This line of crescent moons is jumbled. Back to back, point to back, flip and flop, ring and around, they lie along the land, the edge of an old fault. These connected crescents are a band of hills. The mid point on the back of a crescent is the high point on its ridge, and the curve of the crescent has a sinuous top of progressively lower peaks that extend around the curve to its points. These hills could be the white of the moon, but they are more the texture of bone, dirtied by the elements, in my mind. They are like scabrous, weathered bone; they have a surface of gravelly flakes. The gravel, which adheres to the surface crust of the soil, crunches underfoot, but it gives way easily because powder lies beneath the crust. The powder variations—greenish gray, pinkish tan, dirty bone white—depend, I suppose, as much on light as place and adulteration.
They are simply the White Hills. They lie below Triassic reds and pine green slopes of the Wasatch Plateau. They are a decorative white band that is 35 miles long; they rise sharply from the edge of the flat Sevier Valley. Their bony slopes are the only place where the Arapien blazing star (Mentzelia argillosa) is known. I have been walking the White Hills, the red slopes that lie behind, and the mirroring white hills on the west side of the valley to understand better the distribution of the Arapien blazing star.
This blazing star takes its common name from the substance of the hills; their powdery base of white, with its shades of gray, tan, green and pink, is Arapien Shale. This shale is rich with gypsum. Scattered on the surface of the hills are hunks of selenite, a crystalliferous form of gypsum, which looks like layer after layer of fused, slightly cloudy glass. Gypsum from these hills is mined and processed in a facility, layered in bone white dust, near Sigurd. The powder of gypsum-rich, Arapien shale flows into my shoes with each sideways step I make down steep slopes.
Gypsum (hydrous calcium sulfate) can be advantageously mixed into the soil of one’s garden, but in arid environments of the American West, where it occurs naturally and richly in some soils, it can limit the growth of plants. The intolerance that some plants have for gypsum rich soils, as well as the converse—the attraction to, even restriction to, gypsum-rich soils by so-called gypsophiles, is not well understood. Limitations imposed by gypsum can be at least partly physical: gypsum-rich soils in arid environments tend to form surface crusts that can be difficult for the roots of newly germinated seeds to penetrate (1). High calcium concentrations in soil may also limit the ability of some plant to take-up nutrients like phosphorus and iron (2). Gypsophiles may have adaptations that enhance uptake of these two required nutrients from the soil despite the problems presented by its calcium-richness. For gypsophiles, there may be advantages to living on gypsum-rich soils. The relatively depauperate plant communities on gypsum rich soils can reduce resource competition for gypsophiles. Gypsum crusts can also provide strong thermal insulation, which enhances water retention (3).
The arid western landscape has islands and archipelagos of gypsum-rich outcrops, and gypsophiles can be limited to a single island or small archipelagos of gypsum. Plants that are limited geographically and specialized for life only on a particular kind of substrate are called edaphic endemics (edaphic comes from the Greek edaphos for ground). The Arapien blazing star, which is endemic to the White Hills, may be an edaphic specialist, a plant adapted to the soil conditions of this local environment.
Art Kruckeberg, a great student of soil specialization and plant distribution, suggested that edaphic endemics evolve from ancestors that are less specialized—they could become established and persist on restrictive as well as unrestrictive substrates (4). There are different models for the evolution of edaphic endemics. Peter Raven applied the idea catastrophic selection in which small marginal populations become isolated, suffer a rapid decrease in number of reproductive individuals, and, subsequently, adapt rapidly under strong selection from an extreme environment as narrow edaphic endemics (5). In contrast, Don Levin’s model of ecological speciation portrays edaphic shifts occurring more gradually, and possibly with interbreeding between the newly evolving edaphic specialist and its less specialized ancestral group. Levin’s model calls primarily for shifts to “benign sinks”—environments that are not extreme and can permit the establishment of various species—that require little genetic change. Levin’s model for the gradual evolution of edaphic tolerance and, perhaps, specialization, emphasizes that changes in few genes may be involved (6).
These models emphasize new evolutionary change to account for edaphic endemics. Some have pointed-out that we must also carefully distinguish species that are narrowly distributed because they are newly evolved (so-called neo-endemics) from those that once were more widespread but have become more narrowly distributed over time (so-called paleo-endemics). These paleo-endemics may have narrowed distributions because of climate change or increasing competition for resources. Paleo-endemics may not have evolutionary specializations for life on restrictive substrates but simply may capable of tolerating the restrictive conditions, especially as a refuge from competition or intolerable environmental changes in their former ranges. With paleo-endemics, we should perhaps look for the absence of specializations and for the signs of escape.
