Relationships Between Rare Plants of the White Mountains and the Late Cenozoic Geology of the Colorado Plateau JONATHAN W. LONG Rocky Mountain Research Station, U.S. Forest Service 2500 South Pine Knoll Drive, Flagstaff, AZ 86001 ABSTRACT. A complex geologic history has shaped the distribution of Arizona willow (Salix arizonica Dorn) and the Mogollon paintbrush (Castilleja mogollonica Pennell). These subalpine plants do not appear to be strict substrate specialists, but they do seem to favor coarse-textured and well-watered soils. Most of their occupied habitats were shaped by Quaternary glaciations, but are ultimately derived from felsic substrates formed before the Pliocene period. Populations of Arizona willow have been identified in the White Mountains of Arizona, the High Plateaus of Utah, and in the Southern Rocky Mountains of New Mexico and Colorado. Species closely related to the Mogollon paintbrush also occur in the Utah plateaus and the Southern Rocky Mountains. Genetic dissimilarity among these populations suggest that these taxa likely share an evolutionary history that extends into the Neogene, when tributaries of the ancestral Colorado River connected young volcanic highlands on the margins of the Colorado Plateau. This history points to the likelihood of additional populations of Arizona willow in the San Juan Mountains, and it suggests that these plants have survived dramatic changes in their environments. These patterns demonstrate the value of analyzing geology at a detailed level when interpreting habitat preferences and distributions of rare species. INTRODUCTION The Mogollon paintbrush (Castilleja A few species are endemic to only the mogollonica Pennell) is another rare White Mountains of Arizona, but the list plant that is endemic to the White has grown shorter in recent years. The Mountains of Arizona, but whose status Arizona willow (Salix arizonica Dorn) as a separate species has been was thought to be a member of this questioned. Pennell first described this select group until Robert Dorn yellow-bracted paintbrush as a separate reidentified a specimen in the Rocky species (Pennell 1951). Holmgren Mountain Herbarium that had been (1973) placed it in the Septentrionales collected in 1913 from southern Utah. group, which includes the yellow- This realization led to the recognition of bracted sulphur Indian paintbrush (C. Arizona willow populations in southern sulphurea Rydb.) as well as several Utah, northern New Mexico, and endemic species of the Southwest (Fig. southern Colorado (Thompson et al. 2). National plant databases (the 2003). The scattered populations in these USDA’s PLANTS National Database four states represent the known and the University of North Carolina’s distribution for the species (Fig. 1). Biota of North America Program) currently hold that C. mogollonica is a synonym for C. sulphurea, which is 59 FIGURE 2: Distribution of members of the FIGURE 1: Distribution of Arizona willow in the Septentrionales group of Castilleja in the Colorado Colorado Plateau region. Plateau region based on Holmgen (1973). distributed widely in the Rocky isolated from other high mountain Mountains. However, a taxonomist ranges during the time in which new currently working with the genus holds species have evolved. that C. mogollonica is a valid species, SIGNIFICANCE OF GEOLOGY TO PLANT and instead C. sulphurea should be DISTRIBUTIONS synonymous with C. septentrionalis (Lindl.) (Egger 2004). Reconstructing the geologic evolution of landscapes helps to explain the With the exception of retaining C. distributions of endemic species. mogollonica, plant nomenclature used Geology not only explains the past throughout this paper conforms to the conditions an organism has withstood, PLANTS database (http://plants.usda.gov). but it also regulates present aspects of Some biologists have even questioned habitat including climate (through the status of the flagbearer of rare orographic effects), hydrology, and soil species in the White Mountains, the chemistry. However, many ecologists Apache trout. Some taxonomists studying rare biota in the White contend that this trout should be Mountains of Arizona have either designated merely as a subspecies (i.e., ignored geologic variation or Oncorhynchus gilae ssp. apache) oversimplified it. For example, in (Behnke 2002). These trends reflect in discussing the biogeography of the part the vicissitudes of taxonomy. endemic Mogollon paintbrush, However, they also may reveal a Bainbridge and Warren (1992) described growing recognition that the White the region as “basaltic.” Similarly, the Mountains have not been entirely conservation agreement for the Arizona 60 willow asserted that all but one formed from felsic volcanic flows during population of the plant