Assessing water status of two urban tree species : Acer Platanoides and Tilia Euchlora by very high spatial resolution imagery and field micro-dendrometers measurements

Urban areas are faced with issues of particulate air pollution and urban heat islands, in a context of a growing number of their inhabitants. Ecosystem services provided by urban trees are impacted by the health and functioning mechanism of the trees, in particular the evapotranspiration process. The impact of urban specificities on tree functioning has yet to be fully studied. In recent years, improvement in remote sensing and the availability of very high spatial resolution imagery offer new perspectives and working methods for urban tree. The aim of this study is to explore how Pleiades imagery and field micro-dendrometer measurements can assess the health and water status of the two main tree species present in the city of Dijon: Acer Platanoides and Tilia Euchlora. The work has been leaded in 3 steps. First, the very high spatial resolution Pleiades imagery has been used to identify tree canopy in Dijon city. Generalist and empirical approaches are compared, for instance NDVI, MSAVI2 and EVI vegetation indices. Then, tree canopy, tree species (based on field records), morphological parameters (from topographic data and digital elevation model) and proximity to pollutant emissions are used to select six sites in Dijon. In each site, one or two mature trees (six Tilia Euchlora and five Acer Platanoides overall) are finally equipped with the micro-dendrometer (PepiPIAF system) to record daily stem diameter variations. Variables reflecting the water status of the trees, like the maximum daily shrinkage, are then calculated from these field measurements. The first results are encouraging, a marked response of vegetation indicators to precipitation is observed, with high values after heavy rainy episodes. The next step is to establish the link between vegetation indices obtain via remote sensing and micro-dendrometers measurements. This could in turn be a step forward the modelling of trees’ water status at a high spatial resolution at the scale of the city.