The geography of follicle pubescence in the Physocarpus opulifolius (Rosaceae) complex1,2

Abstract

Taxonomic circumscription is a critical prerequisite for ecological, taxonomic, and systematics research but can be complicated by undetected patterns in morphological variation. Cryptic variation has long confounded the taxonomy of Physocarpus (Cambess.) Raf., with conflicting circumscriptions especially prevalent in the Physocarpus opulifolius (L.) Maxim. complex, which comprises P. opulifolius sensu stricto and Physocarpus intermedius (Rydb.) C.K.Schneid. Here, we assess variation for carpel and follicle pubescence, a key diagnostic character for the P. opulifolius s.l. complex. Data were collected from 748 herbarium accessions obtained from across the range of this complex in North America. We then assessed the statistical and geographic distribution of this variation. As suggested by Rydberg and others, follicle pubescence is bimodally distributed and exhibits a strong geographic signal. An eastern, glabrous (or nearly so) form occurs in temperate forests from the St. Lawrence River valley of Quebec west to the Upper Great Lakes region and south to the southern Appalachians (i.e., P. opulifolius s.s.), while a more western, densely pubescent form occurs from the Interior Highlands north to the Driftless Area, with disjunct populations in the Southeastern Plains of Alabama, Sand Hills of Nebraska, Black Hills of South Dakota, Front Range of Colorado, and Sierra Madre Oriental of northeastern Mexico (i.e., P. intermedius). We conclude that taxonomic recognition is warranted based on variation in follicle pubescence, which also has implications for downstream research (e.g., floristics, phylogenetics, and ecological niche modeling).