The importance of species addition versus replacement varies over succession in plant communities after glacial retreat

Abstract Mechanisms underlying plant succession remain highly debated. A global quantification of the relative importance of species addition versus replacement is lacking due to the local scope of most studies. We quantified their role in the variation of plant communities colonizing the forelands of 46 retreating glaciers distributed worldwide, using both environmental DNA and traditional surveys. Both mechanisms concur in determining community changes over time but their relative importance varied over time along successions. Taxa addition predominated immediately after glacier retreat, as expected in harsh environments, while replacement became more important for late-successional communities. Those changes were aligned with total beta-diversity changes, which were larger between early successional communities than between late-successional communities (>50 years since glacier retreat). Despite the complexity of community assembly over plant succession, our global pattern suggests a generalized shift from the dominance of facilitation and/or stochastic processes in early successional communities to a predominance of competition later on.