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Platanus wrightii

Figure 1—Arizona sycamore on Ash Creek, Yavapai County, Arizona. Image by Johnida Dockens, used with permission.

SUMMARY

Citation:
Fryer, Janet L. 2022. Platanus wrightii, Arizona sycamore. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis/plants/tree/plawri/all.html [].

Table of Contents

• SUMMARY
• INTRODUCTION
• TAXONOMY
• SYNONYMS
• LIFE FORM
• DISTRIBUTION AND OCCURRENCE
• GENERAL DISTRIBUTION
• SITE CHARACTERISTICS
• PLANT COMMUNITIES
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• BOTANICAL DESCRIPTION
• SEASONAL DEVELOPMENT
• REGENERATION PROCESSES
• Vegetative Reproduction and Regeneration
• Pollination and Breeding System
• Seed Production
• Seed Dispersal
• Seed Banking
• Germination
• Seedling Establishment
• Plant Growth and Mortality
• SUCCESSIONAL STATUS
• FIRE ECOLOGY AND MANAGEMENT
• IMMEDIATE FIRE EFFECTS
• FIRE ADAPTATIONS AND PLANT RESPONSE TO FIRE
• FUEL CHARACTERISTICS
• FIRE REGIMES
• FIRE MANAGEMENT CONSIDERATIONS
• OTHER MANAGEMENT CONSIDERATIONS
• FEDERAL LEGAL STATUS
• OTHER STATUS
• IMPORTANCE TO WILDLIFE AND LIVESTOCK
• VALUE FOR REHABILITATION OR RESTORATION
• OTHER USES
• ADDITIONAL MANAGMENT CONSIDERATIONS
• MANAGEMENT UNDER A CHANGING CLIMATE
• APPENDIX
• REFERENCES

INTRODUCTION

FEIS abbreviation:
PLAWRI

Common names:
Arizona sycamore
Arizona planetree
buttonwood
Wright's sycamore

TAXONOMY
The scientific name of Arizona sycamore is Platanus wrightii S. Wats. (Platanaceae) [2,75,101,103,123,202,205]. Arizona sycamore intergrades with California sycamore [75,95,103,139] in southern California [75,103].

Common names are used throughout this Species Review. For scientific names of plant and animal species mentioned in this review and links to other FEIS Species Reviews, see the Appendix.

Reviews cited in this Species Review include: [20,153,173,213].

DISTRIBUTION AND OCCURRENCE

Figure 2—Distribution of Arizona sycamore. Map from Little (1976) [116] and digitized by Thompson et al. (1999) [197].

Arizona sycamore is native to the Southwest and northern Mexico. It occurs from central Arizona east to southwestern New Mexico [75,116,202] and south to northwestern Mexico. Isolated populations occur in Sonora and Sinaloa, Mexico [116] (fig. 2).

States:
United States: AZ, NM [202]
Mexico: Chih, Sin, Son [75,107,116,139]

SITE CHARACTERISTICS
Arizona sycamore is a warm-desert riparian species that grows in lowland and montane areas with bimodal precipitation. Most precipitation falls in summer [92,156]. Arid climate restricts this phreatophyte to moist areas [52,153,158]. It is an obligate riparian species [10,58,91,92,93] that grows only where there is abundant surface or ground water [93]; this includes intermittent, ephemeral, and permanent watercourses [55,133]. Arizona sycamore occurs near large rivers; smaller, higher-elevation first- and second-order streams [10,91,93]; and springs [57,75,213]. On National Forests of Arizona and New Mexico, Arizona sycamore-Arizona walnut communities were associated with west-flowing, low-gradient streams [194]. Arizona sycamore is "very tolerant" of flooding; it may withstand inundation for two or more growing seasons [21]. In the Southwest, high-velocity and long-duration floods can occur in winter and spring, typically after Pacific frontal rain- and snowstorms; floods of varying magnitude and shorter durarion occur frequently after convective thunderstorms during summer monsoon season; and occasionally flooding may occur in fall from dissipating tropical storms [6,172,187].

Arizona sycamore grows in canyons [40,57,75,103,130,153,205], ravines [217], and valleys [205]. It also grows along washes [209] and arroyos and on floodplains [153,166,167], alluvial fans [130], bajadas, and toeslopes [138].