occur on the late Miocene period (Merrill 1974). "basaltic (volcanic) soils" (AWITT Mudflows and lahars down the young 1995). These statements are incorrect, volcano created extensive deposits of and they mislead readers to assume that colluvium that are known as the Sheep the White Mountains are monolithic. Crossing Formation (Merrill 1974). The geologic formations underlying During the Quaternary, four distinct populations of Arizona willow and glacial events sculpted the two highest Mogollon paintbrush in Arizona remnant peaks of the volcano, Mount originated from felsic Tertiary Baldy and Mount Ord. The earliest volcanism, which formed several other glaciation carved out five U-shaped montane regions in the Southwest. Some valleys that flowed to the west, north, researchers have considered the and east (Merrill 1974). Within the past importance of variation among these 3000 years, a very small glacier occurred volcanic landforms as well as the on Mount Ord, while periglacial activity influence of glaciation in evaluating the formed talus deposits in northeastern habitat of rare species in the White drainages of the Mount Baldy volcano Mountains. Ladyman (1996) reported and also shaped some of its south-facing that the endemic Mogollon Clover slopes (Merrill 1974). (Trifolium neurophyllum Greene) Populations of Arizona willow in appeared “to be positively associated Arizona are concentrated on landforms with basalt soils and negatively derived from the felsic Mount Baldy associated with datil soils” [Datil Group volcanics (Long and Medina, this volcanics are predominantly felsic proceedings, Geologic Associations of pyroclastic rocks containing pumice and the Arizona Willow in the White ash located east of Mount Baldy in New Mountains, Arizona). The four Mexico (McIntosh and Chamberlin populations on glacial deposits are the 1994)]. Other researchers have argued largest and highest density populations that Pleistocene glaciation had been a in Arizona, with estimated populations primary factor controlling the in the hundreds or thousands (AWITT distribution of Arizona willow and other 1995). Nearly half of the subpopulations willow species (Price et al. 1996). Rinne are located on sites mapped as the Sheep (2000) hypothesized that both mineral Crossing Formation, which is a composition and glacial history could sedimentary formation also derived from account for reported differences in trout Mount Baldy volcanics. Most of the productivity between streams derived remaining populations occur within four from rocks close to the Mount Baldy kilometers downstream of outcrops of volcano and those farther away. this formation. The Sheep Crossing Formation is texturally indistinguishable GEOLOGIC ASSOCIATIONS OF ARIZONA from glacial tills in the area, although it WILLOW was deposited millions of years earlier White Mountains, Arizona from debris fans and mudflows from A complex series of volcanic flows Mount Baldy (Merrill 1974). created the White Mountains, but the Comparisons of plant densities to central massif of Mount Baldy was substrates show that alluvium, glacial 61 deposits, and Sheep Crossing Formation appears to be the only one in Utah represent the prime habitat for Arizona occurring in a watershed devoid of willow in the White Mountains. Small Tertiary volcanics. However, at least six populations occur on surfaces mapped as other populations have diverse substrates basaltic flows, although these deposits in their watersheds, leading Mead (1996) may be thin enough that underlying to characterize them as being derived in felsic formations yield water and part from sedimentary formations. substrates to the inhabited areas. Southern Rocky Mountains, New High Plateaus, Utah Mexico and Colorado The Utah populations occur in the In northern New Mexico, the Arizona High Plateaus region (Fig. 1) which is willow occurs in at least 21 populations capped by andesitic and dacitic Tertiary in the Sangre de Cristo Mountains and volcanics of the Marysvale volcanic four populations in the Jemez Mountains field as well as some younger basaltic (Atwood 1996; Atwood 1997; Dorn flows (Stokes 1986; Luedke 1993; 1997). The rocks underlying these areas Rowley et al. 1994). Thus, the petrology are more varied and much older than and age of the High Plateaus of Utah are those in the White Mountains of Arizona similar to the White Mountains volcanic or the High Plateaus of Utah due to the field of Arizona. The populations of tremendous uplift of the Rocky Arizona willow in Utah are scattered Mountains. Substrates for Arizona across the plateaus, with the greatest willow habitat appear to become more concentration on the Markagunt Plateau diverse in these taller mountains. In the around Brian Head, a geologic cousin of Sangre de Cristo