Penstemon guadalupensis and P. albidus

Penstemon guadalupensis is a Texas endemic documented from only a handful of counties. It's similar to P. albidus which ranges from Texas to Canada. Both differ from P. cobaea by being considerably smaller overall: smaller flowers, narrower leaves, shorter height. Despite its name, P. albidus (the white-flowered penstemon) comes in a pale lavender version. P. guadalupensis, however, has only been documented as white-flowered.

P. guadalupensis is underdocumented species. And almost certainly has declined in many areas due to land abuse. In 1895, it was very abundant along the Guadalupe River and Town Creek in vicinity of Kerrville. It hasn't been seen in Kerr Co. since.

Here's a slide showing the two side-by-side, a range map, and Heller's original description of P. guadalupensis along with the type specimen:

To distinguish the two, these are my thoughts at the moment, based on observing a lot of P. guadalupensis in person (and all of the observations on iNat) and comparing them visually with several hundred iNat observations of P. albidus from across the western great plains (Canada to Texas).

Based on that, here's my conclusion on how to distinguish them:

P. albidus has leaves that are as wide at the base as they are near the flowers--the basal leaves are not linear. The color of the leaves lean toward bluish green (often with a frosted or matte appearance--which would feel like sandpaper):

P. guadalupensis has leaves that are narrow at the base (linear, strap-like) and increase in width as they approach the flowers. The color of the leaves lean toward yellowish green:

The difference in leaf width from base to flower (or lack thereof) being a character that really stands out to me. And FNA has some outright mistakes in the description and key both. I've since found Heller's original description, and he highlighted the leaf shape as I've described it as a main character. Note: the FNA couplet for these species contains an error in leaf width (compare it with the FNA species description).

There are some specimens that are contentious (i.e., so the reliability of distinguishing characters may not be 100% and there may be hybridization or these species may represent clinal variation of one species, etc.

Posted on April 16, 2023 10:45 PM by pfau_tarleton pfau_tarleton



Posted by observerjosh about 1 year ago

Great work!

Posted by bosqueaaron about 1 year ago

Do you mind either linking to this post or me linking to this post in a comment for a P. guadalupensis observation? I like to favorite observations with identification guides or keys to make it easier to refer back to them (since iNat doesn’t let us favorite journal posts).

Posted by observerjosh about 1 year ago

Yes, feel free to link. However, do understand that this is tentative. And there are specimens west of Abilene that are contentious (e.g.

Posted by pfau_tarleton about 1 year ago

I do understand, especially at the edges of species’ range or where they overlap with closely related species, there may even be some amount of hybridization.

Posted by observerjosh about 1 year ago

It's also worth noting that the sand/limestone connection doesn't seem to hold up. Among the guadalupensis populations that @pfau_tarleton and I studied last week, the ones near Dublin were in typical limestone soil, whereas the ones around Proctor Lake were in sand/sandstone while having the same appearance.

Posted by bosqueaaron about 1 year ago

I tracked down the original species description and added it to the linked slides. Heller's original description of the species points out the main character that I noticed--linear leaves at base becoming wider up near the flowers.

It was described from Kerrville. Interestingly, no iNatter has observed them there. At the junction of Town Creek and Guadalupe River, mentioned in the description, sits Riverside Nature Center. In April and May of 1894, Heller collected 25 specimens that remain in several herbaria around the country. These are the only herbarium records that I know of from Kerr County. Evidently it hasn't been seen there since!

@bacchusrock I see you've been in that neck of the woods recently!
@arlonm Something to keep your eye out for this month.
@s_pi_ky, @kypfer_kyroptera, @codystricker, @bgsmith, @frankmargaret, @plateauville

Posted by pfau_tarleton about 1 year ago

Many thanks, this is great information.

Posted by frankmargaret about 1 year ago

So I walked around the Riverside Nature Center last Thursday. I did not find any P. guadalupensis which isn't surprising considering what has happened to the land in the last 100+ years. It's not very pretty with periodic floods, overeating by deer, invasive non-native species, and a lot of cultivated plots.

Posted by bacchusrock about 1 year ago

Guadalupe Cemetery in Kerrville would be a good place to look for them.,-99.1458942,464m/data=!3m1!1e3

Posted by pfau_tarleton about 1 year ago

I won't be back in the area until May, but I'll see if I have time then to check out the Cemetery.

Posted by bacchusrock about 1 year ago

I added some notes on this observation:

Here are some of my observations from Proctor Lake. The observations I noticed were mostly on rocky, shallow soils, with the exception being some on shallow sandy soil with rocky layer beneath – along the edge of where sandy and rocky conditions meet. In both cases, the soils seem very poor and low in moisture and nutrient capacity (without actually conducting any tests).

I agree 100% with the descriptions of P. guadalupensis above, but I haven’t seen P. alba in person to compare/contrast.

I also added an observation with fruiting since I didn’t see any on iNaturalist yet.

Thanks again for this journal post!

Posted by observerjosh about 1 year ago

I went by Guadalupe cemetery yesterday. Lovely little cemetery with lots of character but no P. guadalupensis. I also look along the riverbank but maybe that wasn't the best place to search.

Posted by bacchusrock about 1 year ago

@bacchusrock, that's sad news for the type locality.

Posted by pfau_tarleton about 1 year ago

@bacchusrock Did you look across Town Creek Rd.? Looking with Google Street View, I see rocky surface soil similar to where I saw it. It looks like there are some sightings about an hour away at Cross Mountain in Fredericksburg.

Posted by observerjosh about 1 year ago

Yes, I walked along Town Creek Road. The other side of the creek had rocky surface but I didn't go to that side. I can try again next week. I don't travel to Fredericksburg. A few weeks ago, I also looked around the area where @sambiology had seen one in 2018 but it was bone dry around there and not much growing at all. If you have other thoughts for around Kerrville, let me know and I'll try to check them out next week.

Posted by bacchusrock about 1 year ago

It could be too dry. And it's also getting late in the season for down there. Up here, about half of them have gone to seed...and they're hard to see without the flowers. Next spring, I'm going to come down and do a lot of driving around in Kerr county.

Posted by pfau_tarleton about 1 year ago

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