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Title:Enhancement of diversity via disturbance and seed addition in two sand prairies dominated by different grasses
Author(s):McNicoll, Molly
Director of Research:Augspurger, Carol K.
Doctoral Committee Chair(s):Augspurger, Carol K.
Doctoral Committee Member(s):Paige, Ken N.; Molano-Flores, Brenda; Taft, John B.
Department / Program:Plant Biology
Discipline:Plant Biology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Bromus inermis
community structure
disturbance
dominant species
grassland diversity
microsite limitation
sand prairie
Schizachyrium scoparium
seed addition
seed bank
seed limitation
species identity
Abstract:Grassland diversity can be reduced by native and non-native dominant species, through heightened competition for resources and low colonization by non-dominant species. Traits of the dominant species may influence these interactions and also the extent to which the dominant is affected by disturbance. Disturbances that reduce dominant species may enhance community structure due to competitive release, but non-dominant species may require additional types of disturbance that create regeneration microsites, such as reduced litter and increased bare soil. In addition, community structure may be limited by seed availability, or a combination of microsite and seed limitation. Studies of these factors aid our basic ecological understanding of the maintenance of diversity in grasslands, but also are pertinent to restoration and management of grasslands. The goal of this research was to gain a better understanding of dominant and non-dominant species coexistence by examining 1) diversity of two communities dominated by contrasting dominant grasses, 2) responses of the communities to manipulative disturbances, and 3) responses to augmentation of microsite and seed availability. First, I address the questions: Does community structure aboveground and in the seed bank differ between communities dominated by contrasting grasses? Do community dynamics in these communities differ over time? In a sand prairie in northwestern Illinois, USA, baseline community structure was compared in two communities dominated by either Schizachyrium scoparium, a native warm-season (C4) grass, or Bromus inermis, a non-native cool-season (C3) grass. Five years of vegetation surveys and a single-year seed bank sampling were conducted. A decline in % cover of non-dominant species with increasing % cover of dominant species suggests suppression of non-dominant species by both dominant grasses. However, greater diversity in the Schizachyrium than Bromus community and loss of species over time from the Bromus community suggests that negative effects on non-dominant species may be stronger in the Bromus community. In addition, differences in seed banks between the two communities indicate possible differences in site history, another potential influence on aboveground community structure beyond the presence of different dominant species. Second, this research asked: How does dominant species identity influence response to disturbance? What is the relative importance of dominant species reduction vs. independent stimulus of non-dominant species? How does frequency of disturbance influence response of dominant and non-dominant species? Separate disturbances were targeted to reduce the dominant grass (mowing) and create regeneration microsites at different frequencies (soil disturbance for a single or five years). Mowing reduced % cover of Bromus more than Schizachyrium, and displaced Bromus, but not Schizachyrium, as the dominant species. Species richness increased less by reduction of the dominant alone and more when combined with the greatest frequency of soil disturbance. However, microsites from soil disturbance were ephemeral, especially in the Bromus community. The extent of dominant species reduction depended on species identity and amount of tissue loss, which in turn determined whether or not non-dominant species abundance increased following disturbance. Newly disturbed microsites fostered colonization by non-dominant species in both communities. The results indicate the importance of species identity when pairing management techniques and dominant reduction. They also show that annual spring mowing is a tool managers may use for control of Bromus. In addition, the independent response of non-dominant community structure to disturbances that reduce competition from those that stimulate new colonization indicates the importance of multifaceted disturbances in restoration and maintenance of non-dominant species diversity in grasslands. Third, this research asked: What is the relative importance of microsite vs. seed limitation in sand prairies dominated by different grasses? This question was addressed in both the Schizachyrium and Bromus communities in a factorial microsite (gap) and seed limitation (seed augmentation) experiment. In gap plots, species richness increased in the Schizachyrium, but not Bromus community. In seed addition plots, species richness did not differ from controls in either community. In seed + gap plots combined, species richness was greatest and species composition diverged most from control plots in both communities. Low recruitment in gaps in the Bromus community may indicate inferior microsites and/or greater seed limitation in that community. In contrast, the low establishment of sown species in undisturbed vegetation showed that traits common to both dominant species, such as high production of biomass and dead vegetation, may make communities difficult to invade, regardless of dominant species identity. Finally, results for combined gap and seed addition plots indicate removal of multiple limitations is necessary to achieve the greatest change in community structure and species composition, independent of dominant species identity.
Issue Date:2013-02-03
URI:http://hdl.handle.net/2142/42384
Rights Information:Copyright 2012 Molly McNicoll
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12


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