Big Bend National Park is not close to anything. So, for most of us, a trip there requires advanced planning. Between scheduling time away from work, booking plane flights, and reserving hotel rooms (there are very few rooms in the park and not many within an hour's drive of the park boundaries), one needs to make arrangements long before knowing the weather. I also was coordinating to meet with Martin Terry, of Sul Ross State University, who very generously showed me many amazing cacti in neighboring Presidio County. I planned my visit for late April 2011, hoping to catch the peak of the cactus flowering season, but it was not meant to be. I arrived almost eight months since the last trace of precipitation and immediately after several record freezes. During my visit, there were a few days when the air was filled with diffuse smoke from many, vast wildfires that stretched from northeast Mexico, through west and central Texas. Yet despite choosing possibly the worst year to visit Big Bend, this was still a magical place, with a few of the larger cacti somehow managing to flower, especially several species of Grusonia, Opuntia, and Echinocereus.
Having never before seen Ariocarpus fissuratus, other than in cultivation, I played the game of trying to identify appropriate habitats while driving. After a few days, I got good at this, easily spotting flat rectangular white limestone rocks that form small gently sloping hills. One of those habitats is a place just inside the park boundary where you can see hundreds of Ariocarpus after a meager five-minute walk along utterly flat sandy terrain, plus a slight gentle uphill on limestone. Even my super-suburban septuagenarian parents would find this a trivial effort.
Enter Big Bend National Park from the northwest, i.e. via Persimmon Gap. This is the road heading south from the town of Marathon and the Marathon Basin, which is itself a worthwhile stop for all the diminutive endemic cacti growing on novaculite outcrops. Six kilometers (four miles) south of the park entrance, on the east side of the road, is a parking lot (a euphemism, given that it may be able to hold three vehicles) for Dog Canyon, a canyon cutting through the Sierra del Carmen (Fig. 1). The Sierra del Carmen is the same range that the Rio Grande cuts through at Boquillas Canyon. Walk along the trail for about a hundred meters. There will be limestone outcrops on both the north and south side of the trail, although I only had time to explore the north one (Fig. 2). Leave the trail and head along the sand for the base of the limestone outcrop. As soon as you leave the sand for limestone, Ariocarpus fissuratus abound, although there is a higher density at the top. Some individuals are dead and papery yellow; some are a beautiful sunburnt rose color; some are classic slate grey (Figs 3–5). Near the base of the outcrop, as close to the trailhead as possible, there are some vertically-oriented pieces of limestone and, sandwiched between them are a few similarly oriented Ariocarpus (Figs 6 and 7).
This ridge contains other cacti that are typically only found in limestone, such as Echinocactus horizonthalonius and Mammillaria lasiacantha (Figs 8 and 9). Both of these species only appear as miniature specimens at this locale. On limestone, Echinocereus dasyacanthus is also extremely small, especially compared with the large specimens that grow in the sand surrounding the outcrop (Figs 10 and 11). However, not everything on this limestone outcrop is diminutive. Echinocereus stramineus, another plant that always grows in limestone, is full-sized here, as is christmas cholla, Cylindropuntia leptocaulis (Figs 12 and 13). Another typical limestone plant found here is ocotillo, Fouquieria splendens (Fig. 14).
10–11
Not confined to the limestone, Echinocereus dasyacanthus survives here, though reduced in size.
The surrounding sandy plain is largely devoid of plants – except the ubiquitous creosote (Larrea tridentata), mesquite (Prosopis sp.), and grasses – but contains a few large clumps of Echinocereus enneacanthus (Fig. 15). Close to the road I stumbled upon a group of half-dozen javelina, but could not get close enough for a good photo (Fig. 16). One odd species near the base of this limestone outcrop is Coryphantha macromeris (Fig. 17). This species often grows in flat sandy areas or in shale, including at Big Bend, but at the Dog Canyon trailhead area seem to only be found in limestone. These are robust plants, but surprisingly are entirely devoid of offsets.
Dog Canyon trailhead is a truly trivial locale to access, with a decent selection of cacti, including many easy-to-find Ariocarpus fissuratus. Figure 18, of Yucca baccata from the limestone outcrop, shows how close the road is, with a view looking north towards the Persimmon Gap park entrance.
