Stewarding Floodplain Forests in a Changing Climate: Assisted Migration and Spring Tree Phenology in an Urban Climate Change Experiment and Monitoring for Floodplain Tree Regeneration

Abstract

As climate change continues to affect the world’s ecosystems, land managers seek to determine the best actions to maintain or adapt their forests to the current and projected climatic shifts. Floodplain forests are an ecosystem of interest for mitigating the effects of climate change, primarily through increased critical habitat and biodiversity, reduced nutrient input in riverine systems, and carbon sequestration. These unique forests are critical in climate action plans, but they are also facing increasing stressors due to severe weather events, nonnative species, and landscape alterations, resulting in the need to support these forests through a changing climate. Chapter 1 of this thesis focuses on the spring phenology of trees planted in an urban floodplain forest climate change experiment in Saint Paul, MN. Specifically, we examined the bud break and leaf out of trees in the Adaptive Silviculture for Climate Change (ASCC) project to determine if resident tree species, southern tree species, and populations of trees from USDA Hardiness Zones 4, 5, and 6 differed in the timing of these phenophases. We found that novel southern tree species who were moved northward via assisted species migration and range expansion leafed out significantly later than all other species. We did not find many significant differences among trees within the same species who were sourced from different hardiness zones, with the exception of eastern cottonwood (Populus deltoides) trees from hardiness zone 5 breaking bud and leafing out later than those from zone 4. Our findings suggest that the movement of species northward from outside of their natural range may have implications for their successful growth and survival due to the potentially altered timing in key leaf phenological events and subsequent mismatch with the growing season at the site of relocation. Chapter 2 is focused on a collaboration with Prairie Island Indian Community to monitor floodplain forests to support the Tribe’s reforestation efforts. The overarching goal of monitoring was to support healthy and resilient floodplain forests through providing information that will assist in identifying successful tree planting locations for diverse and climate-resilient tree species derived from the “Resilience” management approach of the ASCC project. Planting of diverse tree species in canopy openings was desired to maintain a forest canopy and other ecosystem services after the removal of green ash (Fraxinus pennsylvanica) trees impacted by the nonnative insect emerald ash borer (Agrilus planipennis). We developed a monitoring plan to sample the understory vegetation and overstory light levels at sites deemed a priority for planting or for management actions to reduce reed canary grass (Phalaris arundinacea) abundance. While specific results and management recommendations will not be shared in this published thesis, we assessed the potential success of enrichment plantings as well as the impacts of reed canary grass in 19 canopy gaps. These included gaps where ash were harvested, where ash will be harvested, and pre-existing canopy gaps. We identified suitable sites and locations within gaps to plant a diverse set of floodplain tree species that will create and maintain forest canopy, provide habitat, and potentially increase resilience to the impacts of climate change.