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<br />I
<br />t
<br />
<br />growth responses to streamflow. Growth
<br />of cottonwoods in both reach types in-
<br />creased in response to increased flow vol-
<br />ume. although volume of flow explained
<br />slightly more of the variance in annual
<br />growth for trees in the effluent reach. The
<br />instream flow models indicate that a great.
<br />er volume of flow should be released to
<br />maintain healthy cottonwoods in the efflu-
<br />ent reach. These results suggest that stream
<br />?iversion adversely affects riparian popu-
<br />lations in effluent and influent reaches, but
<br />to a lesser extent in effluent reaches. The
<br />rate of groundwater discharge 10 lhe stream
<br />in the influent reach has not been sufficient
<br />to alleviate moisture stress and prevent
<br />growth reduction during low-flow years.
<br />However, further data are needed on this
<br />topic, particularly because rales of ground-
<br />water discharge at the study sites were not
<br />measured,
<br />Within the effluent reach. the black cot-
<br />tonwood population showed a sharper
<br />growth response to streamflow than did
<br />the Fremont cottonwood. The young age
<br />of the black cottonwoods in this reach (an
<br />unavoidable consequence of the limited
<br />availability of trees) may have contributed
<br />to this response. Young cottonwoods at an-
<br />other eastern Sierra Nevada stream, Rush
<br />Creek. also had more annual growth vari-
<br />ance explained by present-year streamflow
<br />than by prior-year variables in com parison
<br />to mature trees (Stromberg and Patten
<br />1990). Growth in juvenile trees appears to
<br />be buffered to a lesser degree by prior-vear
<br />photosynthate allocation. and it appears to
<br />be more responsive to present-year envi-
<br />ronmental conditions.
<br />
<br />000084
<br />
<br />Supplemented-flow Reach
<br />
<br />Growth of trees in the supplemented-
<br />now reach was influenced bv localized in-
<br />dividualistic factors. This i~ indicated by
<br />the low correlations between tree chro-
<br />nologies within the reach and the lack of
<br />significant instream models for the reach
<br />(Fritts and Swetnam 1989). This contrasts
<br />with the diverted reaches, where one site
<br />factor, streamflow volume. had a primary
<br />Influence on annual growth. Trees in the
<br />supplemented-flow reach were not limited
<br />by moisture, as expected given the high
<br />volumes of water released into the stream.
<br />Individualized factors that may influence
<br />annual growth in the reach include vary-
<br />ing levels of competition over time and
<br />annual differences in the extent of soil sat-
<br />uration and anaerobic conditions. The de-
<br />cline in growth by some trees during very
<br />wet years suggests that prolonged satura-
<br />tion negatively affects growth and vigor of
<br />some trees at the site. Other human im-
<br />pacts at the site. perhaps including soil
<br />compaction, as well as the old age of most
<br />of the trees. also may contribute to the low
<br />canopy vigor at the site. The question of
<br />how closely the growth response of trees
<br />in this hydrologically modified. flow-sup-
<br />plemented reach compares to that of trees
<br />in unregulated, free-flowing streams awaits
<br />further study.
<br />
<br />ACKNOWLEDGMENTS
<br />
<br />We thank Southern California Edison, Incor-
<br />porate~, for providing financial support and hy-
<br />drologiC data. Thanks also go to three reviewers
<br />for their constructive reviews of the m"nuscript.
<br />
<br />REFERENCES
<br />
<br />Cook, E. R., and G. C. Jacoby. 1983. Potomac River streamflow since ]730 as reconstructed
<br />by tree rings. Journal of ClJmate and Applied Meteorology 22(10):1659-1672.
<br />Fritts, H. C. 1976. Tree rmgs and climate. New York: Academic Press.
<br />_, and T. W. Swetnam. 1989. Dendroecology: A tool for evaluating variations in past
<br />and present forest environments. Advances in Ecological Research ]9:11]-144.
<br />Gustard, A. 1984. The characterization of flow regimes for assessing the impact of water
<br />resource management on river ecology. Pages 53-60 in A. Lillehammer and S. ]. Saltveit,
<br />editors. Regulated rivers. New York: Columbia University Press.
<br />Harris,.R. R. 1988.. Associations between .st~eam valley geomorphology and riparian vege-
<br />tation as a baSIS for landscape analYSIS In the eastern Sierra Nevada, California, USA.
<br />Environmt.'nlal Management 12(2):219-228.
<br />_, c.~. F~x, and.R. Risser. 1987. Impacts of hydroelectric development on riparian
<br />vegetation In the Sierra Nevada region, California, USA. Environmental Management 11(4):
<br />519-527.
<br />
<br />I~ 10
<br />
<br />January 1991
<br />
<br />Rivers. Volume 2, Number I
<br />
<br />-
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