Floristic mapping data as a proxy for the mean urban heat island

Abstract

Assessment of spatial variability in the long-term urban heat island (UHI) is severely restricted by coverage and availability of measurements. This also limits the opportunities to analyse its relation to surface characteristics. In this study we therefore introduce a new proxy dataset derived from floristic mapping. The basic assumption is that the species composition in an area reflects climatologic conditions of a certain time period. Ellenberg indicator values for temperature (EITs) were processed to summarise the overall temperature preferences of the occurring plant species. The EITs showed a clear heat island pattern, were highly correlated with existing measurements, and showed increased values in densely built urban classes. Hence, they are considered suitable as UHI proxies. Further, they were related to a large number of typical UHI predictors. The normalised difference vegetation index (NDVI) showed the strongest correlation with the derived pattern and was comparatively robust towards cloud contamination. Urban morphology also explained a noticeable proportion of the overall variance. All predictors explained more than 2/3 of the overall spatial variability, while the redundancy was high. Therefore, the predictors at least allow qualitative statements about the differential exposure to heat-related risks in cities. KEY WORDS: Floristic mapping · Urban heat island · Ellenberg indicator values · UHI predictors · Local climate zones · Infrared thermography Resale or republication not permitted without written consent of the publisherClim Res 49: [45][46][47][48][49][50][51][52][53][54][55][56][57][58] 2011 even in a maritime climate. More detailed knowledge about the impact of different urban morphologies on mean air temperature would help to adapt urban infrastructures to climate change-related risks in time. However, the frequent lack of long-term UHI data severely limits such analysis.We therefore examined a new proxy dataset, which has potential to indicate the mean annual to mean decadal heat island intensity for a whole city at a spatial resolution of 1 km in a consistent manner. This dataset was derived from the mapping of spontaneous occurring plant species in Hamburg, Germany, which was conducted over a period of 15 yr. The underlying idea is that the species composition in a certain area yields information about the respective climatic and ecological conditions (Ellenberg et al. 1992). Although this approach might be uncommon in urban climatology, it is appropriate considering the long tradition of phenology in urban climatology, especially in Hamburg (cf. Franken 1955). Gödde & Wittig (1983) presented a thermal division of Münster, Germany, on a vegetational basis which visually represented typical zoning of urban microclimatic conditions. In this study we not only interpreted patterns from the floristic dataset visually, but also conducted an empirical analysis, and further refined the data as a proxy for UHI intensity.To gain more detailed knowledge of the UHI intensity, its ...

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