Forest strata assemblage relationships and tree growth response to flooding in a hydrologically modified floodplain, Southwestern Illinois, United States of America

Objectives of this research were to determine the effect of altered hydrology on species composition and radial growth of trees in floodplain forests of the Lower Kaskaskia River. The Carlyle Dam caused significant changes in hydrology below the site of installation. Flood frequency and duration at a site 6.4 km below the dam was significantly reduced. Flood frequency was more variable at the site 6.4 km below the dam prior to dam installation. Post-dam flood frequency and duration were related to overstory and midstory species composition for all sites, whereas pre-dam hydrology was not related. This indicates that the current species composition has adjusted to post-dam river hydrology. Medium to high post-dam mean annual flood frequency and flood duration, were related to Acer negundo/Acer saccharinum, Acer saccharinum, and Acer saccharinum/Fraxinus pennsylvanica overstory assemblages. Low to medium post-dam mean annual flood frequency and flood duration were related to Betula nigra/Ulmus americana and Celtis occidentalis overstory assemblages. The overstory stratum could potentially succeed to Celtis occidentalis, a shade tolerant species dominant in the midstory strata if current conditions continue.; Increasing % clay was related to Acer negundo/Acer saccharinum, Acer saccharinum, and Acer saccharinum/Fraxinus pennsylvanica overstory assemblages. Increasing % sand was related to Betula nigra/Ulmus americana and Celtis occidentalis overstory assemblages. Soil texture was not significantly related to midstory species composition. Increasing Quercus palustris basal area was related to increasing pre-dam flood duration, and no post-dam Quercus spp. relationships to hydrology were found. Significant relationships of radial growth to flood and climate variables were found, although variables were not always related in the same manner to radial growth. Generally, the relationships of climate and flood factors were different after dam installation than before.; The Carlyle Dam has altered hydrology and changed tree species composition and radial growth along the Lower Kaskaskia River. Further research concerning the central floodplain forest is needed at the landscape scale to address the consequences of hydrologic alterations throughout our large river systems.