Cenozoic losses of tree diversity constrain climatic adaptation of Western and Central European forests.

Paleoclimatic changes during the late Cenozoic led to substantial losses of tree diversity in Western and Central Europe. A modern synthesis linking which taxa were lost, the timing of these losses, and their implications for the adaptation of the region's forest ecosystems to ongoing climate change is needed. Here, we compile a dataset of fossil occurrences of tree genera from Western and Central Europe to analyze tree diversity changes during the Cenozoic (66 Ma - present). We find that tree genus richness declined 75% after its peak in the Miocene (23 - 5.3 Ma), with most losses already occurring during the Plio-Pleistocene transition (ca. 2 - 3 Ma). We then use genus-level niche models to explore how these changes unfolded within climate space. Our results show how the current regional tree flora was progressively filtered towards the colder and drier margins of climate space. This coincided with the losses of most heat-tolerant lineages, including many temperate genera that could have profited under modern climate warming (e.g., Castanea, Juglans, Aesculus). We foresee several ways in which our analyses of paleoecological data can inform forest management to increase regional tree diversity and enhance forest resilience under climate change.