Arizona sycamore grows in coarse, alluvial substrates [10,85,153] including sandy loam [177], sand [133,153,177], gravel [133,153,177], and cobble [153]. It does not tolerant saline soils [169]. Arizona sycamore grows in limestone-derived soils in the Huachuca Mountains, Arizona [215].

Arizona sycamore occurs in low- to midelevation (300–2,450 m) riparian communities [19,49,51,57,75,107,177,191,196] (table 1). In one of the first surveys of Arizona plant communities, Shreve (1917) noted that Arizona sycamore was "near its upper limit" at 1,800 m in riparian areas within evergreen oak and Arizona pine forests of the Santa Catalina Mountains [166,167].

PLANT COMMUNITIES
Arizona sycamore occurs in mixed-deciduous, oak, and pine-oak communities [28,115,205]. On the Coronado National Forest, Arizona, Arizona sycamore averaged 11% total cover across 15 riparian communities [48].

Arizona sycamore occurs in [2,28,129,152] and often dominates [19,141,148,162] mixed-deciduous riparian woodlands and forests. It is considered diagnostic of temporarily flooded deciduous woodlands of the Southwest [137]. Arizona alder, Arizona walnut, box elder, Fremont cottonwood, Goodding's willow, green ash, narrowleaf cottonwood, and velvet ash are frequent codominants or associates [10,20,22,49,51,85,148,151,191,193]. Arizona sycamore, Arizona sycamore-green ash, and Arizona walnut-Arizona sycamore riparian communities are common in Arizona and New Mexico [57,91,133,141,191,193]. In a few canyons and fault lines in west-central Arizona, Arizona sycamore grows in California fan palm-mixed deciduous oases with Fremont cottonwood, honey mesquite, and velvet ash [213].

The understory of mixed-deciduous communities with Arizona sycamore is typically composed of shrubs, including California brickellbrush, catclaw acacia, honey mesquite, and mule-fat [93,133]. The surface layer is composed of bunchgrasses, including sideoats grama and sand dropseed [133].

Arizona sycamore codominates some mixed oak and pine-oak communities [10,58], including scrub oak, oak woodland, and Madrean pine-oak communities near watercourses. Arizona white oak, Emory oak, silverleaf oak, and/or Sonoran scrub oak often codominate these oak communities [9,10,125,131,132,146,154,166,192]. Along with these oaks, Apache pine, and/or Chihuahuan pine also codominate the pine-oak communities [9,10,131,132,146,166]. In southeastern Arizona, Arizona sycamore-Emory oak-Arizona white oak/canyon grape-tasselflower brickellbush riparian communities occur on mesic, low-elevation sites [177]. Arizona sycamore communities on toeslopes may merge into Chihuahuan pine-Arizona white oak communities [10].

Arizona sycamore codominates or occurs in some juniper and cypress woodlands [48,106,142,196,216]. On the Coronado National Forest, Arizona sycamore/juniper (alligator juniper and/or oneseed juniper) communities were "extensive", occurring at high elevations (mean 1,603 m) for Arizona sycamore communities [48]. On the Tonto National Forest in central Arizona, Arizona sycamore codominates with Arizona cypress, and it codominates with smooth Arizona cypress to form a "minor forest type" with a chaparral understory [131]. In the Santa Catalina Mountains, Arizona sycamore codominates with Arizona cypress [138] and is an associate in Arizona cypress-Arizona alder-silverleaf oak woodlands [142].

Arizona sycamore occurs in some midelevation coniferous woodlands. On the Sierra Ancha Experimental Forest of Arizona, Arizona white oak-Arizona sycamore stands occur on sheltered, mesic sites within ponderosa pine forests [154]. Arizona sycamore is sometimes a component of bigtooth maple-white fir riparian communities of Arizona and New Mexico; it was present on 25% of sites surveyed (n = 300 total sites) [191].

Arizona sycamore merges into both desert and wet grasslands. At low elevations, it fingers into desert grasslands dominated by gramas [19,28]. It is occasional in bulrush (includes common threesquare and chairmaker's bulrush) riverbank communities in New Mexico [133].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• BOTANICAL DESCRIPTION
• SEASONAL DEVELOPMENT
• REGENERATION PROCESSES
• SUCCESSIONAL STATUS

