The ongoing climate change is triggering plant community thermophilization. This selection process ought to shift community composition towards species adapted to warmer climates but may also lead to biotic homogenization. The link between thermophilization and homogenization and the community dynamics that drive them (colonization and extinction) remain unknown but is critical for understanding community responses under rapid environmental change. We used 14,167 pairs of plots to study shifts in plant community during 10 years of rising temperature in 80 forest ecoregions of France. We computed community mean thermal optimum (thermophilization) and Δβ-diversity (homogenization) for each ecoregion and partitioned these changes into extinction and colonization dynamics of cold- and warm-adapted species. Forest understorey communities thermophilized on average by 0.12 °C per decade and up to 0.20 °C per decade in warm ecoregions. This rate was entirely driven by extinction dynamics. Extinction of cold-adapted species was a driver of homogenization but it was compensated for by the colonization of rare species and the extinction of common species, resulting in the absence of an apparent homogenization trend. Here we show a dieback of present cold-adapted species rather than an adaptation of communities via the arrival of warm-adapted species, with a mutually cancelling effect on β-diversity. These results suggest that a future loss of biodiversity and delayed biotic homogenization should be considered.

持续的气候变化正在引发植物群落的嗜热化。这种选择过程应该使群落组成转向适应温暖气候的物种，但也可能导致生物同质化。嗜热化和均质化以及驱动它们的群落动态（殖民和灭绝）之间的联系仍然未知，但对于理解快速环境变化下的群落反应至关重要。我们使用 14,167 对样地来研究法国 80 个森林生态区在 10 年气温上升期间植物群落的变化。我们计算了每个生态区的群落平均热最适值（嗜热）和 Δ β多样性（均质化），并将这些变化划分为冷适应和热适应物种的灭绝和殖民动态。森林林下群落平均每十年嗜热0.12°C，在温暖的生态区每十年最高达0.20°C。这个速度完全是由灭绝动态驱动的。适应寒冷的物种的灭绝是同质化的驱动因素，但它被稀有物种的殖民和常见物种的灭绝所补偿，导致不存在明显的同质化趋势。在这里，我们展示了目前适应寒冷的物种的枯死，而不是通过适应温暖的物种的到来而进行的群落适应，这对β多样性具有相互抵消的影响。这些结果表明，应考虑未来生物多样性的丧失和生物同质化的延迟。