Stand Dynamics and Diversity Patterns in Planted and Naturally Regenerating Urban Forests

The world is becoming an increasingly urban planet with 68% of the global population expected to live in cities by 2050 and urban land cover expected to increase by 40%. This urban expansion brings with it a host of environmental and health consequences such as the urban heat island effect, reduced air and water quality, and biodiversity losses. In forested biomes, trees and forests growing within the urban matrix offer a valuable opportunity to offset many of these negative impacts and to provide a suite of additional benefits. In recognition of this opportunity, there is mounting interest in investing in urban forests as a form of green infrastructure. Effectively directing these investments will depend on baseline knowledge of current and potential future conditions, however, urban forest dynamics are poorly understood. In this dissertation, I help to overcome knowledge gaps in urban forest dynamics by examining patterns of nativity, diversity, and species composition in planted and naturally regenerating urban forests. To do this, I draw from two datasets that capture the two sources of future trees in urban settings: natural regeneration and tree planting.In my first two chapters, I use field data from forested natural areas throughout the city of New Haven, CT, USA to examine successional trajectories and regeneration potential in urban forest patches. While previous work has focused on discerning the differences between urban and rural forests, in this work, I focus instead on discerning the range of urban forest types that can be found within a single city. Using patch size as a framework I examine naturally regenerating forests in large (95-126 ha), medium (1-19 ha), and small forest patches (0.05-0.65 ha). In my first chapter, I find that forest structure, composition, and the proportion of native species shifts significantly with forest patch size and by relating these shifts to regeneration patterns in the seedling layer I highlight a suite of distinct successional trajectories. In my second chapter, I build on these findings by examining the regeneration potential of the buried seedbank at these same plots. Similar to findings from my first chapter, I find that the proportion of native species and dominance of individual tree species in the buried seedbank shifts with forest patch size. Together, these two chapters suggest that large patches are following similar successional trajectories to analogous rural forests in the region whereas small patches are exhibiting more distinct and novel successional trajectories. Medium patches are the most challenging patch size to characterize and in some cases resemble large patches and in other instances, small patches. Challenges in distinguishing forests in this patch size highlight the potentially important role that landscape structure, age, land-use history, and anthropogenic impacts – in addition to patch size – play in shaping urban forest dynamics. Indeed, results from regeneration regressions in these two chapters indicate that proximity to surrounding forest cover is a significant positive predictor of the number of native seedlings and germinants in the buried seedbank. This finding suggests that native tree planting may be necessary in more isolated forest patches in order to sustain future cohorts of native trees. Tree planting is the focus of my third chapter. In this chapter, I use survey data from municipalities and non-profit organizations throughout the Northeastern USA to understand how local tree planting programs contribute to regional diversity patterns. I find that cities in the Northeast rely heavily on a narrow suite of species and genera for specific ecosystem services. Specifically, nearly 20% of all shade trees planted in the region are oak species and over 50% of ornamental trees are either cherry species or tree lilac. This overreliance on individual taxa may have implications for future forest resilience. Moreover, tree planting palettes in the region included invasive tree species, one of which (Norway maple) was also a prolific species regenerating in the urban forest patches from chapters one and two. This finding underscores the importance of considering natural regeneration and tree planting in the context of one another as planted trees may serve as seed sources for naturally regenerating trees in natural areas. Collectively, this dissertation illuminates potential future forest conditions in planted and naturally regenerating urban forests. Insights into the future forest are the cornerstone to effective and appropriate forest management and findings from this dissertation can be leveraged to inform management in urban forests throughout the Northeastern, USA. Beyond management, this dissertation also introduces frameworks that can be further honed and developed to enhance our understanding of forest dynamics in urban areas around the world