The effects of altered hydrologic regime on tree growth along the Missouri River in North Dakota

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Abstract
Increment cores were collected to examine effects of changed hydrologic regime on radial growth of floodplain trees downstream of Garrison Dam. Alterations in seasonal streamflow patterns, near elimination of over-bank flooding, and apparent lowering of the water table during the early growing season following completion of the dam in 1953 were implicated in the significant decline in postdam growth of Ulmus americana, Fraxinus pennsylvanica, Acer negundo, and Quercus macrocarpa. The measured decline in Populus deltoides growth was not statistically significant. Trees on terraces at the edge of the floodplain that received concentrated runoff from upland ravines (e.g., Quercus macrocarpa) and those with deep root systems (e.g., P. deltoides) on low terraces close to the water table were least affected. The most pronounced change in tree growth occurred on high terraces that received little upland runoff (e.g., U. americana, A. negundo). Multiple regression analysis for P. deltoides growth showed a distinct change from correlation with spring streamflow in the predam period to correlation with rainfall parameters in the postdam period. Growth of P. deltoides and Q. macrocarpa on reference sites unaffected by damming of the Missouri River increased significantly in the postdam period.