Oregon delphiniums -- easy to collect but hard to
identify, Part I
Early this year, I took on the job of writing a treatment of the genus Delphinium (larkspur) for the Oregon Vascular Plant Checklist. Not having studied the group before, I decided to acquaint myself with its diversity by examining all the herbarium collections of larkspurs at Oregon State University. Now, three months and over 1000 specimens later, I have a newfound respect for the taxonomic complexity of Delphinium! One major difficulty with the genus is deciding where to draw the boundaries between its species. Due to their generally similar floral structure, as well as the absence of genetic barriers to intercrossing, species of larkspur are known to hybridize in many different combinations. We would expect that if gene exchange continues long enough, the morphological gaps between particular taxa may become blurred, and novel combinations of traits may arise. As I suggest below, this appears to have happened more than once among our Oregon larkspurs, and perhaps it even gave rise to a new species of hybrid origin.
Delphinium species occupy a wide range of environments, from arid to moist, high elevations to near sea level, shady forests to open meadows, sagebrush scrub to grasslands. They are found almost throughout Oregon, are showy, and as I said in the title, are easy for botanists to collect. Herbarium specimens make excellent study material, retaining their floral colors for 100 years or more. Important taxonomic traits occur in the flowers, stem pubescence, leaf arrangement on the stem, leaf shape and degree of dissection, and in the underground parts—the roots and rootstocks. Other characters seem to vary with the environment, especially height, degree of branching, and size of inflorescence. In the discussion below, I will focus on the species that are found west of the Cascade Range, saving my comments on the east-side taxa for a second article.
The number of Delphinium taxa that I recognize from west of the Cascade crest is 11, which coincidentally is the same number for this area as one finds in M. E. Peck's Manual of the Higher Plants of Oregon. Peck's list of taxa is not identical to mine, however, and I assign a different geographical range than he did to certain species. For example, Delphinium glareosum (rockslide larkspur), which is best developed in the Olympic Mountains and the Cascades of Washington, comes south into Oregon only on the high peaks of the Coast Range of Clatsop and Tillamook Counties, rather than extending through the Oregon Cascades south to Lane County as previously supposed. Also, Delphinium decorum (coast larkspur), a species of the California Coast Ranges, enters Oregon only on the narrow coastal strip of Curry County and is not in the interior mountains as Peck thought. In my view, the common blue-flowered larkspurs of the Siskiyou Mountains in Oregon all represent D. nuttallianum (upland larkspur), which Peck believed was entirely east of the Cascades. The name D. sonnei, which was used by Peck for much of D. nuttallianum in southwestern Oregon, has dropped into synonymy both in my list and in the new Jepson Manual: Higher Plants of California.
There are three western Oregon larkspur species that are distinctive and show little confusing variability. These are the red-flowered Delphinium nudicaule (red larkspur) of the Siskiyou Mountains; the blue-flowered D. trolliifolium (poison larkspur), a tall, leafy-stemmed species with widely spreading sepals, found from the Columbia Gorge south to northwestern California; and the purple-flowered D. glaucum (mountain larkspur), also tall and leafy-stemmed but with forward-pointing sepals, common in the southern Oregon Cascades and ranging south to the Sierra Nevada and north to Alaska. Another blue-flowered western Oregon species that is reasonably distinctive is D. nuttallii (Nuttall's larkspur), found in the Columbia Gorge and in Clark and Skamania Counties, Washington. It has enlarged, tuber-like roots and resembles the much more common tuber-rooted species D. menziesii (Menzies' larkspur), but it has smaller flowers with forward pointing (rather than widely spreading) sepals, shorter floral pedicels (hence a narrower raceme inflorescence), and a distinctly hairy pair of drooping petals in the center of the flower. These two larkspur species are important to my theory of the origin of D. oreganum (Willamette Valley larkspur), described below.
On the state list of rare, threatened or endangered plants are three additional larkspur species of the Willamette Valley—Delphinium oreganum, D. leucophaeum (white rock larkspur), and D. pavonaceum (peacock larkspur). What surprising coincidence, one wonders, caused this concentration of rare species of a single genus, in such a limited geographical area! The range of D. oreganum is from Clackamas County to Linn County, almost entirely east of the Willamette River; D. leucophaeum is highly localized near the Willamette River just south of Portland; and D. pavonaceum is centered in Polk and Benton Counties, mainly west of the Willamette River. In terms of their morphological features, these three taxa can be characterized as follows: (1) D. oreganum is intermediate, in its floral and inflorescence traits, between D. nuttallii and D. menziesii; (2) D. leucophaeum almost exactly resembles D. nuttallii, but its sepals are white instead of blue; (3) D. pavonaceum is very similar to D. menziesii in all its features except flower color—it has white sepals and blue upper petals, instead of the blue sepals and white upper petals of D. menziesii! It seems evident that these five larkspurs are very closely related and have undergone some rapid evolutionary divergence, perhaps in the recent geological past.
My theory of the evolution
of these fascinating Delphinium taxa is based on the catastrophic
Ice Age flooding that occurred in the Willamette Valley, between
15,000 and 12,800 years ago. An excellent description of these
Pleistocene floods of the Columbia River, called the Bretz Floods
or Spokane Floods, can be found in the book Cataclysms on
the Columbia, by J. E. Allen and M. Burns (Timber Press,
Portland, OR, 1986). At least 40 times, at about 50-year intervals,
a huge glacier-dammed lake in Montana burst forth and inundated
over 16,000 square miles of Washington and Oregon landscape.
The Willamette Valley was flood-scoured at its north end and
filled by a temporary lake, up to an elevation of 400 feet, which
extended south to the present location of Eugene. The repeated
filling and then rapid draining of the lake must have created
large areas of habitat disturbance, fresh deposits of silt and
gravel, alteration of stream courses, and destruction of established
vegetation. New genetic forms of Delphinium, produced
by hybridization and/or mutation, became established in these
disturbed sites, I believe, and evolved to form the three endemic
species, whose ranges are even today exactly limited to the area
of this so-called Lake Allison in the Willamette Valley.
Valuable studies of populations of these rare Willamette Valley larkspurs have been carried out by Gaylee Goodrich (M.S. thesis, University of Oregon, 1983) and by Lynda Boyer (M.S. thesis, Portland State University, 1999). Dr. Keith Karoly of Reed College is doing further research on their genetic differences. These new studies may help to clarify my hypothesis of rapid evolution in habitats that were disturbed by Pleistocene flooding; for example, that D. oreganum arose through genetic stabilization of hybrid populations between D. nuttallii and D. menziesii. The two white-flowered species, D. leucophaeum and D. pavonaceum, seem to have evolved directly from their respective parental blue-flowered taxa, D. nuttallii and D. menziesii. Isolation through altered pollination behaviors by insects, due to the changed flower-color patterns, seems a likely hypothesis in both cases.
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| Illustration by Jeanne R. Janish from Hitchcock et al. 1969, Vascular Plants of the Pacific Northwest, courtesy of University of Washington Press. |