Trees in the street canyon environment: exposure and response under changing micro-climates and water availability

Local scale urban climate modelling utilising urban land surface parameterisations commonly show reductions in air temperature in response to increasing vegetation cover. At the micro-scale however, complex interactions exist between street trees and the surrounding built environment. Such interactions control how effective trees are in providing cooling benefits and improvements to human thermal comfort. In particular, water availability can play a vital role in plant response to urban environmental conditions that at times, can be quite demanding. For trees to provide cooling, they need to sustain healthy canopies and actively transpire. Such intricacies could contribute to the difficulties in modelling latent heat fluxes in urban areas (Grimmond et al. 2011).

This study investigates the potential influences of changing urban environmental conditions on tree function and health. Using the model MAESPA to represent an individual tree, we explore the influence of the unique urban environment (e.g. changing radiation levels, temperature, VPD, CO2) on tree transpiration and shading capacity. The model is tested with data collected from trees in a shallow street canyon in Melbourne, Australia. Within canyon environmental parameters such as radiation and CO2 concentrations were collected, as well as soil moisture, and the MEASPA model outputs are compared with observed leaf assimilation rates and stomatal conductance.

We then undertake a number of scenarios focussed on changing environmental conditions and/or soil moisture to explore the impacts on individual tree transpiration rates and canopy density and infer potential cooling benefits. Such information can help guide irrigation management of street trees under extreme heat conditions, and help inform street tree planting arrangements to maximise within canyon human thermal comfort.