Understanding the radiative performance of urban trees

Built up areas are dominated by man-made materials with low albedo and high heat storage capacities. Solar irradiance paired with anthropogenic inputs from heating, air conditioning or traffic as well as city specific phenomena like Smog or CO2 domes perpetuates a change in the net surface energy balance of urban areas causing a temperature increase within cities, a phenomenon known as the Urban Heat Island effect (UHI). In order to mitigate negative impacts, Cities need the provision of ecosystem services by green areas, a provision largely dependent on the characteristics of the vegetation populating these spaces. The benefits obtained from green spaces cover cultural, provisioning as well as supporting services and importantly include climate regulation i.e. microclimatic influences of plants. Vegetation can provide a natural cooling system not only through evapotranspiration but also through physically shading a sidewalk or building envelope thus by interacting directly with the radiation. Against the backdrop of varied urban environmental conditions, plant responses in cities differ from behaviour under natural habitat conditions, triggering a dynamic response resulting in changes of functional traits. The magnitude of these differences needs to be fully understood on an inter- and intraspecies specific level to provide urban planners with an enhanced understanding of tree specific roles in sustainable urban design strategies. Like all terrestrial objects, plant tissue exchanges heat with the immediate surroundings not only through the modes of convection and conduction but also through radiation thus qualifying spectroscopy as a tool to assess the radiative performance of trees. As such, the focus on the radiative and biological response of trees to near and mid-infrared radiation in an urban context fits with the uptake of urban greening as an engineering solution to climate change.
The research objectives are to systematically quantify the interactions between trees with solar radiation and the urban thermal environment to assess their spectral properties and passive cooling performances. The knowledge will improve the understanding of the benefits and potential issues of vegetation in an urban context, leading to a database of the measured performances and related insights like functional trait influences. Impacts on e.g. shade quality or heat storage potential will aid species choice and decision making about trees in the urban environment.
The project is designed as a doctoral research thesis to be completed in 2017. Currently, a methodology based on spectral measurements is being developed and validated through a series of pre-studies. Leaf samples are being collected throughout the summer growth season and reflectance and transmittance spectra from 0.4 µm to 15 µm will be taken, utilising a hand held spectroradiometer (Spectral evolution SR-2500) as well as a lab based FT-IR Spectrometer (Bruker Optics IFS 66v/S). Dendrological variables and a selection of functional leaf traits will be tested for wavelength correlation. This will inform focus areas of leaf traits and their spectral impacts for further work, combining a novel focus on urban specific environmental influences, dynamic plant traits and radiative performance to further inform the choice of the right tree for the right place.