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11/ ~1 ~ ~ ~ ~ ~ ~ ~ ~ - ~ ~ ~ ~ , , I I I J "&0/- .... Responses or Riparian Cottonwoods to Alluvial VJaterTable Declin~s I '. MICHAEL L. SCOTT. PATRICK B. SHAFROTH GREGOR T. AUBLE United States Geological Survey. Midcontinent Ecological Science Center 45'2 McMurry Avenue Fort Collins, Colorado 80525-3400. USA ABSTRACT I Human demands for surface and shallow allu- ,':"1 groundwater heve contributed to the loss, fragmentation. and simpiifieation of riparian ecosystems. Populus spacies typically dominate riparian ecosystems throughout arid and semiarid regions of North American and efforts to minimize loss of riparian Populus requires an integrated understand- ing of the role of suriace and groundwater dynamics in the establishment of n~. and maintenance of existing, stands. In a controlled. whole.stand field experiment, we Quantified responses of Populus n lorpholcgy, growth. and mortality to water stress resulting from sustained water table decfine fol- lowing in-channel sand mining along an ephemeral sandbed stream in eastern Colorado. USA. We measured live crown volume. radial st., n growth. annual branch increme,t. and mortality of S89live Populus deltoides subsp, men/lilera stems over four years in conjunction with localized water table declines, MeascremenlS began one year prior to min- Ing and included trees ln boll1 affected and unaffected ar- eas. Populus demonstrated a threshold response to weter table declines In medium a1luviai sands: sustained declines ;2:1 m produced leal desiccation and branch dieback within three weeks and signific~nt dec!ines in live crown volume. stem growth, and 88% mortality over a t/lree-year period. Declines in live crown VOlume proved to be a significant leading IncflCator of mortality in the following year. A logistic regressle;>n of tree survival probability against the prior year's live crown volume was significant (-2 log likelihOod = 270, x' with i df = 232, P < 0.0001) and trees with absolute de- clines in live crown volume of ;2:30 e1uring one year had sur- vival probabilities <0.5 in the following year. In ,contras~ more gradual watsr table declines of -0.5 m had no measur- able effect on mortality. Slam growth. or live crown volume and produced significant declines only in annuai branch growth increment.. Developing quantitative information on tha timing and extent of morphologicai responses and mor- tality of Populusto the rate. depth. and duration of water table declines can assist in tha design of management pre- scriptions to minimize impacts of alluvial grounclwater depie- tion on existing riparian Populus forests. . Riparian corridors occupy important landscape posi- tions between upland and aquatic ecosystems and are uniquely productive. physically dynamic, and biologi- cally div= (Brinson and otheIll 1981, Gregory and omers 1991. Naiman and othen 1993). Depletions of surface and shaUow alluvial groundwater have contrib- uted to the loSs. ftagmenllltion., or severe ecological impairment of these system. '(Dynesius and Nilsson 1994. StrOmberg and others 1996). Species of Populus are the most abundant trees of riparian ecosystems throughout arid and semiarid regions of North America. Pojni!u.Hlominated stands provide unique stnlCtural ,habitat (Brimon and others 1981) and ere vulnerable to reductions in surface and groundwater availability. Declines in P<>jJuIUJ forest cover have been observed where severe drought or land and water management KEY WORDS: Colorado; Water stress: Grour'ldwatflr: Gravel mining; Populus daltoid6S; Rlp2Ifan; Water table declines .Author to whom correspondence: should be addrezed. Environmental ManagementVo/. 23, No.3, pp. 347-358 activities have decreased water availability by reducing' surfaA:e flo,,,, or depleting alluvial groundwater aquife.. (Albertson and Weaver 1945, Groeneveld and Griepen. trog 1 985, Rood and Heinze-Milne 1989, Rood and othen 1995, Stromberg and others 1996). For example. approximately 1125 km of once perennial streams in Kansas are now intermittent. in part a consequence of groundwater pumping in the High Plains Aquifer (Lay- her 1986, Luckey and othen 1988), and flow depletion along the .Arkansas River is lUSOciated With loss of riparian trees (Kromm and White 1992). Similarlly, large areas of riparian forest have been lost to groundwa- ter pumping and associated flow depledon in the southwestern United States (Stromberg 1993). Human activities that directly or indirectly influence alluvial groundwater sources include damming and dive..ion of rivers and streams. groundwater pumping, and channel incision resulting from altered flows of water and sediments, bank stabilization. and instrearn gravel min- ing (llravard and others 1997, Kandelf 1994, 1997, Roqd and othen; 1995, Stromberg and others. 1996. 1997).