Levee Lake: A 2012 Floristic and Natural Community Survey and Analysis.

摘要

Levee Lake is a forested wetland system located within a Mississippi River oxbow meander scar in the American Bottom of the Northern Section of the Lower Mississippi River Bottomlands Division (White 1978) that has not been developed or converted to agricultural land. Because of its relatively undisturbed status, Levee Lake is representative of a presettlement wetland. In 1976, Levee Lake was documented with a 93-acre Grade B shrub swamp/marsh/pond (SSMP) community, qualifying for the Illinois Natural Areas Inventory (INAI). Additionally, 129 acres of Grade C wet floodplain forest surrounding the SSMP community was recorded (Nyboer and Reeves 1976).;Nyboer and Reeves (1976) described Levee Lake as the largest complex of marsh, pond, and swamp communities representing presettlement American Bottom conditions. They also identified potential draining efforts at site perimeters. Based on regulatory agency and Illinois Natural History Survey (INHS) documents as well as historical aerial photographs, perimeter portions of the forested wetland system and adjoining properties were subjected to decades of clearing and draining efforts prior to and following the 1976 survey.;This study involved evaluating the current floristic composition, natural communities, and the overall quality of the Levee Lake wetland system. The information from the current evaluation was then compared to the 1976 survey to examine how recorded disturbances have affected the condition, communities, and quality of the Levee Lake wetland system. The hypothesis of this study was that documented clearing, draining, and subsequent hydrologic alterations would likely cause changes in plant composition and natural communities since the 1976 INAI survey.;To accomplish this task, the transect (Transect 1 or T1) that was used for vegetative sampling in 1976 was re-created and used as a benchmark for 2012 vegetative sampling. To compare the plant composition from 1976 to 2012, twenty 0.25 meter (m)2 sampling plots were established along Transect 1. At each plot (T1P1 through T1P20), the relative cover of each vascular plant species was recorded and the resulting species data evaluated. To provide additional plant data for the site, herbaceous, shrub, and tree sampling was conducted via the Critical Trends Assessment Program (CTAP) protocols. Vegetative sampling was conducted in late summer/fall of 2012.;To evaluate the overall plant quality of Levee Lake and the existing natural communities, existing vegetation outside of the aforementioned transects was also recorded in late summer/fall of 2012. The overall site conditions and natural communities were observed and recorded during site visits in 2011, 2012, and 2014. The current natural communities were determined by the vegetative sampling, the overall site observations, as well as available aerial photographs and images.;Based on the research and field work, the native plant composition suffered a reduction in quality. Additionally, communities suffered a reduction in quality and a shift in community type. A reduction in water levels caused severe woody encroachment of the SSMP community identified in 1976. Today, only approximately 0.58-acre [0.23 hectare (ha)] of Grade C marsh/pond community remains. In 1976, the pond community was considered an exceptional feature with the surrounding shrub swamp/marsh community considered a significant feature. Although most of the former 93-acre Grade B SSMP community suffered from severe woody encroachment, recent wetland restorations to the north and to the south appear to have aided in returning hydrology to this community. Herbaceous, shrub, and tree vegetation data collected in the CTAP plots provided further evidence of wetland and swamp conditions. The former SSMP community has evolved into a Grade C swamp/marsh/pond (SMP) community. An approximate 11.73-acre (4.75 ha) Grade D marsh/wet meadow has evolved in the southwestern region that was subjected to decades of clearing and draining disturbance (White 1978; White and Madany 1978). Further evidence of a shift in plant composition and communities between species along Transect 1 in 1976 and in 2012 was shown through the NMDS ordination and an ANOSIM test which showed that plot communities distinctly differ between the two years (Minchin 2013). (Abstract shortened by UMI.).