Geologic Associations of Arizona Willow in the White Mountains, Arizona

JONATHAN W. LONG AND ALVIN L. MEDINA

Rocky Mountain Research Station, U.S. Forest Service 2500 South Pine Knoll Drive, Flagstaff, AZ 86001

ABSTRACT: The Arizona willow (Salix arizonica Dorn) is a rare species growing in isolated populations at the margins of the Colorado Plateau. Although its habitat in the White Mountains of Arizona has been mischaracterized as basaltic, the area is actually a complex mixture of felsic, basaltic and epiclastic formations. Comparing the distribution of the Arizona willow to mapped geologic formations revealed that occupied sites are strongly associated with felsic, coarse- textured Mount Baldy formations. The most robust subpopulations are located in three glaciated reaches, but about half occur in exposures of the Sheep Crossing Formation. Other sites occur in areas mapped as Quaternary basalt, but these lie either downstream from Mount Baldy formations or where basalt and porous cinders form a relatively thin mantle over the Mount Baldy formations. Glacial deposits, the Sheep Crossing Formation, and large alluvial deposits have high hydrologic conductivity that may favor the willow. Despite its affinity for the Mount Baldy formations, the Arizona willow is not a strict substrate specialist, since it has survived when transplanted into basaltic areas in Arizona and it grows in different substrates in New Mexico and Utah. Nonetheless, understanding the geologic associations of this rare plant can help to explain its distribution and to design appropriate conservation measures.

INTRODUCTION was "almost entirely volcanic in origin." The Mount Baldy stands out at the second conservation agreement for the Arizona highest mountain in Arizona (3476 m) and willow asserted that all but one population as a refuge for rare species. The mountain of this plant in New Mexico and Arizona represents the only known habitat in occur on "basaltic (volcanic) soils" (AWITT Arizona and the southernmost habitat 1995). Similarly, a conservation assessment overall for the Arizona willow (Salix characterized the habitat of the Mogollon arizonica Dorn). Because geology has Paintbrush (Castilleja mogollonica Pennell), tremendous influence over climate, which co-occurs with Arizona willow, as topography, hydrology, and soil chemistry, “basalt-derived” (Bainbridge and Warren it is essential to interpreting plant 1992). Failure to recognize the potential biogeography. However, many ecologists confounding effects of geologic variation trying to explain the distribution of plants in can result in faulty inferences concerning the White Mountains have discounted or the distribution and status of species. misrepresented the geologic variation of the STUDY AREA area. For example, Sivinski and Knight Mount Baldy has been studied and (1996) concluded that substrate mapped by geologists beginning well over a specialization was not an important factor century ago, when G. K. Gilbert described governing plant endemism in the Mogollon Mount Baldy as “massive eruptions of Province, which extends from New Mexico trachyte,” from which, “stretch, in every to the White Mountains, because the area direction, long slopes of sanidin-dolerite

