Spatial and temporal variability of urban tree canopy temperature during summer 2010 in Berlin, Germany

Trees form a significant part of the urban vegetation. Their meteorological and climatological effects at all scales in urban environments make them a flexible tool for creating a landscape oriented to the needs of an urban dweller. This study aims at quantifying the spatio-temporal patterns of canopy temperature (T-C) and canopy-to-air temperature difference (Delta T-C) in relation to meteorological conditions and tree-specific (physiological) and urban site-specific characteristics. We observed T-C and Delta T-C of 67 urban trees (18 species) using a high-resolution thermal-infrared (TIR) camera and meteorological measurements in the city of Berlin, Germany. TIR images were recorded at 1-min intervals over a period of 2 months from 1st July to 31st August 2010. The results showed that Delta T-C depends on tree species, leaf size and fraction of impervious surfaces. Average canopy temperature was nearly equal to air temperature. Species-specific maximum Delta T-C varied between 1.9 +/- 0.3 K (Populus nigra), 2.9 +/- 0.3 K (Quercus robur), 3.2 +/- 0.5 K (Fagus sylvatica), 3.9 +/- 1.0 K (Platanus acerifolia), 4.6 +/- 0.2 K (Acer pseudoplatanus), 5.0 +/- 0.5 K (A. platanoides) and 5.6 coproduct 1.1 K (A. campestre). We analysed Delta T-C for a hot and dry period (A) and a warm and wet period (B). The range of species-specific Delta T-C at noon was nearly equal, i.e. 4.4 K for period A and 4.2 K for period B. Trees surrounded by high fraction of impervious surfaces showed consistently higher Delta T-C. Knowledge of species-specific canopy temperature and the impacts of urban structures are essential in order to optimise the benefits from trees in cities. However, comprehensive evaluation and optimisation should take the full range of climatological effects into account.