Connectivity in riparian plants: influence of vegetation type and habitat fragmentation overrides water flow

Spatial genetic patterns can be influenced by a broad range of factors across a landscape. The hypothesis that heterogeneous vegetation and habitat fragmentation rather than water flow influence genetic patterns in two riparian plant species with different niches was tested. Genotyping by sequencing was used to assess the genetic diversity and structure of each species at 12 sites across a river catchment. Generalized dissimilarity modelling examined the relative influence that vegetation type and habitat fragmentation had on patterns of genetic differentiation across the landscape. Restricted gene flow in the widespread species, Callistachys lanceolata, resulted in lower genetic differentiation than that exhibited by Astartea leptophylla, a restricted riparian species with high gene flow. Geographic distance and vegetation type explained the patterns of genetic differentiation in the widespread species, whereas habitat fragmentation and, to a lesser extent, vegetation type explained patterns in the restricted species. Water flow was not found to have significant impacts on patterns of genetic diversity in riparian plant species with restricted and widespread distribution. Impacts of vegetation type on genetic differentiation were most likely due to change in canopy density and associated pollinator communities in the vegetation types across the catchment. Reduced connectivity caused by habitat fragmentation was evident in the restricted riparian species, while reduced connectivity in the widespread species was related to the change of vegetation type between sites. Natural causes of reduced connectivity as well as anthropogenic causes need to be considered in future work to predict persistence and resilience under a changing climate.