49 [basalt]” (Gilbert 1875). Gilbert’s mountains where the streams were less steep description of two distinctive lithologies has (Merrill 1974). A very small glacier remained accurate to the present day, yet occurred on Mount Ord within the past 3000 one of the first regional maps lumped the years, while periglacial activity formed talus two types into “Quaternary-Tertiary basalt” deposits in northeastern drainages of the (Wilson and Moore 1960). Melton (1961) Mount Baldy volcano and also shaped was the first to describe numerous glacial south-facing slopes (Merrill 1974). deposits on Mount Baldy, while Finnell et METHODS al. (1967) observed that the volcano was composed of three major groups of volcanic We prepared a composite geology map of rocks with different compositions. However, the Mount Baldy area based on the maps by Merrill’s dissertation (1974) was the first Merrill (1974), Wrucke and Hibpshman work to map various felsic volcanic, mafic (1981), Condit (1984), and Nealey (1989). volcanic, and epiclastic formations around Onto this map, we plotted the reported the entire mountain. locations of Arizona willow subpopulation (fig. 1). We identified subpopulations based A variety of volcanic and epiclastic flows on groups that occurred on separate created the White Mountains of Arizona. drainages, or that occurred on the same Between 23 to 12 million years ago, drainage but were separated by at least 500 volcanic and volcaniclastic eruptions of meters of apparently unoccupied habitat. For predominantly andesitic to basaltic-andesitic each subpopulation, we identified the petrology flowed across a wide area in east- geologic formations at the site. We also central Arizona (Berry 1976). Atop this determined whether the watershed above the surface, felsic lava flows built the Mount site was derived from felsic Mount Baldy Baldy volcano to an elevation of nearly formations or from basaltic formations. To 4000 m (Merrill 1974; Nealey 1989). The compare the association between the Sheep Crossing Formation resulted from Arizona willow and particular formations, volcaniclastic processes that caused we summed both the number of populations colluvium and tuff to accumulate in the on each type and the estimated total valleys at the base of the volcano (Merrill numbers of Arizona willow on each type. 1974). Starting approximately nine million years ago and continuing into the This overlay approach has several Quaternary, basaltic flows partially covered limitations. First, while all large stands of the older volcanic deposits (Merrill 1974; Arizona willow have probably been Condit 1984). identified, small plants may have been missed. Consequently some subpopulations During the Quaternary Ice Ages, four may not have been identified in small side distinct glacial events sculpted the two drainages. Second, recent human impacts major peaks of the volcano, Mount Baldy such as reservoir construction has altered and Mount Ord (Merrill 1974). The earliest plant distributions. However, in the glaciation was the most extensive, shaping dammed drainages where it occurs, the plant five valleys that flowed in directions to the is still found above and below the reservoirs. north, west, and east. The glaciers sculpted Third, the geologic maps available for the out U-shaped valleys and deposited small area may be inexact, especially in describing amounts of till on the slopes of the two drainage bottoms. Maps sometimes peaks. Most of the material loosened by the overrepresent younger formations that may glaciers was transported far away from the

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FIGURE 1: Three-dimensional view of geologic formations on the eastern face of Mount Baldy, overlaid by subpopulations of Arizona willow (outlined circles in proportion to estimated size). Fine stippling designates felsic volcanic rocks, coarse stippling designates the Sheep Crossing Formation, white designates glacial and alluvial formations, and dark gray and black designate older and younger basaltic formations, respectively. have been eroded and replaced with species. The enumerations of both alluvium from older, upstream formations. subpopulations and individual plants are Despite the limitations of the approach, the used only to demonstrate geologic results offer valuable insights into associations of the plant in relative terms. relationships between plants and the rocks RESULTS beneath them. Subpopulations occur in a 500 m Enumerating subpopulations of Arizona elevation range, with the highest willow is difficult because individual plants subpopulation in Smith Cienega (3050 m) are hard to separate. We used the estimates and the lowest subpopulation in Hughey in the Arizona Willow Conservation Creek (2550 m). Subpopulations are found Agreement (AWITT 1995) and the White between 2600 m and 2900 m in all three Mountain Apache Tribe’s Ecosystem major watersheds draining Mount Baldy Management plan for Arizona willow (White River, Black River, and Little habitat (WMAT 1995). Both approaches Colorado River). Sub-populations occur in applied a rule of counting plants separately drainages that flow north, south, and east, where their stems were spaced more than 1 but not in any drainages that flow primarily meter apart. However, estimated sizes of the westward, except for a single plant on a largest subpopulations are likely to have slope above the North Fork of the White large error terms due to the difficulty in River (fig. 1). counting the prostate form of this clonal

51 The subpopulations are concentrated on are all located on the western half of Mount landforms derived from the Mount Baldy Baldy. Several other populations occur in volcanics (fig. 2), and most of the estimated areas mapped as basaltic, but in valleys individuals occur in glaciated reaches (fig. downstream from Mount Baldy formations 3). The three largest and densest populations where the felsic substrates naturally deposit. are located in large glaciated meadows (fig. All of these are located within 3.8 1 and fig. 3). These populations have been kilometers downstream from exposures of estimated in the hundreds to thousands the Sheep Crossing Formation. (AWITT 1995). Large populations also A few populations occur in areas mapped occur in the glaciated valley of the West as basalt and are not downstream of Fork of the Little Colorado River and in the exposures of Mount Baldy formations (fig. extensive alluvial deposits along the East 3). These populations occur in the Snake Fork of the Little Colorado River. Creek watershed and an adjacent tributary Nearly half of the populations occur in that flows into White Mountain Reservoir. exposures of the Sheep Crossing Formation This area has a more complex geology than (fig. 2), primarily in the upper member of superficial mapping suggests. The younger that formation. The Sheep Crossing basalts, including highly porous cinders, Formation is texturally indistinguishable form a relatively thin mantle over the older from glacial tills in the area, although it was formations, including the Sheep Crossing deposited millions of years earlier from Formation, as shown in cross-sections by debris fans and mudflows from Mount Merrill (1974). Colter Spring (fig. 1), the Baldy (Merrill 1974). The only extensive location of the holotype specimen of outcrops of the Campground Member that Arizona willow, is a prominent example of are not associated with the Arizona willow one of the many springs in this area.

