Cool topoclimates promote cold-adapted plant diversity in temperate mountain forests

Climate strongly influences the composition and diversity of forest plant communities. Recent studies have highlighted the role of tree canopies in shaping understory thermal conditions at small spatial scales (i.e. microclimate), especially in lowland forests. In mountain forests, however, the influence of topography in environmental conditions (i.e., topoclimate) is ought to also influence plants' perceived temperature. Understanding how topography and canopies interactively affect understory temperature is key to identifying stable refugia that could shelter cold-adapted forest specialist plants under climate change. Here we report on growing season understory temperatures using 48 loggers in contrasting topographic features and canopy of a mid-range mountain valley spanning from 475 m a.s.l. to 1203 m a.s.l. in the Vosges Mountains (NE France). We disentangle the relative importance and the effects of topography vs. canopy in determining local temperatures. We then evaluate how topography and canopy-induced variation in temperature drive plant community composition and richness in 306 floristic surveys across the studied mountain valley. Our results show that topography outweighed canopy cover in explaining growing season understory temperatures. Regardless of canopy, the daily mean temperature of the growing season in south-facing ridges was 1.5 degrees C (CI: 0.62 - 2.38 degrees C) warmer than shaded valley bottoms, while dense canopies cooled temperatures by 0.5 degrees C (CI: 0.02 - 0.98 degrees C) compared to less dense canopies. Topoclimate explained community composition as much as elevation and was the only significant predictor of species richness. Cold topoclimates harbored 30% more species than the average species richness across our plots. This increase in species richness was explained by an increase of cold-adapted species, both forest specialist and generalist species. Our findings highlight a stronger role of topography compared to canopy cover on community composition in mountain forests via topoclimatic cooling of north-facing slopes and valley bottoms. The importance of topographic features to explain temperature cooling and diversity underpins their role as present and likely future microrefugia.