Temporal trends of climate-driven lags of mountain plant distributions

Species are shifting their geographical ranges in response to climate change by going locally extinct at sites that become unsuitable and by colonizing newly suitable sites. However, the velocity of these local extinctions and colonizations seem to lag behind recent environmental change for the majority of mountain plant species, entailing so-called extinction debts and colonization credits that will have to be paid off in the future. This implies that further species re-distributions are to be expected based on environmental changes that occurred in the past, even if climate change was halted now. Due to the very limited availability of historical long-term datasets, such range shifts and range lags are so far only reported as temporal snapshots that compare current species distributions to one particular time in the past. The temporal development of species’ local extinctions and colonizations as well as of their extinction debts and colonization credits thus remain elusive. Yet, climate change has accelerated over the recent decades and it seems more and more likely that this trend will continue in the future. The relaxation times of extinction debts and colonization credits in response to climate change (i.e. the time required for the distribution of a species to reach equilibrium with occurred environmental conditions) are thus becoming increasingly important to reliably predict the future trajectory of mountain biodiversity. To address this, we calculated species distribution models with an annual resolution over the last six decades based on >40’000 vegetation plots from all European mountain ranges recorded by a multitude of scientists. We found that even though the number of local extinctions and colonizations increased generally over time, mountain plant species increasingly accumulated extinction debts and colonization credits. Their distributions are thus more and more out of sync with the environment and are increasingly veiling past effects of climate change on mountain biodiversity yet to come, reinforcing warnings about potential climate-driven ecological disruptions in the future.