FIGURE 2: Percentage of estimated total Arizona willow FIGURE 3: Percentages of estimated total Arizona subpopulations located on particular geologic willow individuals located on particular geologic formations. formations

52 DISCUSSION formations generally have more sand, less clay, less silt, and lower pH than soils Many factors, including elevation, derived from basaltic formations (Long et al. temperature, aspect, soil texture, soil 2003). In Utah, all but one of eighteen chemistry, and past glaciation, could Arizona willow populations sampled by constrain the distribution of Arizona willow. Mead (1996) had over 30% sand in the Due to the evolutionary history of Mount uppermost mineral soil horizon (excluding Baldy, most of these factors are inherently sites that lacked mineral soils due to thick correlated and therefore difficult to separate cover of peat). Coarse-textured soils may (Long et al. 2003). However, a combination create more favorable habitat for the of lithology and topography, or a “lithotopo Arizona willow by increasing hydrologic type” (Montgomery 1999), is largely conductivity and aeration. sufficient to explain the distribution of the species. Specifically, the willow inhabits Botanists have noted associations areas where deposits of felsic Mount Baldy between soil aeration and growth of many rocks have collected in wide valleys on the willow species. Tall-growing willows of lower flanks of the volcano. Climatic many species are associated with well- regimes may further constrain the aerated mineral soils, while prostrate forms downstream and upstream limits of Arizona often occur in poorly-drained bogs with willow habitat, since there are alluvial abundant fine organic matter (Brinkman deposits further downstream and at least one 1974). In some cases (e.g., Salix glacial deposit (Bull Cienega on Ord Creek) myrtillifolia Anderss.), the tall- and low- that are not inhabited by the plant. However, growing forms of a single willow species the distribution is not determined simply by have been so distinctive that the two forms elevation, because streams with spring flows were even classified as subspecies (Dorn at suitable elevations in basaltic areas have 1975). For the Arizona willow, taller no evidence of Arizona willow, including specimens were observed in microsites Soldier Creek to the north, Big Bonito Creek where they were sheltered from ungulate to the Southwest, and Burro Creek to the grazing (AWITT 1995). However, the east. occurrence of the prostrate growth form in lightly grazed areas was hypothesized to Influence of Geology on Edaphic reflect the influence of finer-textured, less Conditions aerobic soils (AWITT 1995). Examining Differences in geology exert a variety of sites in Utah, Mead (1996) concluded that influences on soils that could account for the tallest Arizona willows, and 75% of all differences in the suitability of habitat for stands with an average height greater than Arizona willow. However, the most basic 100 cm, grew on Tertiary volcanic parent difference is soil texture. In Arizona, the materials in alluvial flood plains with substrates occupied by Arizona willow are mineral soils. He also found that peat depth derived from Mount Baldy formations, was negatively associated with Arizona which are felsic rocks with greater than 50% willow height. Wetland successional silica content (Merrill 1974; Nealey 1989). processes such as the accumulation of fine As such silica-rich volcanics weather, they sediments and decomposition of organic form relatively deep, acidic soils with low matter may reduce gas exchange and clay content and low organic matter content stimulate growth of bacteria, both of which (Freeman and Dick-Peddie 1970). Riparian meadow soils derived from the Mount Baldy

