Urban tree health monitoring with hyperspectral and chlorophyll fluorescence measurements

Urban trees play key roles for mitigating environmental problems in cities, but they often face harsh environmental conditions. For instance, drought stress is one of the most common stresses urban trees suffer, and urban trees are more prone to water deficits than forest trees. The objective of this study was to monitor the health status of tilia tomentosa trees growing under varied conditions in two cities, using optical measures. Trees were grouped into three treatments in terms of their surface soil availabilities, including (i) boxed, trees planted individually in small rectangular soil surfaces; (ii) linear, several trees grown in soil strips; and (iii) unlimited, trees freely planted in roomy soil surfaces (e.g., parks). We performed leaf reflectance spectroscopy and chlorophyll fluorescence measurements for quantifying tree health status and compared the three treatments. Results showed that boxed trees yielded lower values of the normalized difference water index (NDWI) and physiological reflectance index (PRI) compared to the linear and unlimited trees. NDWI and PRI are the typical optical indicators of leaf water and physiological status changes owing to stresses, respectively, suggesting potential impact of water deficit on the boxed trees. Similarly, spectral indices associated with leaf chlorophyll were significantly lower in the boxed trees, suggesting leaf biochemical changes as a result of limited soil availability. Furthermore, leaf chlorophyll fluorescence measurements showed that boxed and linear trees produced lower Fv/Fm (maximal quantum yield) than unlimited trees, suggesting a reduced photosynthetic efficiency of photosystem II. Our results demonstrate that leaf chlorophyll fluorescence and reflectance parameters are excellent early indicators of tree health status in urban environments. The findings imply potential associations between urban soil functioning and tree growth and health and, therefore provide a baseline for optical assessment of ecosystem services provisioned by urban trees.status: publishe