Climate change is considered as one of the most important threats for biodiversity in the next century. It has been showed that plant communities are already responding to the increase in temperature by changes in their composition. These changes result mainly from local extinction of cold-demanding species and/or colonisation of warm-demanding species. The respective rates of these two underlying processes remain poorly known even though they control the long-term changes in biodiversity. Using 13,090 forest plots surveyed between 1914 and 2014 along a temperature gradient of 2.9-14.7°C covering Mediterranean, temperate lowland and mountain biomes, we investigated the importance of species’ extinctions and colonisations in the trend of communities’ thermophilisation and resulting changes in species diversity.
Communities changes were studied in France by comparing two periods, 1914-1987 and 1997-2014, characterised by a difference of 1.18°C in mean annual temperature. From a set of 123,314 floristic inventories, we generated the 6,545 studied pairs of close historical and recent plots, characterized by a mean distance of 1,598m and a mean difference in elevation of 5m between the 2 plots of a pair. Pairs of plots were also selected in order to avoid any bias in forest canopy cover between studied periods.
Results/Conclusions
We observed a gradient of communities’ thermophilisation from 0 % of the expected communities’ response to climate warming in lowlands to a thermophilisation of 71 % of the communities expected response for the coldest temperatures in mountains. Except for one, this pattern was the same for all the six studied mountain ranges. There was a uniform decrease of cold-demanding species along the whole temperature gradient and an increase of warm demanding species only in highlands. This increase alone explains the gradient of thermophilisation with elevation. The differences in species’ extinctions and colonisations induced a gradient from a decrease of species richness in lowland to a richness increase in highlands.
We showed here a biotic attrition in lowlands where almost only extinctions occurred whereas high colonisation in highlands led to both an increase in assemblage diversity and an adaptation to warming climate. This clearly showed that if high-elevation species and communities are considered at high risk of extinction due to the lack of available habitat to migrate, low-elevation communities are at crucial risk of biodiversity loss in the next decades. This highlight a paradox for high-elevation communities where biodiversity is increasing while the most cold-demanding species are disappearing.