Updated and downscaled European winter hardiness maps including Urban Heat Island effects for urban tree species selection.

Trees provide cooling services in urban areas in summer. Which tree species are suitable in cities in a future climate remains an open question. One of the criteria for tree species selection is winter hardiness. Winter hardiness is an indicator of the lowest temperatures that plants typically experience in an area. The United States Department of Agriculture (USDA) defines winter hardiness as the average annual minimum temperature. Hence, it is a straightforward and commonly used indicator to predict the likelihood of certain plants to experience frost damage in winter. The most recent winter hardiness map for Europe was made in 1984, and does not include the climate change from the past decades, and neglects the urban heat island effect (UHI) that causes warmer winters in urban areas, impacting winter hardiness. In this study, we developed an updated and downscaled version of the European winter hardiness maps using minimum temperature from the ECA&D E-OBS dataset version 26.0, and the European Local Climate Zone (LCZ) map with a 100x100m resolution.

The new maps represent the winter hardiness for five standard normal periods between 1951 and 2020, and represent the years 2030, 2050, and 2085 for the Netherlands using four climate scenarios. These maps show how hardiness zones have moved to the north and east over the past decades. They show that roughly 60% of Europe is now in a different hardiness zone compared to the 1951-1980 average, potentially allowing new tree species to flourish here. They also indicate that the Netherlands can move up to three hardiness zones between the present and 2085. Furthermore, these maps show that many urban areas are in different winter hardiness zones compared to the rural surroundings, which has additional consequences for the tree species that are able to flourish in urban environments.