Some Thoughts on the Flora Of South Texas After Seeing It in Mexico…
In December I flew to San Luis Potosi and spent 12 days doing vigorous field botany throughout the states of San Luis Potosi,
Querétaro, Hidalgo, Mexico, Puebla, and Oaxaca. This consisted of meeting up with two different friends that live there, going out every day and photographing, documenting and filming every wild plant species that I came into contact with. Photographs were uploaded to the site inaturalist as well as to my own cloud database, and every observation of a plant species then had various notes attached to it such as habitat type, elevation, sympatric species, presence of pollinators, geologic substrate (all mostly limestone) and so on.
Throughout the open valley basins, mountainsides, and sketchy winding one-land roads we traversed across, I couldn’t help but notice that so many of the plants that I know from South Texas also grew in Mexico, and that - more importantly - they all looked so much healthier than they did in Texas. As of February 2026, South Texas is in an epic drought, so it’s not hard to imagine that the Texas populations of most Tex-Mex plants look to be in worse shape than their Mexican counterparts. South Texas is also at a much lower elevation - and is thus a much hotter climate year-round - than most of Mexico.
Cordia boissieri, Senegalia berlandieri, Jatropha dioica, Karwinskia humboldtii, Hibiscus martianus, Nahuatlea hypoleuca - all of these looked incredibly healthy in Hidalgo, Mexico, where they grew at 5500’ elevation. In Hidalgo, Senegalia berlandieri and Nahuatlea hypoleuca become single-trunked small-to-medium-sized trees rather than the multi-stemmed scraggly individuals that you only ever see them reaching in South Texas.
Seeing this made me realize that perhaps the only reason that these plants were in South Texas to begin with was due to the late Pleistocene cooling from roughly 2 million to 11 thousand years ago, when all of lower-latitude North America was much cooler and milder of a climate than it is now. This “cooling” likely allowed the plants that thrived in the mountains of Mexico to the South to “come down” to the hotter, lower-elevation South Texas plains to the North. It begs the question though - what grew in the plains and undulating hills of South Texas prior to this cooler period, though?
Did Stenocereus huastecorum - a multi-stemmed columnar cactus that grows only a hundred miles South of the Texas border - at one point grow in Texas along with other freeze-intolerant plants that now only occur in Mexico, such as Decatropis bicolor (rutaceae)?
Keep in mind that Texas is just basically Mexico, in keeping with the floristic affinities that span across very recent geopolitical borders. If you step into the mountains of Nuevo Leon at elevations of 5,000-7000 feet near places like Aramberri, you can catch a glimpse of what the Edwards plateau and Balcones escarpment may have looked like 600 years ago before it was turned into private, inaccessible land surorunded by strip malls and cul-de-sacs. Most of the same plants grow in both places, the geology is still limestone, etc. Many Mexican plants reach their Northern limit in Central Texas, such as Taxodium mucronatum, etc. There are beautiful rolling hills covered in Monarda and Junipers and Oaks in the Nuevo Leon region, as well. In between the higher elevations of the edwards plateau and the Sierra Madre lies the Rio Grande Valley, which is hotter than most places in Mexico. Here there are slightly different ecotypes of widespread plant species, these ecotypes being adapted to and selected for a much hotter, lower-elevation climate. There are also different ecotypes of many different plant species that exist in both South Texas (which is hotter and more humid) and West Texas (which is drier and cooler throughout the year)...examples include : Sidneya tenuifolia, Lophophora willamsii, Jatropha dioica, Hamatocactus hamatacanthus. I am less familiar with the North and East Texas area than I am with the South and West.
"No coviene, por otra parte, perder de vista el hecho de que la reparticion geografica de los organisms por lo general have caso omiso de las divisiones politicas de la corteza de nuestro planeta, en cambio con frecuencia esta ligada con la delimitacion de regiones naturales, definidas por condiciones fisiograficas, climaticas, edaficas, etc. Para ser congruente con tal circumstancia y dar cuenta de muchos endemismos verdaderos, sera necesario en esta capitulo extender las fronteras del pais y asi, se recurrira al termino megamexico, cuando se incluytan las partesde las zonas aridas sonorense, chihuahuaense y tamaulipeca, que pertenecen a los estados unidos de america, al termino de "Megamexico2", cuando se abarque el territoria centroamericano hasta el norte de Nicaragua y al termino "Megamexico 3" para comprender ambas extensiones (veanse los mapas de la figura 2) "- Jerry Rzedowski, Diversidad y Origenes de la Flora Fanerogámica de Mexico
"On the other hand, it is important to remember that the geographic distribution of organisms generally ignores the political divisions of the planet's crust and is instead frequently linked to the delimitation of natural regions defined by physiographic, climatic, edaphic, etc. conditions. To be consistent with such circumstances and to account for many true endemisms, it will be necessary in this chapter to extend the country's borders, and thus the term megamexico will be used when including parts of the arid zones of Sonora, Chihuahua, and Tamaulipas, which belong to the United States of America; the term "Megamexico2" when covering the Central American territory as far north as Nicaragua; and the term "Megamexico 3" to include both areas (see maps in Figure 2)" - Jerzy Rzedowski, Diversidad y Origenes de la Flora Fanerogámica de Mexico
