Microclimate of large solitary trees along rural-to-urban gradients across Europe

Large solitary trees are keystone features for biodiversity in many urban and rural landscapes around the world. Yet, because of their isolation, they do not benefit from the buffering effect of neighbouring trees as in forests. As they are more exposed, solitary trees are more vulnerable to the impacts of climate change, such as extreme droughts, heat waves, and wind gusts. Research on microclimates below solitary trees is scarce and a more detailed understanding is needed to better understand and predict the future impacts of climate change on their associated biodiversity and ecosystem services. Here we quantified air temperatures and vapour pressure deficits below the crown of >200 trees along rural-to-urban gradients for three tree species (oak, ash, and lime) across nine European cities. We recorded microclimate measurements every 30 min for 10 months and analysed the effects of the surrounding built-up area and how different tree species influence microclimatic conditions. The microclimate below trees in more urban areas was overall warmer and drier than below rural trees, whereby 10 % more built-up area caused average summer air temperatures to increase by 0.1 °C and average vapour pressure deficits by 0.02 kPa. Oak and lime were able to dampen the temporal fluctuations of air temperature and vapour pressure deficit more than ash and were able to mitigate maximum summer temperatures 0.55 °C more than ash. Our research thus underpins that solitary trees shape their own species-specific microclimate. We advocate for integrated tree planning to preserve and provide space for solitary trees, and by adopting solitary trees as key components of urban and rural green infrastructures, we can improve microclimatic conditions and enhance biodiversity, ultimately creating more sustainable and liveable landscapes.