White-barked birches hybridize freely, suggesting that current species are too narrowly defined. Even the well-known common native North American paper-bark or canoe birch (Betula papyrifera) causes consternation. Birch expert Hugh McAllister of University of Liverpool and Ness Botanic Garden will explain a bit about the white-barked birches’ propensity to cross-pollinate and thus confound taxonomists, on Tuesday, October 2, from 6:30 – 8 at the Hunnewell Building of the Arnold Arboretum. He will discuss birch distribution around the world while showing images of his research travels and herbarium specimens and posing some as-of-yet unanswered questions that keep this botanist awake at night. Free, but registration requested at www.arboretum.harvard.edu.