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The Role of Native Diversity and Successional Processes on Communityinvasibility in Riparian Primary Forest

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The Role of Native Diversity and Successional Processes on Communityinvasibility in Riparian Primary Forest The Role of Native Diversity and Successional Processes on Communityinvasibility in Riparian Primary Forest by Sean Satterlee Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Biology Program YOUNGSTOWN STATE UNIVERSITY December, 2012 ii The role of native diversity and successional processes on community invasibility riparian primary forest Sean Satterlee I hereby release this thesis to the public. I understand that this thesis will be made available from the OhioLINK ETD Center and the Maag Library Circulation Desk for public access. I also authorize the University or other individuals to make copies of this thesis as needed for scholarly research. Signature: Sean Satterlee, Student Date Approvals: Dr. Thomas Diggins, Thesis Advisor Date Dr. Ian Renne, Committee Member Date Prof Carl Chuey, Committee Member Date Peter J. Kasvinsky, Dean of School of Graduate Studies and Research Date iii ABSTRACT Numerous investigations have elucidated patterns and processes which govern community invasibility but relatively few have examined invasibility in a successional context. Explored here is the potential for biotic resistance to reduce invasibility of riparian successional forests at the landscape scale (~100ha) and address the following: 1) Does exotic species richness and percent cover change across successional time? 2) What is the relationship between native and non-native diversity and does that relationship change through succession? Vegetation surveys were conducted to quantify plant species richness on four raised terraces (understory reinitiation to multi-age old growth), six lower terraces (stem exclusion to understory reinitiation), and seven active channel margins, mid-channel islands, or abandoned channels (stand initiation). Exotic species richness and cover declined throughout succession and no exotic species were found on landforms greater than 136 years of age. However, although native richness remained constant throughout succession, native assemblages changed markedly. Thus landform diversity patterns in Zoar Valley likely reflect contemporary and/or past states of high community invasibility, which suggests that invasibility either does not change throughout succession, or that the regional species pool of native species adapted to particular successional stages is similar in size. This finding suggests that minimally invaded, closed canopy forests is due to a lack of exotic species in the regional species pools that are well adapted for establishing in forest understories and opposes the view that these communities are intrinsically less invasible. Future studies of community invasibility, in forests and other systems, may be better served by examining the traits and life-history strategies to which a community is susceptible to being invaded by. iv ACKNOWLEDGEMENTS I would like to thank Dr. Tom Diggins for being the best thesis advisor that I could hope for. Our many trips to Zoar Valley inspired me to become an ecologist and from our interactions over the past seven years I learned what it was to be a great one. His enthusiasm for science and mentoring, patience, and tirelessness allowed this project to be completed and was instrumental in my personal and professional development. I would also like to thank Dr. Ian Renne for going above and beyond in service as my thesis advisor. His intellectual input was instrumental to making this project the best that it could be. More importantly, the many hours we spent discussing ecology was among the most important drivers of my intellectual development. I am grateful for Professor Carl Chuey serving as a thesis advisor and for allowing me to draw on his considerable botanical knowledge and for his valuable editorial suggestions Finally I would like to thank my family who has supported me in every way possible throughout this process. I am particularly grateful for my parents who taught me the value of hard work and for always making me feel like I could accomplish anything. v Table of contents Abstract…………………………………………………………………………………...iii Acknowledgements…………………………………………………………………….....iv Table of Contents………………………………………………………………………….v List of Figures………………………………………………………………………….….v List of Tables……………………………………………………………………………..vi Introduction………………………………………………………………………………..1 Methods…………………………………………………………………………………..15 Results……………………………………………………………………………………18 Discussion………………………………………………………………………………..24 Works Cited……………………………………………………………………………...29 List of Figures Figure 1. Study site …………………………………………………………………….. 