Studying the interactions between the spectral composition and thermal properties for urban trees

Remotely sensed spectral and thermal measurements are valuable in vegetation related research with a spectrum of applications from ecology to urban landscape studies. Their success depends on our understanding of the complex dynamics between plants and their surroundings as well as the plant’s spectral properties - both are scale dependent challenges. In cases when remote sensing applications are deployed in conditions with sparse vegetation, e.g. trees in urban areas where multiple components within a pixel need to be considered, the contained spectral information can be difficult to interpret.

In this study we present a ground-based experimental layout consisting of a spectrometer and a thermal camera mounted on a portable crane. The experimental layout was deployed in two applications:

(i) thermal images were used to characterize the thermal status of different parts of a dense circular cluster of containerized trees and their spectral reflectance was measured. A statistically significant difference of both VSWIR reflectance and absorption features related to the chlorophyll, carotenoid, and water absorption bands was found between the warmer and cooler parts of the canopy.

(ii) the contribution of the thermal signatures of the tree canopy and the underlying urban surface to the spectral reflectance variation, was studied with two groups of five identically arranged containerised trees placed into two adjacent built and non-built local microenvironments. It was found that strong correlations between the canopy-background temperatures and the spectral reflectance indicate that synergies between thermal and spectral measurements in the fine scale is a promising method for disentangling the combined signal components.

With the resolution of data products from air- and space-borne instruments increasingly improving, results of this study indicate the potential of leveraging the synergy between thermal and spectral measurements for the purpose of more accurately assessing the complex biochemical and biophysical characteristics of measured urban vegetation canopies.