Forest Development And Regeneration Dynamics In The Context Of Global Change And Associated Adaptation Strategies

Over the past century, forested ecosystems in northeastern North America have undergone significant changes characterized by the recovery from intensive land use and impacts from the introduction of invasive insects and disease. These changes, coupled with emerging threats posed by a changing climate present challenges to the maintenance and conservation of biodiversity, ecological functions, and delivery of critical forest ecosystem services. While management strategies to increase forest resilience and mitigate impacts from climate change have been broadly proposed, there are nascent empirical evaluations of their effectiveness. To address this uncertainty, this dissertation couples field manipulations with long-term measurements to examine forest regeneration and stand development in the context of climate adaptation, mitigation, and potential responses to global change. First, we utilized a 69-year record of forest development to investigate the structural and compositional dynamics of transition hardwood and mixedwood forests to understand factors controlling climate adaptation and mitigation potential. Results demonstrate the long-term compositional stability of hardwood stands, yet transient nature of Pinus strobus-Quercus mixedwoods that shifted toward hardwood dominance over time, underscoring the potential reduction in these communities and associated climate benefits without silvicultural intervention. Next, we examined the challenges for adaptive silvicultural strategies aimed at forest compositional transitions by investigating the role of ecological memory and other biophysical constraints on the performance of adaptation plantings (e.g., assisted migration) tested across regional forests. Results show that the performance of future-climate adapted seedling transplants was controlled by species, the strength of vegetative competition, source distance, and extreme climate events, highlighting barriers for managing forests for shifts in composition. Finally, we tested the influence of climate extremes at controlling regeneration and how biotic and edaphic drivers interact or modify these effects. Results indicate that factors other than climate, namely bioturbation of seedbeds, overshadowed the role of precipitation on seedling survival by nearly two-fold. Additionally, the effect of precipitation on survival was closely linked to plant functional traits like seed mass (adjusted R2=0.72, p\u3c0.0001), not the presence of extreme rain events. Collectively, this work highlights the mechanisms of resilience of regional forests and the drivers controlling compositional transitions. Notably, this work emphasizes the strength of factors such as ecological memory, successional processes, or other biophysical factors (e.g., seedbed, functional traits) at potentially outweighing the near-term effects of climate change on these sites. Outcomes from this work suggest that maintenance of northeastern forests via adaptive silvicultural systems may promote adaptive capacity; however, an understanding of the underlying ecosystem dynamics and drivers of regeneration establishment are critical to achieve favorable outcomes