Fifty shades of grey: black beech and mountain beech are genetically distinct but locally admixed

Nothofagaceae (southern beech) dominate two-thirds of the remaining indigenous forest of New Zealand. Four of the five species indigenous to New Zealand belong to the genus Fuscospora, and the remaining species to Lophozonia. Two species, Fuscospora cliffortioides (mountain beech) and Fuscospora solandri (black beech), are characterised by their small, entire leaves, characters not found in other species of the genus. Their taxonomic status and rank have been a persistent problem. While individuals of each species are often well differentiated by leaf shape and growth habit, intermediate plants and populations are common. They have been described as constituting a cline with morphological variation correlated with both elevation and latitude, and have been treated by many botanists as varieties of a single species. We assessed relatedness among populations of the two taxa by NeighborNet analysis and model-based inference of population structure using simple sequence repeat data. Two gene pools corresponding to the two taxa can be recognised, and these co-exist at a regional scale. The strongest correlations between environmental variables and membership of modelled populations corresponding broadly with mountain and black beech were with mean annual temperature, elevation, and longitude. Although past (and presumably ongoing) gene flow between them has occurred in areas of contact, the hybrid zone appears to be narrow, at least in some areas. It is appropriate to recognise these recently diverged taxa at species rank, despite local gene flow.