Korean Mountain Ash (Sorbus alnifolia, Rosaceae) on Great Blue Hill in Canton, Massachusetts

In 2011 while walking to the top of Great Blue Hill in Canton, Massachusetts, the first author found a small tree approximately 2–3 m tall. The leaves of the tree were alternate with doubly serrate margins, somewhat reminiscent of beech or birch but not matching either genus. The bark of the tree displayed prominent lenticels. The tree could not be identified using any of the floras of the region. While conducting research in Harvard’s Arnold Arboretum, the first author noticed that the tree on Great Blue Hill morphologically matched planted specimens of Sorbus alnifolia (Siebold & Zucc.) K. Koch and also fit the description of the species in Dirr (2009). Eight years later, on November 1, 2019, the Great Blue Hill site was revisited, the tree was found still extant and healthy, a specimen was collected, and a twig was brought to the second author, who confirmed the identification. The S. alnifolia occurs on the path from the Blue Hills Trailside Museum to the summit of Great Blue Hill and the Eliot Tower at approximately 4281301.82 00 N, 7180657.85 00 W. The location of this plant seems entirely natural and the S. alnifolia did not appear planted. The Korean mountain ash co-occurs with Pinus strobus L., Vaccinium angustifolium Ait., Acer rubrum L., Comptonia peregrina (L.) Coult., and Betula lenta L. This collection of Sorbus alnifolia is the first record of the tree in Norfolk Co., Massachusetts, and the only occurrence in North America known outside of Boston, Suffolk Co., Massachusetts, in the wild (Cullina et al. 2011). USDA, NRCS (2019) and other continent-wide references do not report the species as naturalized.