Flora of North America species comparison

Previous authors commonly recognized var. cernuum and var. macranthum, based primarily upon size differences. Plants attributed to var. cernuum are found from Delaware and eastern Pennsylvania northward to Newfoundland, while those attributed to var. macranthum are found mainly farther inland into the Midwest. Although there is a tendency for the eastern seaboard plants to be somewhat smaller and more delicate, and the midwestern and far northern plants to be more robust, there is much variation, largely dependent on soil nutrients. There are regional size trends, but based on my observations of this species in Newfoundland, Michigan, Wisconsin, and Minnesota, I do not believe that the two varieties can be maintained.

In the Gray Herbarium, there is a collection by Richardson labeled “Mackenzie River,” which has been cited by H. M. Raup (1947) and others. W. J. Hooker ([1829–]1833–1840) reported Trillium cernuum “from Saskatchewan to Mackenzie River.” Raup stated that “it is the only evidence for the occurrence of...Trillium in the entire Mackenzie Basin.” Other writers have simply quoted that statement. In view of the relatively great disjunction from the known Saskatchewan stations and in the absence of any other supporting specimens from that area, I believe that there might be locality error on the Richardson sheet. However, it is not beyond possibility that T. cernuum could occur there. This apparent disjunct station is not mapped here.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Trillium parviflorum varies from very short, slender, small-bracted plants to tall, broad, umbrella-bracted giants. Regardless of plant or bract sizes, flower and petal sizes are remarkably constant, all plants having small, linear-lanceolate petals. This is not usually the case with T. albidum, the species with which this plant is most likely to be confused. In T. albidum also the plants can be enormous, but when they are, the petals are very long, broad, and conspicuously obovate-diamond-shaped. In large clonal clumps of T. albidum, the larger and more mature offsets show the typical petal shape, while the smaller and presumably youngest offsets sometimes produce smaller, narrower petals, more like those of T. parviflorum.

Some western botanists, more experienced with local populations than I, do not consider Trillium parviflorum to be distinct from T. albidum. They point out that since there is an extensive region of apparent intergradation (as indeed there is, well supported by herbarium vouchers), there exists a morphological cline from the long- and wide-petaled T. albidum to the narrower- and generally shorter-petaled T. parviflorum, and that T. parviflorum, therefore, should not be considered a separate species but rather a subspecies or a variety. Since no one to date has treated T. parviflorum at the subspecific or varietal level, and since in my own limited experience it does appear as a distinct species in Washington north of the Columbia River, I include it here as treated by Soukup. I have seen populations of considerable variation north of Corvallis, Oregon, and agree that there is much overlap with T. albidum. Obviously there is need for a much more extensive study of this situation.

A factor that exacerbates this problem (and many others in Trillium), is that nutrition, age, and even favorable position in the habitat can greatly influence plant and floral organ sizes. In many species, including T. albidum, when a single vigorous clonal clump produces many offsets, the oldest offsets may have flowers with very large petals, sepals, ovary, etc., while the younger offsets may have organs only half the size. In most sessile trilliums particularly, population averages are often more useful than isolated individual measurements, a difficult situation, indeed.

(Discussion copyrighted by Flora of North America; reprinted with permission.)