A multilevel approach to assess the impact of drought on urban tree physiology and ecosystem services: a case study on the city of Karlsruhe.

Urban trees and forests are known to improve microclimate and air quality in cities and increasing tree coverage is thus considered as a potential adaptation strategy. However, modification of environmental conditions due to global climate change, such as extreme droughts, also threaten the vitality and thus the provision of ecosystem services of urban trees. In addition, impervious surfaces are exacerbating these effects by decreasing soil water storage and restricting tree root access to water, thus negatively affecting physiological performance and increasing mortality risks. In the current study, we quantify air temperature regulation and pollutant deposition by accounting for soil water balance and the degree of impervious surfaces below the tree canopy. We therefore apply a multilevel approach using field measurements from an extensive tree inventory in the city of Karlsruhe to apply a physiology-based model (Tree4C). Simulations are compared with tree physiological traits derived from several vegetation-related optical indices (NDVI, EVI) that are provided by a modified Google Earth Engine script for remote sensing analysis on Sentinel-2 imagery. The combined use of inventory, model and remotely sensed data enabled the evaluation of the impact of prolonged drought periods on both tree physiology and resulting decreases in ecosystem services (cooling, pollution uptake). These results provide important support for urban tree management decisions, e.g. the selection of more resilient species or the implementation of efficient irrigation programs.