White Prairie Daisy, Symphyotrichum falcatum, on the Bright Angel Trail, Grand Canyon, Arizona

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These Plants Are Actually Eurybia pulchra, NOT Symphyotrichum falcatum
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I thank Glenn Rink for pointing out my misdetermination here. I'll update this page in May 2009 with the reasoning behind the new determination.

From Dave's Garden Botanary:

Meaning of scientific name: Symphyotrichum: from the Greek symphyos, (growing together) and thrix (hair), referring to the hair-like flowers. falcatum: sickle-shaped
Pronunciation: sim-fy-oh-TRY-kum fal-KAY-tum

If anyone knows what part of the plant is supposedly sickle-shaped, please let me know!

Synonyms (from many sources): white aster, white prairie aster, creeping white prairie aster, falcated aster, western heath aster, white prairie aster; Virgulus falcatus, Aster adsurgens (misapplied), Aster commutatus var. crassulus, Aster commutatus var. polycephalus, Aster cordineri, Aster elegantulus, Aster ericoides var. commutatus, Aster falcatus ssp. crassulus, Aster falcatus var. crassulus, Aster multiflorus var. commutatus, Aster ramulosus, Lasallea falcatus


Table of Contents


Introduction: Apparent Problems With The Determination
Determination
Rhizomed Specimens At Indian Gardens
Additional References
Other Pictures From the Grand Canyon


Introduction: Apparent Problems With The Determination

Symphyotrichum falcatum makes beautiful displays along the upper Bright Angel Trail in September, especially in the wetter areas such as Kolb Springs. Yet it is surprisingly difficult for amateur and professional botanists to be sure of the identification of this common plant due to the following:

The large number of mismatches is quite troubling; correct identifications rarely have so many difficulties. However, surprisingly, in this case the determination is correct.

The source of most of the mismatches is that S. falcatum is a very widespread species, found in every state west of the Mississippi River except for Arkansas, Louisiana, Oregon, Nevada and California. Widespread species often have significantly-different characteristics in part of their range that are often considered to be different taxa, as shown in this case by the many synonyms given above. For example, the Arizona Flora by Kearney and Peebles gives three varieties of A. commutatus in Arizona alone, differing on whether the phyllaries are subequal or conspicuously unequal and whether the stem hairs are spreading or appressed.

Further, when botanists eventually realize that the different variations are all the same species, all too often floras fail to rewrite the species description to include the different variations.

A second source of problems in the determination is that S. falcatum comes in two forms, one rhizomed and one not. Surprisingly, both forms are present on the Bright Angel Trail, looking like different species at a glance (see below).

A third source is that someone, somewhere, apparently picked up the leaf width of 3-6 mm as given in the Illustrated Flora of the Pacific States by Abrams and Ferris for the linear-leaved plants, and turned it into the leaf length. This error appears in at least two separate references (Epple and University of Wisconsin Herbarium: Aster falcatus var. commutatus).

Fortunately, the latest botanical reference, the Flora of North America, has just published its volume on Asteraceae, which is available online. Their description of S. falcatum fits the Grand Canyon plants almost perfectly (see below), and thus there are no problems now with the correct determination of these plants.


Determination

The properties of the phyllaries, needed for the determination, are shown in the next photographs. The flower on the left is older, and its disk flowers have turned from yellow to reddish-brown.

The easiest determination for the plants on the Bright Angel Trail uses the Asteraceae key in the Arizona Flora by Kearney and Peebles, since one only has to separate out species found in Arizona. In the following, my observations replace the key element when they are more restrictive or more definite than the general key element.

Asteraceae key
1' corollas not bilabiate
2' fls not all perfect
3' rays present
6 pappus of bristles .... GROUP F

Group F Key
1 rays white
2' lvs without oil glands
3' pappus of many bristles
4' ray achenes not enveloped by the subtending phyllaries
5' ray flowers with pappus
6' plants not dwarf
7 style tips lanceolate or subulate, acute or acuminate, phyllaries strongly graduated, some partly herbaceous; rays mostly relatively broad .... 27. Aster

Aster Key
1' lvs entire or toothed
4' stems > 10 cm; lvs > 1 cm long
6' upper lvs not tiny and scalelike (although they are much reduced); sts strictly herbaceous
9' lvs not spiny-toothed; heads < 5 cm wide; invol < 15 mm high

In the next couplet, I take both branches since some phyllaries are glandular on the back, and others are glabrous:

10 phyllaries glabrous on back (ciliate on margin)
11' plants per
13 rays white
14 plants hairy, not from rhizomes; heads numerous; (some) phyllaries with a whitish, chartaceous base and an abrupt green tip .... 2. A. commutatus

10' phyllaries glandular on back
18 rays white; heads numerous; phyllaries several-seriate, with a
white-coriaceous base and an abrupt, ~rhombic, green tip.... 2. A. commutatus

Fortunately, both branches led to the same determination.

