Plant Species of San Jacinto Mountain:
Symphyotrichum ascendens, long-leaved aster; and
Symphyotrichum defoliatum, San Bernardino aster
Fig. 1. Left: Symphyotrichum ascendens, long-leaved aster, from Johnson Meadow in May Valley on 3 October 2019.
Right: Symphyotrichum defoliatum, San Bernardino aster, from the Campground at Idyllwild Park on 21 September 2020.
Note the plants are in different bloom stages, and photographed under different lighting conditions, so the difference in the appearance of the flowers is not significant. See Fig. 2 for additional photographs of the variation in the appearance of the flowers.
The important thing to note in these pix is the length of the branches in the inflorescence. S. ascendens branches are generally (not always!) longer than those of S. defoliatum, producing a more open inflorescence.
Pictures by Tom Chester. Click on the pictures for larger versions.
Fig. 2. Some of the variation of the flowers of these two species in the San Jacinto Mountains. Left: Symphyotrichum ascendens, long-leaved aster. Right: Symphyotrichum defoliatum, San Bernardino aster. See also Fig. 1 and Ron Vanderhoff's wonderful pix of S. defoliatum flowers at three stages of bloom (note how open the disk flowers are to see the stage of the bloom for each head).
Pictures by Bruce Watts. Click on the pictures for larger versions.
Symphyotrichum is one of the new genera in which many of our former Aster species are now placed. We have two Symphyotrichum species in the San Jacinto Mountains. S. defoliatum, San Bernardino aster, is endemic to southern California, being found only at SnGb, SnBr, and PR. It is a CNPS List 1B.2 plant, "fairly endangered in California". It was formerly called Aster bernardinus. S. ascendens, long-leaved aster, is widespread throughout the higher elevations of California, and ranges to western Canada, Colorado and northern Arizona. It was formerly called Aster ascendens.
Asters are notorious for being difficult to determine to species. There are a number of species that are morphologically very close, and difficult to reliably separate, in part because many individual species have a wide range of intrinsic variation, and, in some cases, they originated as ancient hybrids between species. See The Flora of North America Treatment of Symphyotrichum for three paragraphs discussing concisely why "Taxonomy of Symphyotrichum is difficult."
Symphyotrichum ascendens and S. defoliatum are very similar species that are often hard to tell apart. There is an excellent reason for this; they both started with the same chromosomes, just in different proportions. S. ascendens is an ancient hybrid between two species, and S. defoliatum is an ancient hybrid from a backcross of S. ascendens with one of its parents1! I.e., both species started out with the same genes from the two parents, but S. defoliatum has two sets of genes from one parent and one set of genes from the other, and S. ascendens has a single copy of genes from each parent.
Since their ancient origin, the two species have gone their separate ways, with separate evolution of their genomes, in part because they cannot easily hybridize due to their different chromosome numbers. Fortunately, their subsequent evolution, which may have included significant alteration of their genes in the first several generations until a stable genome is produced, has resulted in at least some differences that distinguish them.
As far as I know, the only good distinguishing features of these two species are the ones used in the keys. The most useful of those discrimnants for our plants are given in Table 1 and discussed below.
One characteristic used in the keys, the stem height, is not a reliable discriminant in our area. Another characteristic used in the Flora of North America key, the cauline leaf shape and size, has significant overlap, and I haven't yet checked to see whether it is useful in our area.
Table 1. Distinguishing characteristics of S. ascendens and S. defoliatum
Characteristic S. ascendens S. defoliatum Plant habit long-rhizomatous cespitose Presence of leafy shoots in upper axils gen lacking gen present Stem and Leaf Hairs strigose densely short pubescent Inflorescence branches gen > 5 cm gen < 5 cm
Plant habit. S. ascendens forms colonies of stems, since it spreads by relatively long rhizomes, producing regularly-spaced stems. S. defoliatum does not produce colonies of stems. Instead, there are multiple stems very close together coming from the base of the plant, with individual plants not regularly-spaced.
Presence of leafy shoots in upper axils. S. ascendens generally does not have any shoots in its leaf axils below the inflorescence; S. defoliatum generally does; some of the leafy shoots are circled in red in this photograph of S. defoliatum.
Stem and leaf hairs. The stem and leaf hairs of S. ascendens are strigose (short stiff appressed hairs). The stem and leaf hairs of S. defoliatum are densely short pubescent. See Fig. 3.
Fig. 3. Left: S. ascendens, from Johnson Meadow in May Valley on 3 October 2019. Note that the stem and leaf hairs hug the stem and leaf, with no spreading hairs on the stem, and no hairs past the edge of the leaf.
Right: S. defoliatum, from the Campground at Idyllwild Park on 21 September 2020. Note the spreading hairs seen both on the stem and the leaf edges.
Click on the pictures for larger versions.
Inflorescence branches. S. ascendens has a somewhat more open inflorescence, with axillary branches that are generally longer than 5 cm. S. defoliatum has a narrower, more compact inflorescence, with axillary branches that are generally shorter than 5 cm. See Fig. 1.
See also these pictures:
- S. ascendens from Plumas County, California, way outside the range of S. defoliatum
- S. ascendens from Yosemite National Park, way outside the range of S. defoliatum
- S. defoliatum from Santa Ana Mountains, Orange County
- S. defoliatum from San Diego County which also shows axillary leaf clusters (S. ascendens does not occur in San Diego County)
1S. ascendens (2n = 26,52; x = 8 + 5 = 13) was derived from an ancient cross between S. spathulatum (x = 8) and S. falcatum (x = 5). S. defoliatum (2n = 36; x = 8 + 5 + 5 = 18) was derived from a backcross of S. ascendens (x = 13) with S. falcatum (x = 5).
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Updated 30 September 2020.