Urban Trees Phenology and Local Climate Feedbacks of a Residential Area in Berlin.

The role of vegetation in urban climate has been in the spotlight in recent years, as it can play significant roles in carbon sequestration through photosynthesis as well as in the urban energy balance, mainly through evapotranspiration and shading. Based on these, the green infrastructureof cities is considered as a potential solution to lower the urban net CO2 exchange and lower air temperatures, improving the resilience of cities in the context of climate change.Being part of the general physiological responses of trees, the abovementioned mechanismshave been excessively studied in natural environments. However, the quantification of the different effects of these processes in complex and heterogeneous urban landscapes is challenging. In this study, we demonstrate initial results of a year-long observation period of tree vegetation in a residential area in Berlin, Germany, using PhenoCam and flux-tower observations. The phenology curves were extracted from half-hourly PhenoCam images of trees from the Acer, Aesculus, Fagus, and Pinus genera and analysed in combination with comprehensive observations of  thesurface energy balance components, including net radiation, turbulent sensible and latent heat fluxes as well as CO2 fluxes and standard meteorological variables. We showcase the agreement between the gradual development of tree foliage fordeciduous vegetation (which dominates the area) with: a) the upward latent heat flux seasonal maxima observations; and b) the decline of upward CO2 flux values. In particular, the timing of the start of season (SOS), peak of season (POS) and end of season (EOS) is assessed and compared to changes detected in the flux trends. Our data indicates a strong connection of the green-up period of deciduous vegetation with the largest rate of decrease of the CO2 fluxes, leading to a change from CO2 source to sink for a constrained time period. These observations highlight the measurable effect of vegetation-related carbon sequestration that can take place in urban areas with significant vegetation cover under specific/average meteorological conditions.