Journal archives for April 2023

April 16, 2023

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 | 18 comments | Leave a comment