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180 WESTERN NORTH AMERICAN NATLRALtsT [Volume 65 <br />100 <br />v 80. <br />N' <br />E 60 <br />ON <br />x au <br />20 <br />0.30 <br />b <br />dJ 0.25 <br />0.20 <br />94 X <br />0.15 <br />'::v 0.10 <br />UT <br />0.05 <br />Unwatared Waterod <br />t2 *0.44 <br />• <br />NS. P *0.47 <br />p ■ 0.02 <br />y M <br />• • <br />rXa0.35 <br />D <br />NS, p=0.34 <br />P ■ 0.04 <br />y <br />• <br />.0.70 -0.65 -0.60 -0.55 -0.50 -0.45 0 5 10 15 20 25 <br />y`ww.. (MPa) W Water Content (%) <br />Fig. 2. Soil and plant water relations during irrigation. Canopy conductance (C,,; L Hg0 m-s d-1) versus predawn <br />water potential ('f',,, A) and gravimetric soil water content (%, R). "f- specific transpiration (E:i; 1, 1120 or-2 leaf area <br />d -t) versus predawn water potential (Y'". C) and gravimetric soil water content (%, D). For panels ;S and D, watered <br />trees are represented on the right and unwatered trees on the lek <br />respectively) or LA:SA ratios (P = 0.12 and <br />0. 18, respectively, Fig. 4A). Whole -tree hydrau- <br />lic conductance (Kh) was unrelated to DBLC <br />(P = 0.14); however a significant (P = 0.02, r2 <br />= 0:43) inverse power (y = in * x-b) relation- <br />ship was found between Kh and the LA:SA ratio <br />(Fig, 4B). El was also related to Kh, but this is <br />likely driven by the calculation of Kh (eq. 2). <br />Supplemental watering had no detectable <br />effect on leaf abscission during drought; percent <br />leaf area lost was similar between watered and <br />unwatered trees (P = 0.29; data not shown). <br />Furthermore, we found no significant relation- <br />ships between percent leaf area lost and Kh, <br />C., or El (P = 0.90, 0.39, and 0.53, respec- <br />tively). However, leaf area lases was negatively <br />correlated with 'PP., (r2 = 0.37, P = 0.04), <br />suggesting that low water availability may have <br />led to leaf loss. Study trees lost between 0% <br />and 29% of their leaf area during the course of <br />the study (mean leaf loss = 9%, median leaf <br />loss = 4%), and leaf area varied from 75.2 m2 <br />to 505.2 M2 among study trees ('Fable 1). <br />DISCUSSION <br />Previous research has suggested that at cer- <br />tain tames of the year cottonwood trees access <br />water from the unsaturated (vadose) zone. (i.e., <br />part of the soil profile above the groundwater <br />table and the capillary fringe zone), acting as <br />facultative phreatnphytes (Smith et at. 1991, <br />Snyder and Williams 2000). Other research <br />suggests that the principal source of water for <br />tree uptake may shift through a growing sea- <br />son (Zhang et al. 1999), and cottonwood trees <br />are known to have plastic and adaptable root <br />systems (Pregitzer and Friend 1996). However, <br />significant evidence exists to support an alter- <br />native hypothesis that cottonwoods exhibit re- <br />spouse to surface water commensurate with the <br />climatic history of their region. For example, <br />Busch et al. (1992) did not find evidence of <br />soil moisture uptake at a study site that has <br />historically unreliable summer precipitation <br />patterns, and this suggested phreatophytic be- <br />havior. Conversely, Snyder and Williams (2000) <br />found evidence of soil moisture uptake at a <br />study site where summer monsoonal pulse rain- <br />fall events are common. Our study site has a <br />historically predictable summer drought, and <br />our results are consistent with this regional <br />climate hypothesis. <br />Despite (1) successful increases in soil mois- <br />ture within the upper 15 cm of soil (Fig. 1), (2) <br />observations that most study trees showed some <br />