UC Santa Barbara

Trees in cities can make communities more resilient to climate change and provide sustainable infrastructure for heat mitigation and stormwater regulation. Urban areas around the world are growing, and there is potential to plan for urban forestry to be equitable and sustainable. In the Southwestern United States, growing populations along with a higher frequency of drought and heat waves poses a need for urban forestry management in conjunction with efficient water use. 

This dissertation used a mechanistic model, the Regional Eco-Hydrologic Simulation System (RHESSys), to simulate urban tree carbon and water fluxes during a multi-year drought in Southern California. Tree leaf area index (LAI), net primary productivity (NPP), and water use were estimated under different scenarios of temperature, irrigation, and neighboring land cover. In the first chapter, I used high spatial resolution remote sensing data from Santa Barbara, California of tree species, LAI, and changes to the Normalized Differential Vegetation Index during the 2012-2016 drought to create tree species parameter sets for five common urban tree species. With these urban tree parameter sets, I estimated tree drought resistance and resilience for LAI and NPP and showed how they differ in the case of a drought with similar precipitation patterns and different temperatures. I demonstrated how temperature plays an important role for tree productivity post-drought and can increase resilience. Warmer temperatures increased resilience, but only to a certain extent: 1.8C of warming increased post-drought productivity leading to higher resilience compared to the cooler temperature scenarios, but extreme warming of 4C hindered growth. In the second chapter, I used the same tree species parameters to simulate how irrigation inputs to the system would affect drought resilience and water use efficiency (WUE). The response of the average tree NPP to irrigation input during drought was non-linear, with a break-point analysis displaying a steeper decline in NPP beyond irrigation reductions of 25% to 50% depending on tree species. Transpiration was linearly related to irrigation input, and this caused WUE to increase with less irrigation. In the third chapter, I went a step further to explore irrigation effects by incorporating neighboring irrigation on turfgrass. In some city locations, mature trees on their own may not be directly receiving irrigation. However, in this study I found that trees may be receiving excess irrigation water from turfgrass area. This study also highlighted an example of hydrologic modeling with sub-grid water routing, and showed how impervious surfaces disconnected from the stormwater drainage system can increase tree productivity.