Aboveground Description
Arizona sycamore is a large, spreading, deciduous [18] tree. It grows up to 25 m tall [40,75,103,115,123,210] and has diameter of 0.6 to 2 m at maturity [107,205]. It is the tallest tree in many riparian canyons of the Southwest [153]. The trunk is straight to prostrate [75]. Trunks are typically single-stemmed but sometimes multiple-stemmed [2]. Old trees typically have hollow trunks due to decay [59]. Lower branches are thick [75], and the branches tend to arch upward [123,153]. The bark is thin [115]. The outer bark is flakey, exposing the whitish inner bark [59,103,107,115,205]. Leaves are large and flat [59,205]. The inflorescence is a ball-shaped raceme [107] composed of numerous, minute flowers [205,210]. The inflorescence develops into a fruit cluster up to 20 cm in diameter [75] that is composed of achenes [2,22,75,103,123,139,153] (fig. 3). Achenes average about 6 mm long [202] and have basal tufts of hairs [2,103,123,187].

Figure 3—Arizona sycamore leaves and fruit clusters. Image by Sue Carnahan, SEINet Portal Network, used with permission.

Belowground Description
Arizona sycamore is deep rooted [51,55]. Seedlings and young plants may develop adventitious roots [153].

Stand or Population Structure
Arizona sycamore-dominated communities are variable in structure [161] (table 2), sometimes forming gallery forests [26] but often forming woodlands or forests, which may be dense. Mature Arizona sycamore-dominated forests often have a nearly closed [133] to closed [36] canopy. Arizona sycamore cover ranges from about 41% to 74% in communities in which it is dominant [193]. Clumps of Arizona sycamore trees may be clones resulting from sprouting [153].

Understory and surface layers are sparse to well represented in Arizona sycamore communities [10]. In Arizona, Fremont cottonwood-Arizona sycamore communities sometimes have "an almost impenetrable understory" of catclaw acacia and honey mesquite [93]. However, communities in scour zones [141] or that are used by off-highway vehicles (OHVs) [76] may lack or have sparse understories. Durkin et al. (1996) identified an Arizona sycamore community type on sandbars along the Rio Grande, New Mexico, with very little understory vegetation [64]. Population sizes of Arizona sycamore vary across elevational and watercourse-size gradients. In central and southern Arizona, Arizona sycamore populations in large watersheds were mostly composed of small trees (mean trunk diameter of <10 cm), while those at headwater sites were mostly mature trees with trunk diameters of up to 190 cm and ages of up to 235 years [187].

Mixed-deciduous communities with Arizona sycamore are usually uneven-aged [102,128,132] but sometimes even-aged. In Arizona sycamore stands by Sycamore Creek in the Mazatal Mountains, Arizona, overstory Arizona sycamores were 104 to 154 years old, with a cohort of understory Arizona sycamore saplings 13 years old. By Oak Creek on the Colorado Plateau, Arizona, Arizona sycamores were 53 to 73 years old, with no understory regeneration [186]. In a Fremont cottonwood-Arizona sycamore woodland in Santa Cruz County, Arizona, abundance of mature Arizona sycamores (>9 m tall) ranged from 0 to 6 trees per kilometer of stream segment. There were no seedlings (<2 m) or saplings (2-9 m) [85]. Maximum ages attained by Arizona sycamore are unknown [153].

Raunkiaer Life Form
Phanerophyte [150]

SEASONAL DEVELOPMENT
Arizona sycamore is cold-season dormant [36]; dormancy occurs from about mid-November to mid-March. Buds break from mid-March to early April, and leaves expand through late May. Leaf expansion and flowering occur around the same time in spring [16,75], from late March [22] to May [152] (table 3), depending on location and weather. Fruits ripen in fall [16,18,75], starting in October [16,20]. Most fruits disperse from mid-November into winter [16,22], but some disperse in spring [172,187]. Usually, the entire fruit falls, but some seeds may disperse from hanging fruit, especially in February and March [22]. Some fruits disperse during spring floods [20]. Arizona sycamore is winter-deciduous (fig. 1), although trees sometimes retain senescent leaves until spring [22]. Seeds germinate in April and May [187].

Vegetative Reproduction and Regeneration
Sprouting may be Arizona sycamore's primary method of regeneration on most sites [129]. Arizona sycamore sprouts [16,20,85,129,144,153,187] from the root crown, trunk, and/or stump after complete or partial top-kill [15,16,20,85,129,144,153,187]. It spreads by layering [16,60] and sprouting from the roots [85]. Sprout growth is rapid on favorable sites [139]. Sprouts (ramets) may comprise ≥50% of stems in a given population, particularly on sites subject to frequent scouring [187]. Near Sonoita Creek in Arizona, 39% of Arizona sycamore trees had root sprouts, 56% had trunk sprouts, and 22% had both trunk and root sprouts [85].

