Urban growth generates many benefits such as housing and employment opportunities, but also affects the environment's ability to provide ecosystem services including clean air and water. While knowing how urbanization impacts ecosystem services at multiple spatial scales over time is a critical step in the development of sustainable cities, inadequate scientific understanding remains a primary obstacle to this goal.
Doctoral student Adam Berland at the University of Minnesota Twin Cities, under the supervision of Dr. Steven Manson, will explore the spatial and temporal dynamics of urbanization's effects on ecosystem services for an urban core in downtown Minneapolis, MN. The study will take place along a transect that extends from the urban core in downtown Minneapolis through the suburbs to the peri-urban fringe, thus capturing a general gradient of urbanization. The research focuses on the urban forest, a collective term for all trees within the urban area, to determine how previous land cover change has affected the urban forest along an urban-rural gradient, how ecosystem services are associated with the urban forest currently distributed along an urban-rural gradient, and how projected urban expansion will affect ecosystem services in the coming decades. Aerial photographs will be used to document forest canopy change in the transect over the past 70 years with respect to land cover transitions. Regression analyses will assess the relationships among tree canopy cover, land cover type, and distance from the urban core to demonstrate how urban forest cover has fluctuated with land use changes over several decades, thus providing a context for understanding current and future urban forest ecosystem service prospects. Then current ecosystem services in the study transect will be characterized based on detailed vegetation and land cover surveys of 300 field sites. Several modeling approaches will approximate current urban forest provision of services such as air pollution removal, stormwater interception, and carbon storage. Finally, future ecosystem service distributions will be estimated for 50 years under a range of possible land cover change scenarios. Using tree rings and municipal planting records to develop species-level age-to-size relationships for the most common trees in the transect, the study will project tree growth and associated changes in ecosystem service provision. Together, these efforts will highlight past, present, and future ecosystem service distributions along the study transect.
This research will address a key gap in scientific knowledge by demonstrating the distribution of ecosystem services along an urban-rural gradient at multiple spatial scales over time. Given recent increases in regional-scale planning, the study is timely because it analyzes scenarios across several municipalities. Project findings will be directly applicable to regional land managers and urban foresters by highlighting areas of historic gains and losses in ecosystem services, projecting the effects of future land cover change scenarios and urban forest management strategies, and projecting the impacts of future pest outbreaks and natural disasters. Together, the findings will guide local professionals in their short- and long-term efforts to improve urban forest ecosystem service provision in the Twin Cities Metropolitan Area.
We studied the relationship between urbanization and trees in the Twin Cities metropolitan region of Minnesota. We focused specifically on the urban forest—or all trees and woody shrubs in the urban area—to learn how urban development affects tree distributions and sizes. Trees provide many benefits to humans such as clean air, but we do not fully understand how those benefits are distributed along an urbanization gradient from the urban core to the exurban fringe, and this lack of knowledge limits our ability to incorporate urban forest considerations in land use planning decisions. To address this shortcoming, we used a combination of field observations and digital data analysis to assess three key aspects of urbanization-urban forest relationships. First, we focused on residential properties to determine which types of neighborhoods had the greatest urban forest structure, as defined by tree abundance and tree sizes. We visited 150 residential properties, inventoried all trees on the property, and then related that information to property characteristics such as house age, property value, population density, and neighborhood road density. We found that properties near the urban core and at intermediate levels of urbanization had the greatest urban forest structure, while larger properties in newer suburbs had substantially smaller trees. Much of this difference is attributable to time, since newer suburbs have not had enough time to develop mature trees. Second, we compared urban forest characteristics on residential, nonresidential developed properties (e.g., businesses, churches), and undeveloped land. We analyzed urban forest structure, function (carbon storage, carbon sequestration, and air pollution removal), and monetary value (the cost to replace current trees with similar trees). Residential properties and undeveloped lands had similar urban forest characteristics for each comparison, and they always outpaced nonresidential developed properties. In places, the urban forest would cost nearly $400,000 per hectare to replace, which highlights the great value of this resource. For comparison, the nonresidential urban forest is less valuable, as most sites would cost less than $25,000 per hectare to replace. Finally, we developed models to project urban forest structure, function, and value into the future. We knew that newer suburban neighborhoods have smaller trees because they are younger, so we wanted to simulate future urban forest growth in these areas to allow for fair comparisons with older neighborhoods. This work is still in progress, but preliminary results suggest that newer suburbs may have less predictable urban forests in the future due to increased landowner choice in tree planting and a demonstrated preference for planting shorter-lived, smaller trees. The results of this study met the project goals by highlighting new perspectives on the relationships between urbanization and urban forest structure, function, and value. We are sharing results with local urban foresters, who will benefit by learning how their urban forest resources are structured and how they fit into the larger metropolitan regional context. Two undergraduate students worked under this grant, and their exposure to advanced research may motivate them to pursue additional opportunities in education and science. Our work is being prepared for multiple publications in geography and urban ecology journals.
