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i,. ~~ vi anti i~.er rns ntv eeo rY~a <br />• <br />Figure 5. 'IYee survisl in 1993 <br />as a function of change in crown <br />wlmne ind« between 1991 and <br />1992. Solid linen probability of <br />survival (P) in 1999 determined <br />by maximum likelihood regre+ <br />lion logic (P) ~ 9.561 + <br />0.125 • (CLIFF), where logi[ <br />(P1=lnlp/(1~)]andCLIFF= o <br />the aovm wlume in 1992 -live ?` <br />gown wlume in 1991. LYa~hed <br />line ate 95% wn8dence intervals <br />around the survival probability. a <br />floury rrsponsn for mdmduaL <br />trm (N=699) are tallied above <br />and below the tegtmion line with <br />the t»o oB~le valtu:s given N <br />parenthesd. <br />wl~na <br />Cottonwood Respons• Water Table Declines 355 <br /> 60 <br />a0 a <br />J <br />10 <br />I.0 0 OS <br /> ~ ~ <br />0.8 <br /> 0 <br />0.6 <br />O.a E <br />0.2 ~ Z <br />0.0 "~ "~ <br /> <br /> Q 9 <br />O <br /> <br />-70 -fi0 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 <br />Prior Yeer Cftenge In Cram Volurtta Index <br />Annual branch increment was the only measured <br />variable that showed a significant respone to gradual <br />water table declines 50.5 m at transect P, suggesting this <br />growth variable is a senidve indicant of smaller, <br />sublethal declines in the water table. Poptaltrs spetiea <br />exhibit free or indeterminate shoot elongation, which <br />includes expansion of preformed shoos as well a, <br />initiation and elongation of new shoot elements (Ko- <br />zlowski and others ]991). In Po/nslw, indeterminate <br />shoot growth is strongly influenced by site water condi- <br />tions (Liphsrhia and Weisel 1970), and branch growth <br />increments of riparian cottonwoods have been shown to <br />be signiBcandy correlated with stream flows (Willcox <br />and others 1998). At trartsect 9. a shortterm increase in <br />average annual branch growth in 1992 was associated <br />with branch dieback and reducdons in live crown <br />volume and accounu in part for the fact that the <br />average three-year branch increment was not signifr <br />candy differenttiom that of the pooled consols (Figtve <br />8). This apparent growth release in surviving branches <br />tray be related to within-tree improvements in vrata <br />status resulting from selective branch dieback (Zbnmer <br />man 1978. Hinkley and others 1981) and emphasizes <br />the impoRance of interpreting the response of a single <br />morphological variable in the context of integrated <br />whole-plant responses m water saess (Btaame and <br />others 1992). <br />[n contrast m branch elongation, there were no <br />significant reduction in radial stem growth at transect 2 <br />(T=0.85; P=0.405), despite the fact that the final <br />yeaz of the srudy (1994) was the warmest and dries[ in <br />90 years of record at nearby Byers, Colorado. Similarly, <br />Reny and]obnson (1982) observed no significant swm <br />growth declines in Popu[w associated with dam-refaced <br />reductions in spring peak flown and assumed reducdons <br />Table 2. Comparison of specific leaf mass and leaf <br />area for changes in water table depths between 1991 <br />and 1994 <br />Water table cfiange Specific leaf mass Leaf area <br />(m) 1991-1994 (mg/ant) ±SE (cmt) xSE <br />+0.5 to -O.Y5 10.6 ~ 0.4 99.5 2 1.9 <br />-O.Y6 m -1.0 10.7 10.4 91.9 ~ 2.4 <br />>-1.0 11.4 X0.4 90.0 X0.8 <br />F 1.96 1.01 <br />P> F 0.28 0.98 <br />in spring wafer table levels. However, reduced flows in <br />that case were correlated with apparent increased reli- <br />ance of growth on precipitation and temperamre (Reify <br />and Johnson 1982). Increased semitivity of growth to <br />precipitation and temperature would make trees more <br />vulnerable to periods of low prttipitadon and high <br />temperatutu (Smith and others 1991). In more humid <br />regions, Populas is likely m be less sensitive to water table <br />declines because there is less evapoaanpiration with <br />more frequent and abundant addition of moisture from <br />precipitation to the unsaturated soil zone. <br />Significant reducdons in leaf area and increases in <br />leaf thickness have been observed concomitantly with <br />significant reducdons in annual branch growth in <br />chronically water stressed Popvlw (Smith and others <br />1991, Busch and Smith 1995); however, we found no <br />dear xeromorphic modification of leaves. Although <br />leave showed a tendency toward xeromorphy with <br />increasingwater table declines (Table 2), these change <br />were not significant and may have been overridden by <br />more radical morphological adjw[ments such a9 crown <br />dieback in response to severe want stress at trance[ 9. <br />Po/nilvsatatids onalluvial sites with shallowgroundwa <br />