A. commutatus variety key:
1' phyllaries conspicuously unequal, the invol distinctly graduated
2' sts hispid with spreading hairs var. crassulus

The Arizona Flora states this species is:

widely distributed in AZ, 5000 - 8000 feet, mostly in the pine belt, Aug to Oct. Minnesota to British Columbia, south to Texas, New Mexico and Arizona. var. crassulus is the commonest form in Arizona, ranging from northern Apache County to the Kaibab Plateau and south to Cochise and Santa Cruz counties.

The lower elevation of 5000 feet given in the Arizona Flora almost perfectly matches my observations on the Bright Angel Trail of 4800 feet, removing one of the difficulties mentioned above.

The botanical name du jour for A. commutatus var. crassulus is Symphyotrichum falcatum. (By the way, all the Arizona and California species in the genus Aster have now been moved to other genera; there are no remaining Aster species in those two states.) The characteristics of this taxon in the latest description in the Flora of North America match the Grand Canyon plants well, with only the following differences:

This is now an entirely-normal small number of differences, and hence the determination is strongly supported. (Measurements in floras rely on the relatively-small number of voucher specimens, which means it is expected to find at least one characteristic which is outside the range given in a flora. This is especially true for very widespread taxa; it is simply not possible to measure all characteristics for a large number of plants from every location.) In particular, all the difficulties with the physical characteristics of the determination mentioned in the Introduction are now resolved.

The remaining two difficulties are also easily resolved. First, the fact that the Annotated Checklist of Vascular Plants of Grand Canyon National Park (Monograph No. 7) doesn't mention this taxon as being present below 6800 feet, is easily understood. The Grand Canyon is a huge place, and it is not possible for any group of botanists to thoroughly survey it. The Annotated Checklist therefore derives its distribution information primarily from voucher specimens collected by others. Apparently, no botanist has ever undertaken a vouchered survey of the Bright Angel Trail to establish elevation limits for the taxa there.

Second, the habitat quoted in the above references that seems so incongruous with the observed habitat here is mostly pine forests. A little thought about what that environment represents resolves the difficulty. Forests exist where there is enough moisture to support the existence of trees, and hence indicate a mesic (wet) environment. Moisture is usually the key determinant for where asters grow. The Annotated Checklist gives this insight: one of the most mesic forest habitats of the South Rim is actually found just below the rim of the Canyon on steep slopes and ledges of the Toroweap Formation. The sun seldom shines in these microhabitats, protected by the cliffs from drying summer winds; yet these areas are well watered by winter snows and summer thundershowers. This is almost precisely the environment in which the specimens of S. falcatum grow along the Bright Angel Trail. There aren't many mesic locations like this in Arizona, so it is not surprising this unusual habitat isn't mentioned in the floras.

Additional support for this determination comes from the following analysis, which I performed before finding the online Flora of North America treatment. On 14-17 September 2006, I took pictures of every species in bloom that I didn't recognize in the following locations:

This gave me a fairly complete set of species that bloom in September.

I then took the Asteraceae species from the 1 October 2005 Grand Canyon Plant List available here, and added the following information to the excel file: elevation min and max, flower time min and max, whether the flower had ligules, and the color of the ligules. (This file is available here.)

I then sorted the file to find species with white ligules that bloomed in August or September, and which were found above 5000 feet. The only candidates found were Symphyotrichum falcatum, Chaetopappa ericoides (Leucelene ericoides), Erigeron divergens, and Erigeron speciosus var. macranthus. Of these, the only possible determination for these plants on the Bright Angel Trail is Symphyotrichum falcatum. As mentioned above, Chaetopappa ericoides is rejected since it is a much smaller plant, which is also true for the two Erigeron species. In addition, the two Erigeron species have many more ligules; 75-150 is given for both in the Flora of North America.

Furthermore, I found no other specimens in my survey that could fit Symphyotrichum falcatum.

This again is very strong support for the determination, as long as the Grand Canyon Plant List is not missing a large showy species widespread on the Bright Angel Trail, a very safe assumption.

Unfortunately, there is a wild card in the determination from any of the above. The Flora of North America reports that Symphyotrichum ericoides is now known from Arizona, even though it wasn't present in Kearney and Peebles or in the 1987 Grand Canyon Checklist. Using the key and descriptions in the Flora of North America, one cannot assign these Grand Canyon plants to either species! The length of the involucre, 6.0-6.5 mm, and the number of disk florets, ~22, fit only S. falcatum, but the # of ray florets, 9-16, and the length of the ligules, 7 mm, fit only S. ericoides.

