Remote sensing reveals heterogeneous responses of urban vegetation types to compound heat and drought events in Hamburg, Germany

During the past decade, Europe has experienced an exceptional number of compounding hot and dry summers, with substantial consequences for agriculture, economy, human health, and natural ecosystems. Especially in highly fragmented urban environments, healthy vegetation plays a key role in mitigating the impacts of heat, yet large-scale studies on the ecological effects of drought often overlook fine-scale patterns needed for local adaptation planning. In this study, we analyze the short-term responses of semi-natural and urban vegetation types in Hamburg, Germany, to the compounding hot and dry summers of 2018, 2020, and 2022 using high-resolution Sentinel-2 NDVI (Normalized Difference Vegetation Index) time series. We find that herbaceous vegetation, such as grasslands, exhibited reductions in greenness during the drought summers, with stronger deviations in locations characterized by drier site conditions. Woody vegetation and wetlands showed no consistent decline, indicating limited short-term responsiveness. Although these patterns align with studies from larger spatial scales, our results reveal considerable variability of responses within vegetation cover types as well as pronounced small-scale spatial heterogeneity of responses. These findings suggest a complex interaction of local site characteristics, plant species composition, and the specific timing of droughts. The observed fine-scale variability of responses emphasizes the importance of high-resolution assessments for identifying locally vulnerable vegetation types and informing targeted adaptation measures in urban landscapes. By integrating two widely used and globally applicable metrics – the NDVI and the Standardized Precipitation Evapotranspiration Index (SPEI) – our approach ensures high transferability across sensor platforms and climatic regions. At the same time, the use of high-resolution Sentinel-2 imagery demonstrates its effectiveness for capturing drought responses in fine-scale urban vegetation patches. • Herbaceous vegetation responds to compounding heat and drought with a reduction in greenness, with more pronounced reductions on dry sites. • In woody vegetation, no short-term reduction in greenness can be detected, with the exception of young and newly planted forests and parks. • The spatial heterogeneity of responses of vegetation of the same type highlights the need to assess drought impacts on detailed spatial scales.