Herbarium specimens reveal a cryptic invasion of tetraploid Centaurea stoebe in Europe

Abstract Numerous plant species are expanding their native ranges due to anthropogenic environmental change. Because cytotypes of polyploid complexes show often similar morphologies, there may be unnoticed range expansions (i.e., cryptic invasions) of one cytotype into regions where only the other cytotype is native. We critically revised 13,078 herbarium specimens of diploid and tetraploid Centaurea stoebe, collected across Europe between 1790 and 2023. Based on their distribution in relictual habitats, we suggest that diploids are native across their entire European range, whereas tetraploids are native only to South-Eastern Europe and have recently expanded their range toward Central Europe. The proportion of tetraploids exponentially increased over time in their expanded but not in their native range. This cryptic invasion took predominantly place in ruderal habitats and enlarged the climatic niche of tetraploids toward a more oceanic climate. Our differentiation between native and expanded ranges conflicts with dozens of previous studies on C. stoebe. Thus, herbarium specimens can prevent erroneous assumptions on the native ranges of species, which has fundamental implications for designing research studies and assessing biodiversity trends. Moreover, we demonstrate the value of spatio-temporally explicit data in formulating and testing hypotheses regarding the superior colonization abilities of polyploids in ruderal habitats.