Studying the descriptions and distribution maps of these two species in the Flora of North America reveals that these two species are suspiciously similar. I've written the author of that treatment for his insights into how separate those species are. Until I learn more about these two species, I will stick with S. falcatum for the determination of these plants.


Rhizomed Specimens At Indian Gardens

One of the species I found on my 14-17 September 2006 survey was a clearly-rhizomed Aster at Indian Gardens. These plants looked so different from the non-rhizomed Symphyotrichum falcatum plants in the upper elevations of the Bright Angel Trail that it never entered my mind that these were the same taxon. Imagine my surprise when I looked at my pictures in detail and found that the plants were identical in all respects other than the habit! These rhizomed plants key to the same determination in the same way. The only difference in the specimens at Indian Gardens, compared to the specimens along the upper Bright Angel Trail, is the presence of the rhizomes, which results in the stems being erect and well-spaced.

The following photographs show the habit of the plants:

The picture on the right is a blow-up of the lower middle section of the picture on the left. The plants here were just beginning to bloom, so there are few flowers in this picture.

A picture of the flower from these plants is shown above, the right hand picture in the Determination section showing the phyllaries. The only apparent difference between this picture and the picture of the non-rhizomed plant flowers on the left is the age of the flower. The flower on the left is older, and its yellow disk corollas have turned reddish-brown.

Once again, the description in the Flora of North America fits perfectly: colonial or cespitose; with branched rhizomes or with cormoid, branched, woody caudices. The non-rhizomed plants are cespitose with cormoid, branched woody caudices (see this plant voucher from the Grand Canyon South Rim); the rhizomed plants are colonial with branched rhizomes, seen clearly in the closely-spaced stems of the plants at Indian Gardens.

These two forms have been defined as separate varieties in the Flora of North America treatment; var. falcatum is non-rhizomed, and var. commutatum is rhizomed. This treatment as two different varieties is perfectly understandable after seeing the plants along the Bright Angel Trail. However, if the difference in these forms is simply due to the amount of available water, and intermediate forms exist, it might be better simply to recognize that this is a single variable species. There is no geographic separation to these varieties (see the distribution maps at the Flora of North America), and certainly not much separation between the different forms on the Bright Angel Trail.


Additional References

Flora of North America: Symphyotrichum falcatum

USDA Plants Profile: Symphyotrichum falcatum; var. falcatum and var. commutatum.

Southwest Colorado Wildflowers, Virgulus falcatus shows the variety with rhizomes and very leafy phyllaries. The picture here of the phyllaries, and the number of ligules, initially caused me great concern about the proper determination of the Grand Canyon plants. It is certainly not unexpected that plants growing in Foothills, montane. Woodland openings, fields, roadsides in southwest Colorado might look different from plants growing amidst the rocks in the Grand Canyon.

GRIN Taxonomy for Plants: Symphyotrichum falcatum.

Nesom, G. L. 1994. Review of the taxonomy of Aster sensu lato (Asteraceae: Astereae), emphasizing the New World species. Phytologia 77:281.

List of Aster synonyms. This page states: The genus Aster (sunflower family - Asteraceae) is now generally restricted to the Old World species. The other species have now been reclassified as Almutaster, Canadanthus, Doellingeria, Eucephalus, Eurybia, Ionactis, Oligoneuron, Oreostemma, Sericocarpus and Symphyotrichum.

By the way, as the result of these taxonomic changes, we have only two Aster species left in North America: Aster alpinus (native to east Siberia and northern North America) and Aster tataricus (introduced) (Flora of North America: Aster)

Pix of an Arizona plant from Ponderosa Pine Forest at Rio de Flag, showing a few-liguled plant like ours.

University of Wisconsin Herbarium: Aster falcatus var. commutatus gives this as having only linear leaves, as also described in Abrams. It also quotes the curious leaf length as being "less than 1/4 inch long", which was also given in Epple. This page is especially interesting, since it also gives an illustration of the species that shows leaves much longer than 1/4 inches.

ASU Vascular Herbarium pages:


Other Pictures From the Grand Canyon

Plants on the S. Kaibab Trail: hanging garden plant, plant growing below the previous plant, close-up of flowers.

Another view of rhizomed Indian Gardens plants

Bright Angel Trail at ~4800 feet: plant growing alongside trail; view of stems, leaves and flowers; view of involucre; view of leaves.

Kolb Spring at 6360 feet: view of single plant; mosaic view of plants below spring along trail (panorama created from three images by autostitch; the spring is the dark area in the upper right).


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Copyright © 2006-2009 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
http://tchester.org/gc/plants/species/symphyotrichum_falcatum.html
Comments and feedback: Tom Chester
Last Update: 22 September 2006 (misdetermination note added 21 April 2009)