Sprouting is of particular importance to stands on upper banks. Reichhardt (1990) stated that the Arizona "sycamore populations on the higher terraces are able to persist and reproduce indefinitely by basal sprouting where surface moisture is otherwise not sufficient for seed germination" [153].

Pollination and Breeding System
Arizona sycamore is monoecious [2,16,18,20,107,153,205]. It is cross-pollinated [18,80] by wind [153].

Seed Production
Mature Arizona sycamore trees typically produce numerous flowers [103] and fruits. At the Appleton-Whittell Research Ranch in Arizona, Bock and Bock (1989) found fruit clusters on Arizona sycamore averaged 667 seeds each, with trees dispersing a mean of 449 seeds/m² [16].

Seed Dispersal
Wind [22,153,187] and water disperse the tufted Arizona sycamore achenes (in essence, seeds), which usually remain in clusters at dispersal [153]. Most fruit clusters drop under or near the parent tree; they may break apart upon hitting the ground [16]. Seeds disperse over several months [20]. Seeds that break away from fruit clusters that are still on the tree are most likely to be wind dispersed [22]. Water disperses some seeds during floods [20]. At the Appleton-Whittell Research Ranch, 90% of Arizona sycamore seeds dispersed from 10 m upstream to 16 m downstream of parent plants [16].

Seed Banking
Arizona sycamore may have a transient soil seed bank [158,172]; however, density and longevity of seeds in the soil seed bank have not been reported, and seeds do not retain viability over time (see Germination). In Cochise County, Arizona, Arizona sycamore seeds were present in soil samples collected from 0- to 5-cm depths. Viable Arizona sycamore seedlings emerged when the seeds were planted in a growth chamber [158].

Germination
Arizona sycamore seeds are germinable upon dispersal but lose viability rapidly [16,20,169]. Bock and Bock (1985) reported 90% viability of fresh seeds collected in Lyle Canyon, Arizona (Bock and Bock 1985, cited in [16]). Water stress, temperature fluctuations, and seed aging reduce viability; viability drops about 5% for every 2 weeks of seed age [20]. In the laboratory, seed viability averaged 60% for fresh seed and remained above 50% for seeds <6 weeks old, but viability dropped to nearly 0% after 28 weeks [169]. Seeds collected in southeastern Arizona had 0% viability after 1 year [16].

Seeds require soaking to germinate [153]. Scoured sand and gravel bars are favorable sites for Arizona sycamore germination [153]. Temperature range for germination is narrow. In the greenhouse, optimal germination occurred at 27 °C with no water stress (100 kPa water potential) and soil pH from 5 to 7 [169]. In the field, coincidence of requirements for Arizona sycamore germination and seedling establishment is probably rare [153].

Seedling Establishment
Seedlings require a continually moist substrate to establish, but also require protection from scouring [20,81]. Seedlings can establish with flooding if it is not prolonged. They cannot survive desiccation, prolonged drought, or rapid drops in groundwater [16,153,182]. Establishment is positively associated with wet years and high stream flows [81].

Seedlings apparently establish episodically, during years of high winter-spring flooding and low summer flooding [16,187]. In central and southern Arizona, Arizona sycamore stands went 10 to 40 years without substantial seedling establishment. Seedling establishment was positively associated with winter flood size, annual flow rate, and spring precipitation; and it was weakly negatively associated with summer flood size [187]. In central Arizona, a pulse of Arizona sycamore establishment occurred after high winter and spring flooding in 1993, and again after high spring flooding in 1995. Most seedlings (96%) established on active stream channels, where their density averaged 0.8 seedling/m² [81]. Seedlings and saplings usually die if soil moisture levels drop [153]. In Lyle Canyon, Arizona, "thousands of seedlings" died from desiccation during May and June, before summer rains began [16]. Seedlings may establish in clumps where the fruits break apart upon landing. These clumps thin over time, so only a few saplings remain [16].

