Concept and methodology of characterising infrared radiative performance of urban trees using tree crown spectroscopy

Urban trees play an important role in cooling urban microclimates and regulating outdoor thermal comfort. To better understand their contribution to these processes, it is crucial to elucidate urban trees' radiative thermal performance, especially in the infrared (IR) region (approximately 50% of solar radiation). Yet, owing to significant conceptual and methodological challenges, studies on the radiative performance of trees have mainly focused on individual leaves rather than crown-level characteristics. Here we applied a novel conceptual and methodological framework to characterise the crown-level IR radiative performance of 10 lime trees (Tilia cordata), a common urban tree in the UK and Europe. Our results show that reflected and transmitted solar energy from leaves is dominated (>70%) by IR radiation. At the leaf level, transmission and reflection spectra are similar between trees (differences typically <10% in IR region), including those under significantly different urban stress conditions. However, at the crown-level, substantial variations in IR transflectance spectra (maximum difference >40% in IR region) were found between trees. These variations were largely due to crown structural differences (leaf number, density, angles), rather than leaf solar interaction character (leaf-level transmittance or reflectance, leaf colour). Crown transflectance measured from the four cardinal directions was significantly different in the IR region (maximum differences circa 30%), and changed substantially with solar time. Hence, a tree's surroundings received very different, and time dependent, levels of solar IR radiation. These findings have significant implications for species selection and control of environmental stress factors in urban microclimates.