Erigeron trifidus (three-lobed daisy, Asteraceae), a rare western Canadian endemic plant

Figure 1. Plate of Erigeron trifidus from the original description of the species publication in 1834 (Fl. Bor.-Amer. 2:17, plate 120). On the lower right side of the plate is a blowup of a leaf divided into three only once.

Figure 2. Erigeron trifidus (Douglas & Penny #13191). Note the stout leaves with three shallow lobes.

Erigeron trifidus Hook. (three-lobed daisy, Asteraceae) is a rare red-listed and narrowly distributed plant known only from high alpine areas along the spine of the Canadian Rocky Mountains in  British Columbia and Alberta. After reviewing the herbarium specimens identified as E. trifidus housed at the UBC herbarium it became apparent that even expert botanists are confusing this rare species with another high alpine, but widely distributed species called Erigeron compositus Pursh (cut-leaved daisy). However, these two species can easily be told apart based on their leaves alone. Erigeron trifidus is aptly named because the species epithet “trifidus” is a Latin word meaning “three lobed”, and the tips of the leaves of this species are indeed divided into three lobed only (Figs. 1 & 2). The leaves of E. compositus at first glance may seem similar but upon closer inspection the tips of the leaves have more than three lobes, and there are often multiples sets of three lobes. The key in Flora of North America (volume 20. p. 265) uses the term 2-3 times ternateley lobed, which is just a fancy way of saying that the leaves are dissected into two to three set of three lobes (i.e. they have six to nine lobes; Fig. 3).

Figure 3. Detail of leaves of Erigeron compositus, collected by C. Sears on Bullmoose Mt. B.C. in July of 2012. Leaves of this individual are 2 times ternately lobed (2 times divided in 3′s)

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