Twin Cities urbanization and implications for urban forest ecosystem services.

Abstract

Urbanization affects ecological structure and function by impacting the provision of ecosystem services, or benefits we derive from the environment. It is broadly acknowledged that ecosystem services should be formally considered in land management decisions, but inadequate scientific understanding of urban ecological systems is a key obstacle to achieving this goal. In this dissertation, I address this shortcoming by assessing the relationships between urbanization and the urban forest, a key urban ecological component. The three studies described here demonstrate spatial and temporal effects of urbanization on urban forest structure, function, and value in Minnesota's Twin Cities Metropolitan Area. In the first study, I used historical air photos to analyze past trends in tree canopy cover related to urbanization and other land cover changes. Urbanization events generally reduced tree canopy cover, but urban sites rapidly afforested following development. Older urban neighborhoods typically had higher tree canopy cover than newly developed areas. In the second study, I used factor analysis on a suite of urbanization indicator variables to derive an urbanization gradient that is more sophisticated than a simple urban-rural distance-based gradient. This synthetic gradient was strongly related to more types of urban forest structural variables than the distance-based gradient, highlighting the influence of secondary urbanization trends on urban forest structure. In the final study, I stratified the study area by property parcel land use, and compared estimated urban forest structure, function, and value across land use classes. Residential and undeveloped areas both had higher urban forest values than non-residential developed areas, but were not statistically different from one another. This study showed which types of urban land uses promote good urban forest structure and function, and the results can be used to guide future urban forest study designs. All three studies demonstrate the need to consider complexities associated with human-environmental systems. Two major themes were the importance of temporally lagged tree growth and nonlinear urban-ecological relationships. By making these complexities more visible, this research will improve the design of future work, so that we can develop a more complete and nuanced understanding of the effects of urbanization on the urban forest.