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Vegetation structure and inferred patterns of functional group attrition in a shrub encroached old field and tallgrass prairie mosaic

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Title: Vegetation structure and inferred patterns of functional group attrition in a shrub encroached old field and tallgrass prairie mosaic
Author(s): Kron, Zachary P.
Advisor(s): Taft, John B.
Department / Program: Plant Biology
Discipline: Plant Biology
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Thesis
Subject(s): Tallgrass prairie prairie remnants disassembly shrub encroachment plant functional groups
Abstract: Tallgrass prairie is an endangered ecosystem and encroachment by woody species threatens many remnants. Insights are needed into the differences in diversity and species and functional group composition along a gradient of woody encroachment to help gauge restoration potential and gain insights into patterns of disassembly in grassland communities. The study site is a 65 ha (160 acre) tallgrass prairie and old field mosaic in Lake County, Illinois. The three main objectives in this study are to: (1) analyze and classify plant communities, (2) explore seed bank dynamics and its contribution to old field colonization, and (3) determine the patterns of species and functional group richness and cover in a tallgrass prairie:old field mosaic with varying levels of shrub invasion and assess whether there are ordered patterns of loss in richness and cover with increasing shrub canopy cover. Ground layer and shrub layer data were collected from 45 sample plots including 37 located on stratified transects and eight located randomly in high-quality reference prairie habitat. Two community types were identified through field observation and reinforced by cluster analysis, indicator species analysis, and Nonmetric Multidimensional Scaling (NMS). The communities differed significantly in species density, species richness, ground layer cover, floristic quality indices, shrub canopy cover, and percent bare ground. To assess whether germination from the seed bank was limited by the lack of fire at the site, soil samples were heat treated prior to placement in greenhouse flats and germination rates were compared to a control. Heat shock had a variable effect on germination, and the species germinating from the seed bank were dependant upon the treatment. Sørensen Similarity Index indicated that there was very little similarity between the species present in the seed bank and the standing vegetation. To determine if prairie remnants were responsible for the recolonization of the site after agricultural disturbance, species data were examined on a distance gradient to the nearest remnant. Overall species richness, proportion of prairie species, and Floristic Quality Index had no relationship with distance to remnants. Results suggests that many areas of the site are seed limited, further complicating the restoration of plant communities. Possible causes of seed bank failure as a refugium could be attributed to the past history of rigorous cultivation at the site and the recent history of shrub encroachment. Species composition data were converted to plant functional groups based on species traits to assess whether increasing shrub canopy cover leads to loss or decline in richness and cover in species with shared traits. The relationships between functional group richness and ground layer cover to shrub canopy cover were examined with linear regression, discriminant analysis, ANOVA followed by Tukey post-hoc tests, and NMS. Cover of C4 grass, perennial legume, perennial forb, perennial sedge, C3 grass and annual forb functional groups and richness of C4 grass, perennial legume, and perennial forb functional groups follow ordered decline with increasing shrub canopy cover and differences among canopy cover classes were significant. NMS provides a graphical summary indicating functional groups representative of prairie communities are associated with low canopy cover plots compared with closed canopy plots. Comparisons with previous studies at this site suggest shrub species have increased in density three fold in the past fourteen years. Results from this study highlight ordered patterns of losses in the cover and richness of plant functional groups that can be used as a guideline to evaluate sites undergoing shrub encroachment that have important management implications for restoration and management of grassland ecosystems.
Issue Date: 2011-08-25
URI: http://hdl.handle.net/2142/26056
Rights Information: Copyright 2011 Zachary P. Kron
Date Available in IDEALS: 2011-08-25
Date Deposited: 2011-08
 

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