Co-occurring Luzula species (Juncaceae) of different ploidies exhibit weak ecological differentiation at local scale in alpine grasslands of the Eastern Alps

Luzula sect. Luzula is a taxonomically challenging group of angiosperms, whose evolutionary history has been shaped by polyploidy and agmatoploidy (fragmentation of holocentric chromosomes). Several species with different chromosome sizes and numbers, ranging from diploids to hexaploids, occur above the timberline in the Eastern Alps. Species of different ploidies frequently co-occur in the same habitats, but the extent of ecological divergence and niche partitioning among them remains elusive, partly due to their high morphological similarity impeding reliable identification. Here, we focused on three mixed-ploidy sites in the Eastern Alps, where the morphologically similar alpine species L. exspectata (diploid), L. alpina (tetraploid) and L. multiflora (its hexaploid populations) co-occur in close vicinity. We inferred their ploidy via flow cytometry and characterised their small-scale ecological preferences using different ecological indicators like mean Landolt indicator values of accompanying plant species. While diploid L. exspectata is associated with slightly more basic microsite conditions, as it mostly occurs over limestone, no such differentiation was observed between tetraploid L. alpina and hexaploid L. multiflora. We provide evidence for broader niches of tetraploids compared to diploids, whereas tetra- and hexaploids exhibited no differences in niche width. Our results indicate that small-scale co-occurrence of different cytotypes within Luzula sect. Luzula in alpine habitats is accompanied by only a slight niche partitioning, whereas there were significant differences in ecological parameters among the sites. These findings emphasise the influence of geography and geology on ecological microsite conditions and suggest that in this species group, local niche divergence between ploidies is negligible compared to site-specific effects. Different ploidies thus likely have more divergent ecology at a distribution-wide scale than at a local scale.