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~. .~ ..... .... ... .ra o,,, tea.., oYea - • Figure 3. Cattomvood xesponses over flit mining uife[VaI velBLLl change in groundwater leael. Syrtt boll rtpresen[ averages for geomor- phic surfaces (surtaca) widen each transectas fogows: ~ = 1, O = 2, t = S, ~ = 4,. Control transecu 1 and 4 axe always solid, mining af- (ecmd ~~~~ 2 and 3 are open 0 signifimxtdy different than the pooled mnrrott All values are based on the difference between measuro- mend at the end of the pretxvning growing season, 1991, and the end of the growing eeawn in 1994. Stem increment u thus a titres-year mal and branch increment is athree- year mal as a percentage of a prem- ising duce-year mtaL Growth re apoxtses aPPNonly m trees that survived the entire rxrining pcxiod. € ~ U 0 LL a1 -10 a 'n0 m U -W ett:a~i aKtJ Cottonwood ResponseTo Water Table Declines • xso Stem Increment • E m 100 ~.7~ . a i1 ~ 150 e °E 1 a $ Gowri Volume U 700 d ~ i s S0 `, tao f n ~ • m n t N w Survivorship ~ ~ e 8 o -75 -1A -OS 0.0 ~ O l i i r g eD Branch Increment r c 70 ~ W i o E ~~ i e ~ m 50 t[ o° 5 ~ °~ 1 ~Q OS -i5 -1.0 -0.5 OA CFlange in Groundwater Elevation (m) logistic regression of the probability of tree survival in 1998 against the prior yeaz's change in live crown wlume (Figure 5) was highly significant (-2 log likeli- hood =270, XZ with 1 dJ= 292, P < 0.0001). Trees with increases or no prior year change in the live crown wlume had negligible motraliry (probability ofaurvival >97rYo), wheirav ¢ees with prior year declines in canopy volume of?90 had survival probabilities less than 0.5. The most immediate visible responses of Populvs to rapid initial water table destines z] m at tansect 9 included leaf desiccation and branch dieback within three weeks. This response was reflected in decreases in live crown volume that persisted as progressive year-co- year declines For surviving aces over the study period (1991-1994). In contrast, live crown wlume values for trarrsect 2 and con¢ol transects 1 and 4 remained essentially unchanged through the same period. Oveiall pre- and postsnitsing differences in branch sad stem growth contained considerable year-to-year variation. For example, at the crassest moldy heavily affected by mining (transect 3), branch growth in 199$ on surviving ¢ees increased relative m the premising average; however, growth declined sharply in 1999 sad 1994. Sy comparison, branch growth remained below premising averages ac the other «ansects (transects 1, 2, and 4) from 1992-1994. On the other hand, stem growth for surviving ¢ees at ¢ansect 9 averaged 1 ant/yr in the first~two yeas after mining (1992-1998) and increased to 10 em1/yr in 1994. In contrast, consistently higher ales of annual stem growth were observed at the other tansects, ranging from 19 to 56 CIn1/yr at tranaects 2 and 4, respectively. DISCUSSIOfI Drought Response Patterns of Riparian Populusto Wa[erTable Declines I~oo7 353 OS Our experiaaental results demonstrated that Popultu forests are vulnerable w relatively small declines in shallow alluvial water tables. Sustained, gradual water table declines of 50.5 m in the alluvial sands of Coal Creek were associated with reduced branch growth but ]itde mortality; abrupt declines ?1 m produced leaf desiccation and blanch dieback within three weeks and significant reductions in ]ive crown volume (Figure 4A and B), stem gruwth, and 883o mortality over athree- yeazperiod. Significant branch dieback, measured as a reduction in live crown volume, occurred relatively gtrickly following rapid water table declines at uansect 3 and served as a significant predictor of ¢ce mortality in the following year (Figure 5). Blanch diebark is a common response of ¢ees to water s¢ess (Albertson and Weaver 1945) and may enhance tree survival during severe drought by reducing ¢anspirational sur- face area (~immerrnan 1978, Bradford and Hsiao 1982, Braame and others 1996). However, acute water avess associated with rapid water table declines can induce cavitation of xylem vessels and subsequent branch dieback (Tyree and others 1994). Pt>ptelus spp. are particularly vulnerable [o losses in hydaWic conductiv- ity resulting from drought-induced cavitation of xylem vessels, and cavitation has been implicated in the decline of Populm forests dovms¢eam of some dams and diversions (Tyree and others 1994).