Salix arizonica

Recognized as a distinct species only since 1975; often confused with S. pseudocordata (=S. boothii) prior to its recognition as a new species (Dorn 1975). S. arizonica is apparently able to hybridize with a variety of co-occurring Salix species; putative hybrids between S. arizonica and S. brachycarpa, S. boothii, S. geyeriana, S. planifolia, S. wolfii, and S. monticola have been reported (Decker 2006).

Salix arizonica is a rare willow known in the western United States from the margins of the Colorado Plateau in the White Mountains of east-central Arizona, the High Plateaus of south-central Utah, and the southern Rocky Mountains of northern New Mexico and southern Colorado. There are approximately 107 occurrences of this species, though there is evidence of hybridization with S. boothii, particularly for populations in Utah, and there may ultimately be fewer occurrences known. Most populations are located on U.S. Forest Service lands in subalpine wet meadows and riparian areas. The total population size is believed to be approximately 22,000-55,000 individuals, with the largest occurrences (consisting of thousands to tens of thousands of plants) located in Utah. These large Utah occurrences are believed to be stable. Other occurrences in Utah and elsewhere tend to be smaller (10-1,000 plants), and many are declining. Browsing by domestic and wild ungulates (primarily by elk and cattle) is the primary threat to Salix arizonica, although some conservation efforts such as browsing exclosures have been established in some populations. Hydrological alterations (e.g. from diversions and impoundments) and climate change are also a primary threats. Other threats include timber harvesting, road construction and maintenance, recreational use, development and maintenance of ski resort facilities, and disease. Despite its distribution in four states, populations are widely separated by unsuitable habitat and have high genetic differentiation. A monitoring protocol has been developed under the Conservation Assessment, but trends for this species remain unclear.

Salix arizonica occurs in the western United States, in the White Mountains of Arizona, as well as southern Utah, New Mexico, and Colorado (FNA 2010). Populations are concentrated near the margins of the Colorado Plateau in southern Utah, eastern Arizona, northern New Mexico, and southern Colorado, with Utah and New Mexico being the principal areas of distribution (Welsh et al. 2008). Known occurrences are found in three primary clusters separated by 300-500 km (Decker 2006, Argus 2007): (1) the White Mountains of east-central Arizona (where restricted to 15-20 drainages around Mount Baldy); (2) the High Plateaus of south-central Utah (including the Markagunt Plateau near Brian Head Peak, the Paunsagunt Plateau along the East Fork of the Sevier River, the vicinity of Boulder Mountain, the Monroe Mountains, and Fishlake Mountain); and (3) the Southern Rocky Mountains of northern New Mexico and southern Colorado (where concentrated in the southern Sangre de Cristos, Nacamiento Mountains, and southern San Juan Mountains). However, In Utah, the Dixie National Forest botanist notes that the locations of Arizona willow reported near Boulder Mountain in the Escalante and Teasdale Ranger Districts are suspect and look to be more closely aligned to Booth's willow than Arizona willow (UNHP 2021). Range extent was estimated using data from the Utah Rare Plant Database, herbarium specimens, photo-based observations, and NatureServe Network occurrence data collected between 1994 and 2025 (Utah Natural Heritage Program 2021, GBIF 2025, iNaturalist 2025, NatureServe 2025, RARECAT 2025, SEINet 2025).

By applying a 1 km separation distance to data from the Utah Rare Plant Database, herbarium specimens, photo-based observations, and NatureServe Network occurrence data collected between 1994 and 2025, it is estimated that there are 107 occurrences rangewide (Utah Natural Heritage Program 2021, GBIF 2025, iNaturalist 2025, NatureServe 2025, RARECAT 2025, SEINet 2025). Though there may be less than this, depending on the genetic status of questionable occurrences in Utah and New Mexico, which could be more closely aligned with S. boothii (UNHP 2021, Tonne 2002).

The Colorado Natural Heritage Program tracks three known occurrences of this species, all of which have been observed within the last 20 years (CNHP 2023). Occurrences have not been comprehensively mapped in Utah but are known to number more than 30; several dozen occurrences are known from the Fishlake National Forest alone, and 36 locations (some of which might be mapped as the same occurrence) have been reported in the Dixie National Forest (Decker 2006). However, an unknown number of these (particularly the Boulder Mountain populations), show evidence of hybridization with Salix boothii based on preliminary molecular analysis and morphology (UNHP 2021, Lencioni et al. 2024). In Arizona, potential habitat has been thoroughly surveyed and no additional populations are expected (Rutman 1992). However, in other parts of the range such as the San Juan mountains, additional occurrences may yet be found (Decker 2006). In New Mexico, there were 15 to 20 occurrences estimated in 2002 (Tonne 2002).

Browsing by domestic and wild ungulates (primarily elk and cattle) is the primary threat to Salix arizonica across its range (Arizona Willow Interagency Technical Team 1995, Decker 2006, Tonne 2002, Lencioni et al. 2024). Nearly all occurrences on public lands are part of active sheep or cattle grazing allotments. Although this species evolved with native herbivores, the effect of domestic livestock in combination with increasing pressure from wildlife (mainly due to increases in elk numbers), may be beyond its tolerance (Decker 2006). According to Strohmeyer and Maschinski (1995), nearly 62% of Arizona willow populations had reduced size and vigor attributed to grazing by ungulates, with cattle and elk being the primary herbivores observed in an experimental study. However, this threat has been somewhat reduced with recent conservation agreements (AGFD 2002). Since the mid-1990s, lands managers have attempted to mitigate this threat by installing ungulate enclosures around populations, which have been maintained for almost three decades, though caged individuals still show low-levels of browse (Lencioni et al. 2024). Overgrazing also has indirect impacts to S. arizonica by degrading wetland habitats and introducing non-native vegetation (Mygatt 1999). Excessive use of habitat by livestock also causes soil compaction, erosion, and stream channel downcutting (Tonne 2002). Because numbers of cattle and elk are relatively high in New Mexico and Arizona, the populations there may be threatened at a higher extent (Maschinski no date).

Hydrologic alteration (e.g. from diversions and impoundments or elimination of beaver populations) is also a primary threat. In Arizona, habitat has been altered by the construction of high elevation recreational fishing ponds, or tanks for livestock (Decker 2006). Additional threats to this species listed in the Arizona Willow Conservation Agreement and Strategy included alteration of hydrology, disease, timber harvest, and recreation (Arizona Willow Interagency Technical Team 1995). Other threats to this species include impacts from timber harvesting, road construction and maintenance, impacts from recreational use (off-road vehicles and snowmobiles), development and maintenance of ski resort facilities, and disease (particularly infection by fungal rust belonging to the species complex Melampsora epitea), and climate change (Mygatt 1999, AGFD 2002, Decker 2006, Fertig and Reynolds 2009, Maschinski no date). Many of these threats are synergistic; for example, stressed habitat conditions tend to promote rust outbreaks (Arizona Rare Plant Committee 2001). Climate change in particular is a significant threat to Salix arizonica (Decker 2006), and this species is considered to be extremely vulnerable to its effects (Handwerk et al. 2015).