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<br />348 M. L. Scott and olhiss
<br />mGJl.i JKh_J
<br />Effective management of riparian Populw ecosystems
<br />requires an integrated understanding oC surface and
<br />alluvial groundwater dynamic in relation to reproduc-
<br />tion and maintenance of existing stands. Controlled,
<br />whole-plant experiments arc needed to qua ratify relation-
<br />ships be tween the severity of water stress and integrated
<br />patterns of tree response (Braame and others 1992). To
<br />date, quantitative information on the extent and timing
<br />of functional responses and morality of Populw to
<br />groundwater depletion have rarely been temporally
<br />matched with quantitative information on the rate,
<br />depth, and duration of water table declines in the field
<br />(Stromberg and others 1996). Establishing such relation-
<br />ships ran assist management efforts to minimize im-
<br />pacts of alluvial groundwater depletion on existing
<br />riparian Populw ecosystems.
<br />Our objectives were to quantify changes in morphol-
<br />ogy, growth, and mortality of mature riparian Popultu
<br />ddtoides subsp. +noniGfda in response to measured de-
<br />clines in the alluvial water table. In a controlled,
<br />who)a-stand field experiment using 689 live trees, we
<br />measured the response of live crown volume, radial
<br />scero growth, annual branch increment, and stand
<br />mortality m sustained water table declines resulting
<br />from inchannel sand mining. Measurements began in
<br />1990, one year prior [o mining, and ended in the third
<br />year following mining.
<br />Water Relations of Populus
<br />Despite widespread occurrence in acid and semiarid
<br />landscapes, Populw species require continually moist
<br />substrates for arab]ishment (Read 1958, Friedman and
<br />others 1995), are susceptible to drought-induced cavita-
<br />tion of xylem vessles (Tyree and others 1994) ,and suffer
<br />higher mortality during drought than several eastern
<br />deciduous forest species (ICaylor and others 1935, Albett-
<br />son and Weaver 1945). Ruderal traits of Populus contrib-
<br />ute to the persisrcnce of this droughtsensidve species in
<br />drought-prone regions (Friedman ec ai. 1997); these
<br />train include small wind dispersed seeds, rapid germina-
<br />tion and growth, and the ability m tolerate Flooding and
<br />physical disturbance. Populusu generally restricted to
<br />alluvial soils with shallow groundwater (Meinur 1927,
<br />Robinson 1956).
<br />On coarse substrates in dry regions, early establish-
<br />mentand growth of Populw spp. seedlings may require
<br />water tables within 1-2 m of the establishment surface
<br />(McBride and Strahan 1984, Mahoney and Rood 1992,
<br />Segelquist and others 1993, Svomberg and others
<br />1996). Following establishment, root growth allows
<br />young trees to survive gradual water table declines.
<br />Depth to the water table may increase as a result of
<br />•
<br />subsequent floodplain accretion or channel incision
<br />(Everitt 1968, Hereford 1986), and Populw species have
<br />been observed az sites where depth to the water table is
<br />7-9 m (Robinson 1958); however, mature riparian
<br />Populw are typically found in riparian settings where
<br />depth to the water table u 53.5 m (Busch and others 1992,
<br />Scott and others 1997, Stromberg and others 1997).
<br />The response of a plant to gradually increasing water
<br />stress involves progressive and integrated physiological
<br />and morphological responses, beginning with stomatal
<br />closure, reduced leaf and canopy development, and
<br />ending with death (Bradford and Hsiao 1982, Braame
<br />and others 1992). Mild water stress can reduce plan[
<br />productivity by limiting COp assimilation through stoma-
<br />tal closure, loavering net photosynthesis, and through
<br />the death of leaves and fine coon. Under more severe
<br />drought conditions, trees exhibit reduced radial stem
<br />increments, wilting and abscission of leaves, and branch
<br />death. Tree mortality may follow direcdy or secondarily
<br />as the result of insecn or other pathogens (Albertson
<br />and Weaver 1945). Because these changes occur a[
<br />different levels of water stress and on different time
<br />stales, accurate quantification of longer-term water
<br />stress is problematic (Pallardy et al. 1991).
<br />In water-stressed Populus speces, Snrith and others
<br />(1991) found significandy reduced stomatal conduc-
<br />tance and reduced midday leaf water potential (Y+r) Cor
<br />P. tticirocarpa compared with nonstressed trees. These
<br />vends were particularly pronounced for juvenile trees.
<br />Busch and Smith (1995) found moderately higher rotes
<br />oCstomatal conductance and uanspiration and slighdy
<br />higher predawn and midday Ytt in comparing Populus
<br />frnaontii and Salix gooddingii from a gaining reach with
<br />those from a losing reach of the Bill Williams River,
<br />Arizona. Riparian Populus can exhibit morphological
<br />and growth responses to chronic water stress, including
<br />reduced leaf size, increased leaf thickness, reduced leaf
<br />area, reduced annual stem elongation, and reduced
<br />radial stem incremenn (Smith and others 1991, Strom-
<br />berg and Patten 1941, Busch and Smith 1995). Under
<br />tondidons of acute water stress associated with severe
<br />climatic drought or water table declines, Populw display
<br />more extreme morphological responses such as crown
<br />dieback and ultimately srahd mortality (Ellison and
<br />Woolfolk 1937, Albertson and Weaver 1945, Saomberg
<br />1993, Rood and others 1995).
<br />Study Site
<br />The study site teas a 4.8-km reach of Coal Creek, an
<br />ephemeral, sandbed stream within the South Platte
<br />River drainage in the Colorado Piedmont province of
<br />central Colorado (Figure 1). A tributary of Sand Creek,
<br />~ 002
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