Differentiation of Gutierrezia texana and Amphiachyris species in North Central Texas

Introduction

Gutierrezia texana is often confused with two Amphiachyris species with which it is sympatric, Amphiachyris dracunculoides and Amphiachyris amoena, and they have historically sometimes been treated in the same genus. For whatever reason Amphiachyris dracunculoides seems to have become the default choice both in the minds of amateur botanists and for the algorithm on iNaturalist (apparently because the other two have not met the requirements to be included in any of the computer vision models as of yet). All three plants have similar small capitula with yellow ray and disc flowers. The branching patterns can also be similar and the sizes often overlap. Amphiachyris dracunculoides often has a distinctive appearance when it exhibits its classic rounded shape with heads in dense corymbiform arrays, while both Gutierrezia texana and Amphiachyris amoena typically have more open paniculiform arrays. However, variations in the number of capitula caused by any range of factors can cause the appearances to be similar enough to be easily confused. Due to these similarities, it is necessary to examine other details of the plants to make a correct identification.

Differentiation via the phyllaries

The most convenient way for observers to differentiate the two genera in north central Texas is by observation of the phyllaries. As noted in the Flora of North America treatment of Amphiachyris, Amphiachyris species have "abaxial nerves of the phyllaries without green borders." The result is that the phyllaries on Amphiachyris species appear to have a uniform color from edge to edge, and thus often appear wider than the phyllaries on Gutierrezia species. Unfortunately, the green nerve borders in Gutierrezia texana are not always present. They often do present as a narrow dark green band bordering the phyllary nerve for much of the length of the nerve, but many times it also only occurs at the phyllary tips or under some weather and seasonal variations it may not be noticeable at all. The overall result however is the edges of the phyllaries on G. texana are often difficult to discern at all, and at best the phyllaries actually look a lot narrower than they really are.

Phyllaries of Gutierrezia texana


Phyllaries of Amphiachyris amoena


Phyllaries of Amphiachyris dracunculoides


Differentiation via the pappus

The best way to differentiate the genera in Texas is to examine the pappus of the disc flowers. In Amphiachyris, the pappus of the disc flowers consists of a few noticeably long scales. A hand lens may be useful to see them more clearly, but they are often visible with the naked eye.


In contrast, on Gutierrezia texana the pappus on both ray and disc flowers is short or absent, often not noticeable at flowering time. It is most easily noticed on achenes, if one is lucky enough to find a specimen with some intact.

Corymbiform versus Paniculiform arrays

While these flowering patterns don't differentiate the genera, they can still be helpful for differentiating A. dracunculoides, which is corymbiform, from the other two species, which are paniculiform. A corymbiform array is one in which all the flowers (or in this case heads) appear to be roughly at the same level. Specimens like these appear flat topped or rounded. Paniculiform arrays are more difficult to describe, but in general the heads do not appear at the same level. In practice, this can be difficult to ascertain when a photo simply appears to be a mass of yellow flowers, but sometimes one is able to isolate a specific branch and see which description applies. More often than not one can see heads much further down on a branch, making it paniculiform, and ruling out A. dracunculoides.

Corymbiform A. dracunculoides - note how the heads are roughly the same level


Paniculiform G. texana - note how there are heads at various levels of the branches

Disc Flower Style Branch Appendage Length

According to Nesom, the disc flowers of Amphiachyris species are functionally staminate and he also notes that the style branch appendages are fused. What this means visually is that the style branches in Amphiachyris disc flowers (when exposed) appear quite short. On the other hand, this is not the case in G. texana, so if one happens to find a capitulum where the style branches are exposed, they appear quite long. While presence of these longer style branches on disc flowers is indicative of G. texana, its absence may simply indicate that no flowers are in that stage of pollination. Additionally, one has to be certain of looking at a disc flower and not a ray flower where both genera can have long style branches.

Longer style branches of G. texana, often forming sort of a loop


Habitat and Distribution

In my experience, in the Austin area (Travis, Williamson, and Burnet counties), G. texana is much more prevalent than either Amphiachyris species in areas accessible by the public, and is often weedy on the borders of hiking trails. Amphiachyris dracunculoides is more often found in grazed pastures. All three plants do occur in the general area.

Observation Links

Amphiachyris dracunculoides
https://www.inaturalist.org/observations/36339681
https://www.inaturalist.org/observations/8626126
Amphiachyris amoena
https://www.inaturalist.org/observations/8486953
Gutierrezia texana
https://www.inaturalist.org/observations/33987832
https://www.inaturalist.org/observations/35425536
https://www.inaturalist.org/observations/35765857
Russell Pfau's comparisons
https://www.inaturalist.org/observations/34926031
https://www.inaturalist.org/observations/34926032
https://www.inaturalist.org/observations/35627415
https://www.inaturalist.org/observations/35627414

Sources

Nesom, Guy L. 2006. Amphiachyris. In: Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 20+ vols. New York and Oxford. Vol. 20. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=101427
Nesom, Guy L. 2006. Gutierrezia. In: Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 20+ vols. New York and Oxford. Vol. 20. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=114211

Publicado por rymcdaniel rymcdaniel, 05 de junio de 2020

Comentarios

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Yes. :)

Many many many thanks, Ryan!

Publicado por sambiology hace alrededor de 1 mes (Marca)
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@sambiology Hopefully it will be helpful, though I know it is likely that most of the observations still won't capture enough data to be identifiable. :)

Publicado por rymcdaniel hace alrededor de 1 mes (Marca)
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Very true, but this is a journal post that I'll copy and past on the observations to let them know about the important differences. :) So it's super helpful!

Publicado por sambiology hace alrededor de 1 mes (Marca)

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