Plant Growth and Mortality
Seedlings and saplings may grow rapidly on favorable sites [139], but prolonged drought or flooding hinders growth. Highest growth rates probably occur in spring [36], on sites with high groundwater tables. On nine sites in central and southern Arizona, annual radial growth of Arizona sycamore was greatest where annual groundwater fluctuation was ≤1 m, and groundwater depth averaged ≤2 m below ground during the growing season and ≤0.5 m below stream thalwegs [182,186]. By West Clear Creek in central Arizona, annual radial growth of Arizona sycamore averaged 6.6 mm/year. High stream flow on upper reaches of the stream were positively associated with annual radial growth, while high stream flow on the floodplain was negatively associated with annual radial growth [81]. On Sycamore Creek in central Arizona, Arizona sycamore growth was negatively associated with moisture stress [36]. Arizona sycamore produces new secondary and adventitious roots with prolonged flooding [21], assisting postflood recovery and growth.

Browsing may cause considerable juvenile [130,153] and sprout mortality. In particular, deer [187] and cattle [153] may greatly reduce recruitment of young plants and sprouts. However, Bock and Bock (1989) concluded that cattle browsing did not appreciably contribute to summer mortality of Arizona sycamore seedlings in southeastern Arizona. For seedlings that survived late spring drought, most losses occurred during flash floods in August [16].

Adult Arizona sycamores are typically infested with root rot fungi, which eventually results in tree death [59,159].

SUCCESSIONAL STATUS
Arizona sycamore is described as a "disturbance-dependent pioneer riparian tree species" [158]. If flooding is not intense, it pioneers on sandbars and gravel bars [8,97,153,182]. Its abundance tends to decrease in the absence of flooding or other disturbances [8]. In general, Arizona sycamore is considered intermediate in shade tolerance [153]. It can establish beneath its own canopy [133]; however, establishment is best on open sites, and saplings are more common away from tree canopies [22,153,172].

Flooding was historically the primary disturbance in mixed-deciduous riparian communities of the Southwest [72,171,181,212]. Scour, erosion, and deposition during floods kill and top-kill established plants and set back succession [37,128,212]. Early-successional stages are common in mixed-deciduous woodlands [37,212], and trees on heavily scoured sites may never reach maturity [212] (i.e., the stem exclusion stage). Flood intervals for Arizona sycamore woodlands in New Mexico average about 10 years [133]. Based on data collected in Arizona and New Mexico, Szaro (1990) suggested that Arizona sycamore/green ash stands were positively associated with areas with frequent or severe disturbance [194].

FIRE ECOLOGY AND MANAGEMENT

No information was available on the effects of fire to Arizona sycamore seeds, but seeds buried in soil may survive fire.

Postfire Regeneration Strategy
Tree with a sprouting root crown [179]

Other Possible Strategies:
Ground residual colonizer (on site, initial community)
Crown residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources) [179]

FIRE ADAPTATIONS AND PLANT RESPONSE TO FIRE
Few studies were available with information on Arizona sycamore’s response to fire. Fire adaptations of Arizona sycamore are not well known [12], but adaptations to frequent floods (i.e., sprouting and seeding establishment) also enable Arizona sycamore to establish when postfire conditions are favorable (see Regeneration Processes). Arizona sycamore sprouts from the root crown and/or bole after partial to complete top-kill by fire [15,213]. It may also establish from seed from off- and on-site seed sources, although this was not documented.

In 2003–2004 and again in 2012–2013, Bock and Bock (2014) measured the condition of riparian trees at the Appleton-Whittell Research Ranch, in three canyons above the San Pedro River that were burned by wildfire in 2002, 2009, or both. Among five deciduous tree species, Arizona sycamore and velvet ash had the largest percentage of individuals without noticeable damage after one fire. One Arizona sycamore was dead on the second sample date (after one or two fires), but most were completely or partially top-killed and sprouted. For Arizona sycamores on sites that burned once, 43% of survived as mature trees, 0% were killed, and 57% were completely or partially top-killed and sprouted. For Arizona sycamores on sites that burned once or twice, 21% survived as mature trees, 4% were killed, and 75% were completely or partially top-killed and sprouted [15]. See the Research Project Summary of this study for more information on the fire characteristics and response of Arizona sycamore and associated trees (Arizona walnut, desert-willow, Fremont cottonwood, and velvet ash) to these fires.

FUEL CHARACTERISTICS
Historically, fuels in low-elevation deserts of the Southwest were likely mostly sparse and discontinuous, although information on this is limited [65,108,212]. Keane et al. (2000) provide crown bulk density estimates for various structural stages of riparian forests and several other plant communities on the Gila National Forest. See the publication for further information [102].

