Notes on the Viola nuttallii Complex and its Segregate Taxa in Montana, the Rockies and Intermountain West
This post can hopefully be used by folks trying to identify or understand the distinction between the species formerly included in an expanded concept of Viola nuttallii. As used on iNaturalist, the name Viola nuttallii is used in the strict sense and applies to a species that is primarily found on the Great Plains. However, the name has continued to be applied more generally to plants across the western U.S. instead of only to the species from the Plains. I am posting this as part of my attempt to "clean-up" the data on iNaturalist for the 3 taxa below.
The Viola nuttallii Complex in MT and the Interior West can be segregated into:
Viola nuttallii s.s.: A species of the Great Plains
Viola praemorsa: A species of the Mountains and Intermountain Valleys
Viola vallicola: A species of the Plains and Mountains
Generally, the 3 taxa appear to be relatively distinct morphologically with some geographical and ecological distinction. Geographically, V. nuttallii s.s. is a Great Plains species that does not occur (as far I can tell) west of the Continental Divide in MT nor in ID or UT or further west. V. vallicola occurs extensively in the Rockies and Intermountain West and occasionally onto the plains. V. praemorsa is a species of the mountains from moderate to high elevations. V. praemorsa and V. vallicola overlap extensively in the Rockies and Intermountain West. Generally, V. vallicola is in drier and lower elevation settings and V. praemorsa in more mesic or higher elevation settings though they overlap in distribution and somewhat in habitat as well. I find most specimens with good photographs of the mature leaves can be readily identified, though some observations are still difficult to place.
Morphologically V. nuttallii and V. vallicola may be confused where they co-occur, and Viola praemorsa and Viola vallicola may be confused as well, while V. praemorsa is generally distinct from V. nuttalli with little to no morphological overlap. Particularly, I find some specimens difficult to place confidently with either V. praemorsa or V. vallicola. Whether this is due to morphological overlap between the two taxa, hybridization (probably not based on my understanding) or my evolving understanding of the morphological boundaries of the two I am not sure. I have noted that individuals growing in open sunlight vs those in partial shade may account for some of the of the observed variability in leaf characteristics with both V. praemorsa and V. vallicola. But that is only a thought at this point...
To help with distinguishing these species, below is an excerpt from a key to Viola's from an upcoming Identification Manual on the Vascular Plants of Montana that I have been working on:
8. Some leaves long-lanceolate, blades >3x as long as wide with bases cuneate and long tapering, forming narrow wings along the petiole , minutely puberulent or glabrous; grasslands and open gravelly or sandy gen well-drained sites; plains…..V. nuttallii Pursh Tetraploid. Primarily or perhaps exclusively east of the Continental Divide, most common in the eastern 1/3 of the state and becoming less common further west.
8. Leaves mostly elliptic or ovate with bases truncate, rounded and/or short tapering to petiole, blades <3x as long as wide, puberulent, sparsely hairy or glabrous
9. Leaf bases mostly rounded to short-tapering (some may be truncate) to petiole, blades elliptical to ovate, margins often crenate to serrate at least above, glabrous to sparsely pubescent, gen appearing green when fully developed, vein pattern mostly somewhat reticulate with midvein not esp distinct to whitish and distinct; open, often gravelly or rocky slopes, grasslands, steppe; montane to alpine…..V. praemorsa Doug. ex Lindl. [V. nuttallii var. praemorsa] Hexaploid. This species and the next are sometimes treated as varieties of V. nuttallii (Hitchcock and Cronquist 2018, Lesica 2012, 2022). However, they are gen readily separable based on the vegetative traits in the key as well as differences in habitat, geography and chromosome number though some overlap in morphology and geography exists. Two varieties (var. flavovirens and var. linguaefolia) are sometimes recognized as occurring in MT based on differences in morphology, cytology, and leaf flavonoid chemistry (Little and McKinney in FNA). However, they overlap greatly in morphology and geography and as a result are not recognized here.
9. Leaf bases mostly truncate or subcordate, margins entire to subentire and gen ciliate, blades narrowly ovate to ovate, puberulent to glabrous, often appearing gray-green, vein pattern gen not reticulate, midvein often whitish and prominent on upper surface, secondary veins not as prominent; various open sites from dry to moist including grasslands, steppe; plains, valleys to subalpine…..V. vallicola A. Nels. [V. nuttallii var. vallicola] Diploid. Some smaller or later, cauline leaves may have cuneate bases that taper to the petiole leading to confusion with V. nuttallii.
Other References:
Flora of North America
Fabijan etal. 1987. The taxonomy of the Viola nuttallii complex. Can. J. Bot. 65 : 2562-2580.
Ballard etal. 2023. A taxonomic treatment of the violets (Violaceae) of the northeastern United States and adjacent Canada: Provides a good synopsis of the Viola nuttallii Complex. Available here: https://bioone.org/journals/the-journal-of-the-torrey-botanical-society/volume-150/issue-1/TORREY-D-22-00029.1/A-taxonomic-treatment-of-the-violets-Violaceae-of-the-northeastern/10.3159/TORREY-D-22-00029.1.full
