The mineral gypsum appears all over the world, but is rarely seen on the surface anywhere except in deserts. Gypsum, is a form of Calcium Sulphate in which each CaSO4 molecule is chemically combined with two water molecules in a solid crystalline form. It is more soluble than most other minerals, so it tends to accumulate, and/or be exposed only in very dry environments where it may persist in the form of crusts, geologic layers, or scattered as tiny crystals in the soil. In moderate amounts, it provides plants with two vital nutrients — Calcium and Sulphur. Gypsum is widely used in agriculture as a fertilizer, a soil amendment, and sometimes in treating runoff from irrigated fields.
But what is good in moderate amounts can become undesirable when present in high concentrations. High levels of calcium (Ca) and sulfur (S) can be toxic to many plants. Additionally, ion concentration in gypsic soils can be high enough to impede the osmotic flow of water into roots. Moreover, gypsum tends to be deposited on the soil surface where it forms hard surface crusts that prevent seedlings from becoming established.
But as is often the case in nature, some plants have evolved an ability to tolerate gypsum, and a few have even evolved to require it. Those that tolerate gypsum are known as gypsovags; those that require it gypsophiles.
Gypsovags are non-specialist plants that can live on gypsum soils when the physical crust is absent or reduced. They are often stress-tolerant refugees showing a limited ability to counteract the high S, Ca, and Mg concentrations in the soil; moreover, they are not particularly good at extracting the limited amounts of N and P that are available. Chemical analysis of their leaves reveals these weaknesses. But their ability to tolerate gypsic conditions gives them a significant evolutionary advantage over other plants competing in the area.
Gypsophiles, on the other hand, are usually widespread and may even be dominant in the region they inhabit. These specialists can germinate on the physical crusts that typify gypsum-laden soils. In addition, they usually have physiological adjustments to help them cope with the chemical limitations imposed by gypsum soils. Many gypsophiles have succulent-like leaves that help them dilute undesirable ions. Others have specialized structures in the roots that block the uptake of undesirable ions. For example, the Mariposa cactus (Echinomastus mariposensis,) is so good at blocking gypsum uptake, gypsum may crystallize on its root surfaces. And many plants show adaptations similar to those used by salt-tolerant plants to sequester and/or excrete gypsum. Almost all gypsophiles are perennials and several generations of plants are frequently found growing together.
Nevertheless, surprisingly little is actually known about gypsophiles. Few studies have been conducted on the subject (and most of those in Spain) and many studies present contradictory conclusions. DNA analysis is probably the most active area of research at present.
Unexpectedly, DNA analysis tells us that each of the gypsophilic floras evolved independently from local plants; gypsum tolerance may well be a latent genetic trait of many of them. Moreover, it appears that some plants may have developed gypsophilic traits repeatedly throughout their evolutionary history.
Not all plant families contain gypsophiles – in fact, most gypsophiles come from just a few plant clades. (A clade is a group of plants that are known to have descended from a common ancestor) and gypsophilic clades tend to contain mostly gypsophilic species. For example the Acleisanthes (trumpet flowers) clade contains 6 distinct groups of gypsophiles. The Namas (crinklemats) have 8 gypsophilic taxa. The Nerisyrenia (Mustards) clade has 11.
Most true gypsophiles are not very good at distributing their seeds, so they often become isolated on gypsum outcroppings. There they may grow in great numbers but nowhere else. Plants that occur in numbers but that are limited in geographic scope are known as endemics; many endemic species are quite rare. For example, the newly identified Sophora gypsophyla var guadalupensis occurs only in the Guadalupe Mountains and in a single disjunct site in Chihuahua, Mexico. The past few years have yielded several new species that must be classified as gypsophiles.
My own interest in gypsophilic plants began with the discovery of this little plant on the old government road in Fresno Canyon in the Big Bend Ranch State Park. The year was 2011.
I couldn’t figure out what it was, and nobody else was able to do so from the photograph. But in 2015 I found one in much better condition and it was blooming!
It turned out to be Anulocaulis leiosolenus, var lasianthus, or Chihuahuan Ringstem. This plant grows on the rocky gypsic soils found between Big Bend National Park and the Big Bend Ranch State Park. The area begins around Study Butte, includes the Terlingua Ghost town environs, and extends west to the western edge of the state park. Its north/south range is probably little more than 40 miles or so. This a classic rare, endemic, gypsophilic plant!
Because of increasing desertification around the world, I believe we can expect to see increasing attention paid to this unique group of plants as we seek to understand how they deal with difficult environmental conditions.