Neither fuel loads in plant communities dominated by Arizona sycamore, nor the flammability of Arizona sycamore, were quantitively described. Fuels may be abundant in riparian woodlands with Arizona sycamore because subcanopy layers are often well developed [57] (see Plant Communities and Stand or Population Structure). Arizona sycamore's flakey bark [59,103,107,115,205], in addition to its large leaves, contribute substantially to the litter layer. LeRoy (2014) provides information on decay rates of Arizona sycamore litter in years of normal rainfall and drought [111].

Some mixed-deciduous riparian communities with Arizona sycamore have climbing ladder fuels. Canyon grape [10,177], western white clematis, and Virginia creeper—which often grow into upper canopies—are common in Arizona sycamore woodlands of southern Arizona [10].

Nonnative species can alter fuel distributions and loads. When riparian vegetation composition and structure change as a result of nonnative species invasions in the overstory and/or understory, fuel characteristics may also change. In turn, this can alter fire intensity, severity, and/or rates of spread [212] (see Fire Regimes and Management Under a Changing Climate).

FIRE REGIMES
In shrub and mixed-deciduous riparian communities of the Southwest and northern Mexico, wildfires are ignited by lightning in June and July [12]. Contemporary fires often spread into riparian areas from adjacent uplands [15,45], but fires can also start in riparian zones [212]. It is uncertain how frequent fires were within riparian areas of these deserts historically. Fires in small drainages likely had different frequencies and behaviors than fires in larger drainages where riparian zones were wider and vegetation more developed [15,143]. Fire intervals may be long and severity low in washes and riparian mixed-deciduous woodlands where periodic flooding scours the understory, limiting buildup of fine fuels and litter [212,213].

Information on historical fire regimes in riparian areas of the Southwest is limited due to lack of data [79,171,213], and the variability of historical fire regimes within and among desert riparian areas is not well known. The dynamic nature of riparian ecosystems makes their fire regimes hard to characterize [212,213]. Historically, sparse surface fuels and the variable patchiness of desert plant communities probably limited fire size, severity, and intensity, including in riparian zones [65,108,212]. Webb (2017, 2019) suggests that historical fires varied in severity and extent across southwestern riparian ecosystems. Most fires were likely patchy and of low severity due to the patchiness of vegetation in desert landscapes [110,212,213]; however, mixed- and high-severity fires likely occurred as well [110]. Fire might have been historically uncommon in some riparian areas [65,79,173,176,212,213]. In many cases, fires that started in the uplands might have slowed or stopped in riparian zones because temperatures were cooler and live fuel moistures higher [24,65,79,212]. For example, fires would often burn out when they reached borders of Arizona walnut-Arizona sycamore-velvet ash woodlands [51]. However, riparian pine-oak woodlands and forests in canyons might have facilitated fire spread between desert grasslands and higher-elevation forests [99,100].

In many areas, stand structure and function of mixed-deciduous riparian woodlands have been significantly altered since European-American exploration and settlement began [212]. American Indians likely burned riparian zones, although the extent of such burning is unknown [126,127]. Riparian areas experienced rapid ecological change [52,212,213] during European-American exploration (1540–1821) and settlement (1822–1912) periods [52]. In Arizona, for example, Spanish explorers imported livestock to the San Pedro Valley as early as the 1500s; European-Americans cut timber throughout the settlement period; miners diverted water from the San Pedro River to process ore in the 1800s; and fur trappers hunted American beavers until the animals were extirpated from the valley [212,213].

Changes in hydrology and vegetation have undoubtedly affected fire regimes of southwestern riparian communities and surrounding uplands. However, the degree to which fire regimes are changing—or what the effects might be for riparian ecosystem structure, function, and resilience—are not well known [212,213]. The greatest ecological change in riparian areas since large-scale livestock ranching was introduced is probably a shift in fire regimes from mostly patchy surface fires to larger crown fires [7,212]. Some research suggests that fire frequency and severity are increasing in some southwestern riparian ecosystems [33,34,63,67,79,96,188,190,213], although the extent of these increases are largely unknown. Wildfire became a significant disturbance agent in southwestern riparian areas at the end of the 20th century. With the reduction in frequency and magnitude of floods, litter and debris accumulated in the forest understory. This accumulation, along with increased density of native and nonnative vegetation, resulted in greater fuel loads, fire sizes, and fire intensities than existed prior to riparian modification (review by [173]).