As simple as the models sound, these studies that test evolution, specialization, and distributions have an inherent complexity. Some studies have been consistent with Raven’s ideas of rapid edaphic specialization, but others may be more consistent with Levin’s model.
I take the steps to understand why the Arapien blazing star is living only on the White Hills. I walk the hills to find the limits of its distribution and to core the soil for later tests of chemical composition. Our studies of evolutionary relationships among species of blazing stars show so far that the Arapien blazing star is in a group of species that live on high calcium substrates (7). These insights on evolutionary relationships may be among the first signs that the Arapien blazing star is not a neo-endemic; instead, it may be part of a paleo-endemic complex, one that evolved from an ancestor that was already specialized to high calcium soils. This calciphile ancestor may have been geographically fragmented (or dispersed) under pressures of climate change, which led to the reproductive isolation of regional groups of populations. If that hypothesis is correct, then speciation—leading to the origin of the Arapien blazing star among others in its complex—may have simply followed geographic isolation, and its adoption of the gypsum-rich White Hills may not have been an evolutionary stimulus. For the Arapien blazing star, the bony crescents of the White Hills may be as much refuge as a source of adaptation.
Notes
(1) For more information on gypsophily and gypsum soil crusts see the following:
Borselli, L., R. Biancalani, C. Giordani, S. Carnicelli, and G. A. Ferrari, 1996. Effect of gypsum on seedling emergence in a kaolinitic crusting soil. Soil Technology 9: 71-81.
Meyer, S. E. 1986. The ecology of gypsophile endemism in the eastern Mojave desert. Ecology 67: 1303-1313.
Parsons, R. F. 1976. Gypsophily in plants—a review. American Midland Naturalist 96: 1-20.
(2) For more information on nutrient limitations that may result in calcium-rich soils see the following:
Gries, D. and M. Runge, 1995. Responses of calcicole and calcifuge Poaceae species to iron-limiting conditions. Botanica Acta 108: 482-489.
Misra, A. and G. Tyler, 1999. Influence of soil moisture on soil solution chemistry and concentrations of minerals in the calcicoles Phleum phleoides and Veronica spicata grown on a limestone soil. Annals of Botany 84: 401-410.
Tyler, G. 1992. Inability to solubilize phosphate in limestone soils—key factor controlling calcifuge habit of plants. Plant and Soil 145: 65-70.
Tyler, G. and L. Ström, 1995. Differing organic acid exudation pattern explains calcifuge and acidfuge behavior of plants. Annals of Botany 75: 75-78.
Zohlen, A. and G. Tyler, 2004. Soluble inorganic tissue phosphorus and calcicole-calcifuge behavior of plants. Annals of Botany 94: 427-432.
(3) For more information on thermal insulation and water availability in gypsum soils see the following:
Meyer, S. E. and E. García-Moya, 1986. Gypsum grasslands of northern San Luis Potosi, Mexico: patterns of vegetation, soil, and seasonal moisture availability. In Occasional Papers of the Chihuahuan Desert Research Institute, Alpine, Texas.
Muller-Stoll, W. R. and G. Lerch, 1963. Model tests on the ecological effect of vapor movement and condensation in soil due to temperature gradients. Pp. 65-82 in Water relations of plants, Rutter, A. J. and F. H. Whitehead (eds.). Wiley and Sons, New York.
Verheye, W. H. and T. G. Boyadgiev, 1997. Evaluating the land use potential of gypsiferous soils from field pedogenic characteristics. Soil Use Management 13: 97-103.
(4) For examples of Art Kruckeberg’s work see the following:
Kruckeberg, A. R. 1984. California serpentines: flora, vegetation, geology, soils, and management problems. University of California Press, Berkeley.
Kruckeberg, A. R. 1986. An essay: The stimulus of unsual geologies for plant speciation. Systematic Botany 11: 455-463.
Kruckeberg, A. R. 2002. Geology and plant life. Univ. of Washington Press, Seattle.
Kruckeberg, A. R. and D. Rabinowitz, 1985. Biological aspects of endemism in higher plants. Annual Review of Systematics and Ecology 16: 447-479.
(5) Raven, P. H. 1964. Catastrophic evolution and edaphic endemism. Evolution 18: 336-338.
(6) The Levin ideas on edaphic specialization and ecological speciation are from the following:
Levin, D. A. 2004. Ecological speciation: crossing the divide. Systematic Botany 29: 807-816.
Levin, D. A. 2005. Niche shifts: the primary driver of novelty within angiosperm genera. Systematic Botany 30: 9-15.
(7) The studies on evolutionary relationships in the group of blazing stars that include Mentzelia argillosa are part of the ongoing research of my student JS.