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• 356 M. L. Scott and others tuna:,i JACJ ter are vulnerable m water table declines due in part m the limited soil volume for tree roots above the water table. The form of root systems in some deciduous tree species is variable and influenced by site conditions (Biswell 1935); for Populus, overall root structure is responsive to the site specific distribution of soil mois- ture (Yeager 1995). Popular is reported to extend rooa into the water table or tension saturated zone (capillary Fringe) above the water table surface (Meinzer 1927), and trees growing on shallow groundwater sites exhibit shallow, laterally spreading root structures in close association with the tension saturated zone (Condra 1944, Sprackling and Read 1979). At Coal Creek, all trees were associated with a shallow water table surface (53.2 m) that exhibited li[de inter- or intraannuaJ variation in depth. Rapid lowering of the water table in earl}'Jrtne at transec[ S produced visible water stress in trees within three weeks. Under conditions of drought, shallowly rooted trees exhibit greater water stress (more negative water potentials) than more deeply rooted trees (Hinkley and others 1978, Oosterbaan and Nabuurs 1991). During the drought years 1983-1941, mortality of shallow•rooted Popular on floodplaias and surfaces occurred within the first few months of severe drought and following rapid water table declines (Albertson and Weaver 1945). Some drought-tolerant apecies are able to adjust to water limitations imposed by water table decline through modification of aboveground structure (Stromberg and others 1993) or by growing rooa through drained soils to access deeper, saturated soil (Sorenson and ochcn 1991). However, Popular in [his study were unable to adjust to water stress associatedwi[h apid decline of the water table in coarse alluvial sands. As [he water table and associated tension-saturated zone decline, water drains from the soil by gravity, leaving behind water bound by capillarity [o soil pore spaces. Because of a high proportion of noncapillary pore space, the plant- available water content of urisacurated, coarse soils is typically three to four times less than that of finer textured soils (Brady 1974). Rapid water table declines in coarse alluvial soils have been correlated with re- duced transpiration and growth of Popultu cuttings (Mahoney and Rood 1992). Water table declines ?1 m at Coal Creek were linked with progressive structural changes in mature Popular including leaf desiccation, branch dieback; reduced stem growth, and subsequent mortality, suggesting that these trees were unable to reestablish root contact with deeper saturated soils. Similarity, mortality of mature Populus following water table declines in coarse floodplain soils was attributed to an inability of veer to root more deeply because of limited soil water following drawdown (Condra 1944, •- Groeneveld and Griepenvog 1985). In contrast, the greater water storage wpaciry of fine-textured soils may serve as a buffer agairut rapid water table declines, allowing some groundwater-dependent plants to extend rooa and access stored water in deeper soil layers (Sorenson and others 1991). The rate, depth, and duration of water table decline and the water holding charatteristia of the soil interest with water demand (i.e., temperature, humidity, wind speed) to influence the intensity and duration of water stress in groundwater-dependent plants. Furthermore, because Popular root systems appear [o be tailored to site-specific groundwater environments (Yeager 1935), [he historical stability of the water table would influence root distribution, which may in [urn condition the response of trees to subsequent water table changes (Ooscerbaan and Nabuurs 1991). Thus, vees~growing in association with a formerly stable water table may be more sensitive [o declines than trees formerly associated with a more variable water table environment This study was a manipulative field experiment where whole- plant responses of mature Popular were assessed in the context of sustained declines in a formerly stable wamr cable, in medium alluvial sands. Clearly, other combina- tions of antecedent water table environments, meteoro- logical conditions, drawdown patterns, and soil charac- teristics aze possible and beyond the xope of this study. For example, the extent m which a gradual decline in the water-table at Coal Creek to similar depths would have mitigated observed water svess is unknown. Fur- thermore, anecdotal evidence suggests that rapid, deep water table declines of short duration may provide sufficient residual soil moisture to allow some surviaal of riparian Popnlus (M. L. Scott personal observation). Additional controlled studies of whole-tree and whole- stand responses, over a range of conditions, would be desirable. From a management perspective, this study is impor- tant because it represents conditions of water table decline and water stress that mature riparian Popular forests would experience as the result of in-channel gravel mining, channel incision, surface water diver- sion, or groundwa[erpumping. Our results suggest that mature Popultu forests associated with sandy alluvial soils and a relatively stable groundwater environment are vulnerable to groundwater depletion and would be direcdy threatened by human activity resulting in a sustained lowering of the water table ?1 m. More gradual declines of'0.5 m produced measurable reduc- tion only in annual branch increments; however, such veer may be more wlnerable co subsequent periods of Inw preripitadon and high temperatures. Developing quantitative information on the extent and timing of lQJ 010