53 promote anaerobic conditions (AWITT thin aquifer (Merrill 1974), storing and 1995). conducting shallow groundwater to seeps More complex differences in soil and springs at the base of the mountain chemistry also could affect the physiology where the willows congregate. The Arizona of the willow, yet it demonstrates willow most likely draws its water from the considerable tolerance. Soil analyses hyporheic zone rather than from runoff, as revealed that Arizona willow sites in Utah has been shown for its cousin and common are highly variable in chemical constituents, associate, the serviceberry willow (Salix none of which seem to be constraining the monticola Bebb) (Alstad et al. 1999). species (Mead 1996). An isolated Drainage Connections subpopulation has persisted at a site with Lithology and topography are not high surface soil pH and low nutrient sufficient to explain the distribution of the concentrations on the East Fork of the plant, however, because there are extensive Sevier River (Mead 1996), demonstrating exposures of felsic rocks, glacial deposits, the species has the capacity to colonize a and Sheep Crossing Formation on the wide range of coarse substrates. western slopes of Mount Baldy that are Consequently, substrate preferences of the unoccupied. Aspect may partly explain this willow are more likely to be attributable to pattern, since the last major glaciation hydrologic properties than to chemical formed only a small glacier on the west- differences, although it is impracticable to facing slope (at the head of the East Fork of entirely separate these attributes. the White River), while large glaciers Furthermore, while most transplants of occurred in three north- and east-facing Arizona willow into purely basaltic areas in drainages. This pattern suggests that north- the White Mountains apparently died and east-facing drainages were colder and/or (AWITT 1995), some have survived in moister. However, Arizona willow occurs in Burro Creek (Granfelt 2003). Thus, several south-facing drainages (e.g., available evidence suggests that the Arizona Hurricance, Hughey, and Pacheta Creeks, willow prefers coarse-textured soils, but it is fig. 1), indicating that the plant can tolerate not a strict substrate specialist. warmer, drier aspects. Consequently, Topography may reinforce lithology in topographic barriers and drainage maintaining favorable hydrologic conditions connections may explain why the plant does for Arizona willow. While basalt flows are not occur on the western slopes of Mount typically flat, felsic volcanics form steep Baldy. Subpopulations along Ord Creek are domes (Stokes 1986). Glaciers, mudflows, blocked from drainages to the west by the and landslides sculpt such steep landforms 300 m tall ridge extending from Mount Ord to form extensive deposits of gravels, sands, (fig. 1). Similar topographic obstacles may and boulders. Rinne (2000) reported that explain the southwestward limit on Arizona streams draining the Mount Baldy volcano, willow populations. The next drainage west including glaciated drainages and those of Hughey Creek is Big Bonito Creek, below the Sheep Crossing Formation, had whose canyon has incised about 150 meters higher concentrations of fine gravels than below the Mount Baldy formations into pre- streams in areas of younger basalts. High Mount Baldy volcanic rocks. permeability of the Sheep Crossing We hypothesize that Arizona willow Formation relative to the surrounding came to the White Mountains via the Little volcanic formations enables it to serve as a Colorado River, and that low drainage

54 divides and headwater stream transfers periods are evident well beyond these facilitated extension to the White and Black drainages. During the last glacial maximum, River watersheds. The drainage divides which ended about 14,000 years ago, between the North Fork of the White River temperatures were at least 5° C lower, and the Little Colorado River and between precipitation may have been 20 to 25% the Little Colorado River and the West Fork higher, and tree-line was at least 800 m of the Black River are both very low (<30 lower (Merrill 1974). Conditions were even m), as are most of the divides between colder during the previous two glacial smaller tributaries in that area. periods (Merrill 1974). Consequently, Consequently, the eastern flank of Mount almost all of the present habitat occupied by Baldy is a relatively gentle landform with Arizona willow would have been above shallowly incised streams. The steeper tree-line during the three major glacial canyons of the North Fork of the White episodes. River and Big Bonito Creek may have The environmental conditions associated captured streams on the edge of this plateau with glaciation interacted with the pre- after the Arizona willow had become glacial landscape to produce the habitats established there. Snake Creek and Becker presently occupied by the willow. For many Creek turn abruptly west to meet the North willow species, factors that promote early Fork of the White River, which has incised seral conditions such as an open canopy and 150 m over the past 1.5 million years mineral soils may provide the best (Condit 1984). Behnke (1979) presumed opportunity for seedling establishment. that headwater transfers were important Consequently, the retreat of glaciers would mechanisms in distributing fish species in have created optimal habitat for the Arizona the White Mountains. willow within the occupied drainages, while The Role of Glaciation in Forming periglacial activity such as frost action and Arizona Willow Habitat rock glaciers formed talus piles in other Some ecologists have argued that drainages, notably Becker Creek (Merrill Pleistocene glaciation was a primary factor 1974). Heavy snows and rock slides controlling the distribution of Salix rejuvenate willow shrubs and promote a arizonica and other willow species in the more stable age structure of ramets (Price et Southwest (Price et al. 1996). However, al. 1996). Extensive reworking of these alpine conditions such as high elevation, glacial and proglacial deposits would have cold temperatures, and increased created good habitat for willow precipitation not only are associated with colonization, as demonstrated by the strong past glaciation, but are also intrinsic to the correlations between willow pollen and volcanic formations that cap many of the influxes of sediment in Holocene records of plateaus and peaks of the Southwest. Given willows reported from the San Juan this inherent correlation and the limited Mountains of Colorado (Andrews et al. fossil evidence of pre-glacial conditions, it 1975). is difficult to determine the role of glaciation Implications for Sustainability of Arizona in distributing the modern flora. Glaciation willow subpopulations directly affected only five drainages in the In the White Mountains, Arizona White Mountains (Merrill 1974), and willow is presently well-distributed Arizona willow occurs in only three of the throughout the glacial tills and exposed five. However, the climatic effects of glacial Sheep Crossing Formation except on the

