Distribution and phenology of Gonolobus suberosus (Apocynaceae, Asclepiadoideae) and its varieties in North America

Recent morphological and molecular studies resulted in the recognition of two varieties of Gonolobus suberosus in the United States. The objectives of the present study were to (1) provide a list of exsiccatae and (2) present phenological data, as captured from herbarium specimens. Based on queries to sixty-seven herbaria, 578 herbarium specimens of G. suberosus from North America were examined, annotated, and catalogued. A critical re-evaluation of four apparently disjunct populations of var. granulatus in the East led to the hypothesis that eastern plants exhibiting uniformly colored corolla lobes represent mutants of var. suberosus, rather than true disjunctions of var. granulatus. This hypothesis is supported by similarities in the coloration and shape of laminar dorsal anther appendages as well as vestiture of the adaxial corolla lobes shared by putative mutant and wildtype var. suberosus, but not var. granulatus. There appears to be no separation of reproductive phenological activity between the two varieties. Over the period 1895 to 2005, in contrast to previous reports for other taxa, a positive correlation between year of collection and number of specimens with flowers was found for both varieties. The significance of the correlation suggests that collecting effort for flowering material has actually been increasing for both varieties over the past century. No significant correlation was found between year of collection and specimens with fruits. Differences in rates of flowering and fruiting collections may be the result not of collector preference or environmental factors, but of temporal differences in collecting activity coinciding with shifts from flowering to fruiting. Images and a key to the varieties are provided to facilitate identification.