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if./u;, U1 THU 15:15 F.~L ".-'rJ °'S °451 `:LSC/SAES <br />a • III IIIIIIIIIIIII III • <br />ss9 <br />Responses of Riparian Cottonwoods <br />to Alluvial V•Jater Table Declines <br />MICHAEL L. SCOTT' <br />PATRICK B. SHAFRO7H <br />,~ G <br />3 <br />5~~3~oi <br />n?n. e~-~ <br />~ 001 <br />GREGOR T. AUBLE <br />United States Geological Survey, <br />Midcontinent Ecological Science Center <br />4572 McMurry Avenue <br />Fort Collins, Colorado 80525-34170. USA <br />ABSTRACT !Human demands for surface and shallow allu- <br />a:al groundwater have conVibuted to the loss, fragmentation, <br />and simplification of riparian ecosystems. Populus species <br />typically dominate riparian ecosystems throughout Arid and <br />semiarid regions of North American and efforts to minimize <br />loss of riparian Populus requires an integrated understand- <br />ing of the role of surface and groundwater dynamics in the <br />establishment of new. and maintenance of existing, stands. <br />In a controlled, whole-stand field experiment, we quantified <br />responses of Populus morphology, growth, and mortality to <br />water stress resulting from sustained water table decline fol- <br />lowing in-channel sand mining along an ephemeral sandbed <br />stream in eastern Colorado. USA. We measured live crown <br />volume, radial stein growth, annual branch increment, and <br />mortality of 6891ive Populus delfoides subsp. monilifera <br />Riparian corridors occupy important landscape posi- <br />tions between upland and aquatic ecosystems and are <br />uniquely productive, phy5ical]}• dynamic, and biologi- <br />cally diverse (Brinson and others 1981, Gregory and <br />others 1991, Neiman and odtets 1993). Depletions of <br />surface and shallow alluvial groundwater have contritr <br />used to the loss, fragmentation, or severe ecological <br />impairment of these systems (Dynesius and Nilsson <br />1994. Svomberg and others 1996). Species of Pppulur <br />are the most abundant veer of riparian ecosysums <br />throughou t aria and semiarid regions of North America. <br />Populus-dominated stands provide unique structural <br />habitat (Brinson and others 1981) and are vulnerable to <br />reductions in surface and groundwater availability. <br />Declines in Populus forest cover have been observed <br />where severe drought or land and water management <br />KEY Wt7ROS: Calorado~. Water stress. Groursdwater, Gravel mining; <br />PopuNS de/raider: Ripadan; Water table tleclines <br />'Author ro whom correspondence should he addressed. <br />stems over four years In conjunction with localized water <br />sable declines. Measurements began one year prior to min- <br />ing and included trees in both affected and unaffected ar- <br />eas. Populus demonstrated a threshold response to water <br />table declines in medium alluvial sands: sustained declines <br />?7 m produced leaf desiccation and branch tlieback within <br />three weeks and signifcant dec!;nes in live crown volume, <br />stem growth, and 1j8q mortality over athree-year period. <br />Declines in live crown volume proved to be a significant <br />leading indicator of mortality in the following year. A Iegistic <br />regression of Tree survival probability against the prior year's <br />live crown volume was significant (-21og likelihood = 270. <br />~ with 1 dl = 232, P < 0.0001) and trees with absolute de- <br />clines in live crown volume of >30 during one year had sur- <br />vival probabilities <0.5 in the following year. In contrast, <br />Hare gradual water table declines of -0.5 m bed no measur- <br />able effect on mortality, stem growth, or live crown volume <br />and pmduced significant declines only in annual branch <br />growth increments. Developing quantitative information on <br />the timing and extent of morphological responses and mor• <br />taliry of Populus to the rate, depth, and duration of water <br />table declines can assist in the design of management pre- <br />scriptions to minimize impacts of alluvial groundwater deple- <br />tion on existing riparian Populus forests. . <br />activities have decreased water availability by reducing <br />surface flows or depleting alluvial grotmdwater aquifers <br />(Albertson and Weaver 1945, Groeneveld and Criepen- <br />vog 1985, Rood and Heinze-Milne 1989. Rood and <br />others 1995, Svomberg and others 1996). For example, <br />approximately 1125 knt of once perennial streams in <br />Kansas are now intermittent, in part a consequence of <br />groundwater pumping in the High Plains Aquifer (lay- <br />her 1986, Luckey and others 1988), and flow depletion <br />along the Arkansas River is associated with loss of <br />riparian vees (Hromm and White 1992). Similarity, <br />large areas of riparian forest have been lost to groundwa- <br />ter pumping and associated flow depletion in [he <br />southwestern United States (Svomberg 1993). Human <br />activities that directly or induecdy influence alluvial <br />groundwater sources include damming and diversion of <br />rivers and svcams, groundwater pumping, and channel <br />incision resulting fiom altered flows of Hater and <br />sediments, bank stabilvation, and insveam gravel mitt <br />ing (Btavard and others 1997, I{ondolf 1994. 1997, Rood <br />and others 1995, Strombetg and others 1996, 1997). <br />Environmental Management Vol. 23, No. 3, pp. 347-358 <br />