Joint effects of climate change and urbanization accelerate plant biodiversity loss across urban green spaces

Urbanization and climate change are rapidly reshaping biodiversity worldwide, yet their combined impacts on ecosystems remain poorly understood. Urban green spaces (UGS) underpin urban biodiversity, but differ widely in their structure, management, and exposure to global change. Here, we examine how these two drivers jointly influence plant communities across five major UGS types (allotments, private gardens, residential green spaces, parks, and ruderal areas) in three socio-culturally distinct Swiss cities. By combining vegetation surveys from 180 sites (60 sites per city) with species-specific thermal niches, high-resolution projections of historical and future urban microclimates, and land-use change scenarios, we trace how climate-driven species loss interacts with the shifting availability of urban habitats to shape biodiversity outcomes by mid-century. We found that UGS types differed markedly in their contributions to biodiversity, with allotments and ruderal areas supporting disproportionately high compositional uniqueness. However, climate change alone was projected to drive substantial plant diversity declines across all UGS types, with 40-80% of species exceeding their thermal niches by mid-century based on climate change scenario and UGS type. Although allotments exhibited relatively greater tolerance due to broader community-level thermal preferences, this advantage was undermined by their rapid decline under urbanization. When considered jointly, climate change and urbanization generated widespread biodiversity losses through complementary mechanisms: climate change drove species declines, while urbanization amplified these losses by disproportionately reducing high-value habitats. These results reveal a consistent trajectory of biodiversity decline across urban landscapes and highlight the need to conserve high-diversity yet shrinking UGS, particularly allotments, while enhancing the ecological value and climate resilience of expanding UGS, such as parks and residential areas, to safeguard future urban biodiversity under global change.