From urban trees to watersheds: an evaluation of green infrastructure integration for effective climate adaptation

Abstract

Green infrastructure (GI) has the potential to provide multiple ecosystem services to cities, via stormwater management and urban heat mitigation, but its biophysical response to its surrounding environment and hydrological outcome considering its within-watershed placement can create large margins when estimating its realized benefits. This is in part due to the inherent heterogeneity of urban landscapes (e.g., land cover types, gray infrastructure distribution), leading to wide-ranging scenarios of growing conditions and watershed-level impact. To better quantify green infrastructure's climate adaptation potential, we need to first understand green infrastructure's functions at a fine-resolution, and relate its outcome when distributed within an urban watershed. The series of work presented in this dissertation evaluates GI's integration to its physical environments at two scales: one at the tree level, offering a biophysical lens into how trees respond to and simultaneously change their environment, and the other at the watershed scale, examining how the hydrological processes of distributed GI interact with existing gray infrastructure. By first developing a novel method to de-centralize an individual tree's water use measurement (i.e., sap flux), I conducted a multi-year monitoring study on urban ash trees in St. Paul, Minnesota, USA relating their water uptake to their health and their physical environments. I found that healthier trees not only used more water but also tended to conserve their water usage particularly during drought, whereas sick trees used less water and lacked regulation. Then I modeled the hydrological outcome of a distributed system of GI (i.e., bioretention cells), and found that the gray infrastructure’s spatial configuration can introduce tradeoffs between increased peak flow and increased flooding, and further interacts with GI coverage and placement to reduce peak flow and flooding at low rainfall intensity. Findings from this work suggested that GI is not a cure-all solution for climate adaptation, as the environmental conditions (e.g., heat stress, water availability, precipitation extremes) strongly affect GI functions---both on its own and in combination within a watershed, and in turn benefits. To effectively plan for climate change, cities must invest in both forestry maintenance and gray infrastructure expansion, in conjunction with spatially strategizing GI expansion, to help GI reach its potential.