Plant recruitment dynamics in urban forests: consequences for seed and seedling establishment

Plant recruitment dynamics in urban ecosystems can be influenced by multiple co-occurring and often exacerbated conditions and stressors common to most cities. Urban associated habitat fragmentation and land transformation, altered climate and microclimates, biotic invasion, pollution (e.g. elevated atmospheric carbon and nitrogen), and human activity, both direct and indirect, are all observed to have species-specific effects that may limit or facilitate recruitment success. A core question is whether urban ecosystems are functionally different, and if so, what that means for management and restoration practice. This dissertation investigates the application of plant life history frameworks and analyses in the context of urban forest populations. Chapter 1 presents a review of recent literature and a modified recruitment limitation framework for understanding plant recruitment dynamics within urban ecosystems. In the subsequent chapters (2-5) I present a series of related studies that examine seed and site limitation in urban and rural oak-hickory forests located in the New York City metropolitan area. Chapter 2 compares the advance regeneration of native tree species in these sites and the implications for urban forest management. I found urban forests to be seedling limited, however advance regeneration stages were not different from rural forests. Chapter 3 uses the recruitment limitation framework to investigate the relative contribution of seed and site as limiting factors for seedling establishment. Urban and rural forests were both strongly site limited. The native canopy in urban forests is limited by site factors, not seed availability. Chapter 4 presents findings from a seed addition experiment that tested the influence of seed predation and herbivory on early seedling establishment in urban and rural forests. This experiment confirms that seed predation and herbivory pressure is greater in urban forests and contributes to the observed differences in urban and rural forest seedling recruitment. Finally, Chapter 5 presents a multi-year experiment of seed predation dynamics that identifies differences in the temporal pattern of seed removal in urban and rural forests; a potentially critical shift in forest function. Collectively, this dissertation identifies ecological mechanisms that limit plant recruitment in urban forests. A comprehensive understanding of urban recruitment limitation, as a scientific foundation with respect to plant abundance and distribution, will allow us to successfully conserve, manage, and restore plant communities to enhance the human experience of nature in cities.Ph.D.Includes bibliographical reference