Plants on the Llano Estacado

Rhus aromatica and R. trilobata have been treated as distinct species and the same species over the years with little explanation in the Texas flora as to why. Although a much more thorough investigation into the literature would be preferable, I'll just go through the classic Texas references (I would ordinarily include FNA, but the Anacardiaceae treatment hasn't come out yet). Firstly, here are the three names and their equivalents under the two taxonomies:

R. aromatica var. serotina = R. aromatica var. serotina
R. aromatica var. flabelliformis = R. trilobata var. trilobata
R. aromatica var. pilosissima = R. trilobata var. pilosissima

Manual of the Vascular Plants of Texas considers R. trilobata a synonym of R. aromatica with little explanation.
Flora of North Central Texas considers the two species separate but notes the following: "This taxon [R. trilobata] is distinguished in some instances with difficulty from R. aromatica and is possibly only a variety of that species."
Atlas of the Vascular Plants of Texas synonymizes R. trilobata under R. aromatica without any explanation.
Flowering Plants of Trans-Pecos Texas and Adjacent Areas considers R. trilobata a synonym citing Atlas of the Vascular Plants of Texas.
The taxon merge on iNaturalist was made citing this: https://ucjeps.berkeley.edu/eflora/eflora_display.php?tid=41175. Jepson cites this: https://www.jstor.org/stable/25064252?seq=1. The phylogeny paper gives no indication that the two should be treated as synonyms. Indeed, there is not enough populational data mapped in the paper to give a conclusion one way or another meaning that the decision is based on the experience of the authors (John M. Miller & Dieter H. Wilken). This is fine, but it still doesn't offer any populational explanation as to what is going on in the species and why the decision to synonymize was made.

Until a more thorough study is done or I find some better references, I'll be following suit and not recognizing R. trilobata. Please let me know if you have a reference that offers a view with greater depth than the above as there doesn't seem to be much in the way of explanation in the Texas literature on Rhus.

The best distinctions are given in Manual of the Vascular Plants of Texas and are as follows:
Terminal leaflets 25-60 mm long, more or less narrowed at apex.....................R. aromatica var. serotina
Terminal leaflets 15-33 mm long, abruptly narrowed to truncate at apex
Mature leaves glabrous....................R. aromatica var. flabelliformis
Mature leaves densely pubescent.....................R. aromatica var. pilosissima

Rhus aromatica var. serotina doesn't appear to occur on the High Plains/Rolling Plains.

Rhus aromatica var. flabelliformis in truly glabrous form appears to be somewhat uncommon in the region, but many individuals have leaves so sparsly pubescent that it is hard to consider them R. aromatica var. pilosissima. As good varieties, there is going to be intergradation. Here is one of the few glabrous examples in the area: https://www.inaturalist.org/observations/13131983. Even it has a little hair.

R. aromatica var. pilosissima seems to be the most common variety on the Llano Estacado and adjacent areas. The most extreme and obvious examples of this variety are when the hairs are densely pilose and produce a velvety texture. This form is common. less ↑