5 Figure 2. Stand initiation…………………………………………………………… 8-10 Figure 3. Stem exclusion……………………………………………………………… 11 Figure 4. Understory reinitiation……………………………………………............12-13 Figure 5. Multi-age old-growth……………………………………………………….. 14 Figure 6. Regression of total richness, exotic richness, and native richness vs. age…. 19 Figure 7. Regression of proportion exotic vs. age……………………………………. 20 Figure 8. Regression of native richness vs. exotic richness…………………………... 21 Figure 9. Cover vs. age (Honeysuckle and shade tolerant exotics)…………………... 22 Figure 10. Total richness vs. Area……………………………………………………... 23 vi List of Tables Table 1. Landform, area, overstory stand age, native richness, exotic richness, total richness, and proportion exotic…………………………………………………………. 17 1 Introduction The establishment and spread of invasive species are of great concern as they present ecological (Vitousek et al. 1997) and economic (Pimentel et al. 2005) problems, but can also serve as natural experiments for addressing fundamental questions in ecology (Sax et al. 2007). Numerous studies examine how recipient community traits affect invasibility and primarily focus on those that resist invasion. Elton’s (1958) niche-based hypothesis proposes species- and functionally- rich communities are less susceptible to invasion because high diversity assemblages limit establishment opportunities for invaders by reducing access to resources. Subsequent investigations have emphasized the importance of temporal resource availability, disturbance, and competitive interactions in determining community invasibility (Davis et al. 2000, Davis and Pelsor 2001, Tillman 2004, Renne et al. 2006). The view that species richness increases community invasion resistance has been generally supported by small scale experimental studies (i.e., plots ≤ 10m2; Fridley et al. 2007) and species richness is positively correlated with low resource availability (Tillman 1997, Levine 2000, 2001, Naeem et al. 2000, and Kennedy et al. 2002). However, despite experimental and theoretical support for the Eltonian view, observational studies at larger scales (plots > 10m2) have generally found opposing results (Fridley et al. 2007). Stohlgren et al. (1998, 1999, 2003) argue that plot-scale experiments are not supported by observations at landscape scale because their studies, the strong effects of 2 competition observed in small plot studies become dominated by landscape scale processes (also see Levine 2000, Brown and Peet 2003). At larger spatial scales, community composition is more likely to be driven by immigration processes such as high propagule pressure or low-intensity disturbance (Stohlgren et al. 1999, Levine 2000, Brown and Peet 2003). Other hypotheses suggest that extrinsic, covarying factors across large spatial scales promote both native and non-native species diversity (Levine and D’Antonio 1999, Shea and Chesson 2002) and that resource heterogeneity increases with scale (Davies et al. 2005). Within sites (i.e. at small scales), densities of individuals and their competitive interactions limit diversity of both native and non-native species (Levine 2000). In contrast, among sites (i.e. at large scales) resource heterogeneity contributes to higher species richness for both native and non-native species (Shea and Chesson 2002, Davies et al. 2005). Stohlgren et al. (2003) suggests that environmental factors that promote native species diversity also contribute to exotic species diversity. The positive correlation between native and nonnative species richness may also emerge with the presence of “weak invaders” (nonnative species that are present but do not dominate habitats), while a negative correlation exists in environments with high native species richness and presence of dominant nonnative species (Ortega and Pearson 2005). While the diversity of native and non-native assemblages is often thought to reflect exogenous influences (thus driving a positive relationship between native and non- native diversity), relatively few studies have examined the relationship in communities where native assemblages are largely driven by endogenous (biotic) processes, particularly strongly directional primary successional communities. Davis et al. (2001) suggests that there has been a historical disassociation between invasion ecology and 3 successional ecology, and that insights into the fundamental drivers of community invasibility may be gained from merging these approaches. Interestingly, strongly directional successional communities are minimally disturbed, but instead are structured by biotic interactions (factors which promote Eltonian resistance), however the community may be inherently invasible by native non-residents through time. Understanding the drivers of invasibility in these communities and identifying potential differences in the susceptibility of the community to establishment by non-native residents may help to address the fundamental drivers of the establishment
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