55 western slopes of Mount Baldy and Mount consideration of other rare plants in the Ord. The distribution does not suggest that White Mountains demonstrates that rare Arizona willow habitat has been eliminated plant-geology associations are often from a broader area that includes purely individualistic. For example, the Bebb basaltic drainages, as some other researchers willow (Salix bebbiana Sarg.) is most have suggested (AWITT 1995). Researchers commonly found in basaltic meadows (Long have reported death of individual plants and et al. 2003), and it overlaps with Arizona extirpation of small populations following willow only at the lower edge of the latter’s severe infections of a fungus (Melamspora range (Granfelt 2004). While the epitea) (AWITT 1995). Differences in conservation agreement for the Arizona habitat quality associated with geologic willow (AWITT 1995) states that the willow variation may influence the vulnerability of “occurs in the same ecosystem” as the particular subpopulations. The White endemic Mogollon clover (Trifolium Mountains have experienced significant neurophyllum Greene), others have observed warming and drying in the past decade, the clover only in association with basaltic resulting in severe defoliation of spruce soils (Ladyman 1996). On the other hand, trees in high-elevation forests (Lynch 2004). the Mogollon clover’s close relative, the This period coincides with the reports of pygmy clover (Trifolium longipes Nutt. var. deteriorating Arizona willow habitat. Lower pygmaeum (Gray) J. Gillett), does co-occur elevation subpopulations more commonly with Arizona willow in both Arizona (JWL, assume the tall growth form, which is personal observation) and in Utah (Mead associated with lower water tables (AWITT 1996). Gillet (1969) suggested that because 1995; Mead 1996). Consequently, the two closely related clovers are both subpopulations that occur at lower found in the White Mountains, they would elevations in basaltic areas may more be likely to hybridize. However, geologic vulnerable to drought, disease, insects, and segregation might reduce the potential for ungulate impacts due to naturally warmer interbreeding. and drier conditions. The glacial deposits The Arizona willow’s association with that harbor the largest subpopulations of epiclastic formations on the flanks of Mount Arizona willow are colder and wetter. These Baldy has important implications for subpopulations are mostly in the prostrate conservation of the species. Monitoring of form, similar to low-growing willow species the species’ status should distinguish that are accustomed to alpine environments. between subpopulations in marginal habitats Because they occur at higher elevations, at low elevations in basaltic areas and these populations may be better buffered subpopulations in the preferred lithotopo from the impacts of warming. type. Low elevation populations will Other ecologists have observed that probably continue to suffer under a warmer, extensive coarse-textured alluvial and drier climate, while the glaciated meadows colluvial deposits may have special will serve as refugia for this ancient species. biological significance because they Efforts to stimulate reproduction or expand maintain extensive hyporheic exchange populations of this plant will be more likely zones (Jensen and Goodman 2001). This to succeed in areas with sandy-gravelly association may help to understand the substrates and abundant groundwater distribution of the Arizona willow and the supplies. Efforts to conserve the biodiversity Mogollon paintbrush. However, of the White Mountains and other high-

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