environment near Ogden. UT- A, leaf-specific transpiration (EI; liters H2O 01-4 leaf area d-1) plotted with vapor pressure <br />deficit (VPD); 11, gravimetric soil %vatercontent (SWC) plotted with the amount of water added (L) to,,vatured fivess; C. <br />Predawn and midday (1P,jd) water potentials. Error bars represent I standard error, <br />the soil to the root zone of the control trees. <br />Watered trees were at least 50 m from un- <br />watered trees and were separated by a 30-cm- <br />deep trench or a road. Watering treatment <br />continued until the end of the study on 5 Sep- <br />tember (DOY 248). By this time each treat- <br />nient tree had received 935 L of water more <br />than each control tree, averaged over the entire <br />period that the irrigation treatment was applied, <br />this amount is equivalent to an increase in <br />water addition of 42.5 L tree-1 d-1. If com- <br />pletely transpired, this would equal about 2 <br />min d-1 for a tree with a crown area of 20 m2 <br />(median for our watered treatment), This tran- <br />spirational rate could easily occur at our site <br />given that potential evapotranspiration aver- <br />aged 5.2 mm d-1 ± 0.14 (s) over the study <br />period. <br />Gravimetric soil water content (1050C, 48 <br />hours) was measured 4 times throughout the <br />course of the study. Measurements were taken <br />once at the beginning of the study, once just <br />before treatment began, once midway through <br />treatment, and once again at the end of the <br />study. Soil samples (0-15 cm depth) were taken <br />within the same area of each tree, 0.5 m north <br />