Factors that can increase fire severity include drought, altered flood disturbance, anthropogenic ignitions in high-use areas [24,33,63,67,190,212,218], flood and fire exclusion and associated high fuel loads, and more continuous fuels [24,63,67,79,84,212,218] resulting, in part, from spread of nonnative plants [24,33,63,190,212]. Severe or intense fires can induce uniform changes in structure and composition across riparian zones by causing widespread mortality of native trees [11,212], increasing the likelihood of postfire establishment of nonnative plants [79,212,213]. Severe fires may be less common in areas with natural flood regimes [67,212,213].

Drought and/or human engineering can cause changes in hydrology (e.g., surface flows, groundwater levels, and channel and floodplain morphology) that favor nonnative plant species over native ones [34,63,79,96,155,213,221]. Nonnative annual grasses—particularly red brome, Mediterranean grass, and cheatgrass [23,24]—and the nonnative trees Russian-olive [220] and tamarisk (saltcedar and five-stamen tamarisk [219] and potentially Athel tamarisk [206] and French tamarisk [35] can increase fuel continuity and alter fire regimes in riparian communities of the Southwest [23,24] see Fire Management Considerations). Fires may be larger, more frequent, and more severe in invaded communities [24]. For example, fire-induced mortality of native riparian trees can increase dramatically when tamarisk occurs in the prefire plant community [212]. On 30 sites across the Southwest, high fire consumption and postfire mortality of Fremont cottonwoods and willows were positively associated with high prefire tamarisk cover. Fire was more severe where tamarisk was present than where it was not, resulting in more tissue damage to native trees. When prefire tamarisk cover was >50%, fire completely consumed the fine tissues of all native trees present [63].

Presettlement fire regimes of mixed-deciduous riparian communities of the Southwest likely varied among sites [212,213]. LANDFIRE (2008) models classify fire regimes of warm desert riparian Biophysical Settings dominated by cottonwood-willow communities in the Southwest into two groups: Fire Regime Group I (<35-year fire intervals, mixed severity) and Fire Regime Group V (>200-year fire intervals, replacement severity). Communities in the frequent-fire group are generally those along larger rivers, and those in the infrequent fire group are generally smaller desert riparian areas (i.e., stringers) [110]. Madrean oak (encinal) communities with Arizona sycamore likely experienced mostly frequent, low- and mixed-severity fires [100,110,156]. LANDFIRE (2008) models classify fire regimes of southwestern Madrean pine-oak-juniper Biophysical Settings in Fire Regime Groups I and III (35- to 200-year fire intervals, low and mixed severity) [110], although those are general models and not specific to riparian areas with Arizona sycamore.

• Desert riparian
• South-central oak

FIRE MANAGEMENT CONSIDERATIONS
There is limited research on the impacts of fire on southwestern riparian communities [213]. Dams, waterway diversions, and climate change have greatly altered the hydrology of many streams and rivers. In the absence of natural flooding and with lowering water tables, fire is becoming the dominant disturbance in some riparian ecosystems of the Southwest. Groundwater depth and streamflow greatly influence vegetation type and extent in riparian ecosystems [33,84,96,212], and fire regimes seem to be changing in some lowland riparian communities with altered hydrology. Changing fire regimes are likely to have drastic, potentially irreversible effects on biodiversity and function of affected riparian communities. Alterations in plant community structure and plant and animal community composition may occur, including declines in wildlife species of concern [213].

More severe, intense, and/or frequent fire can interact with altered hydrology to accelerate the replacement of native plants with nonnative plants [114,212]. In many riparian areas in the Southwest, there have been increases in tamarisk, Russian-olive, Mediterranean grass, and other nonnative plants that have different fuel characteristics, fire tolerances, and postfire fire responses than native species. Establishment and spread of tamarisk [157,219] and other nonnative species have changed fuel and fire regime characteristics in many riparian areas, and increased fire risk has been associated with high cover of tamarisk and Russian-olive [33,63,134,155,160,212,213]. Where groundwater levels have dropped, tamarisk has a competitive advantage over cottonwoods [114,212]. Its higher tolerance to drying soils [178] and frequent fire are driving changes in riparian stand structure and fire regimes, resulting in decreasing cover of formerly dominant, native trees such as Arizona sycamore, cottonwoods, and willows [213,219]. Fire intervals in mixed-deciduous woodlands may be as frequent as 15 years in areas where tamarisk cover is high [63,109,149,212,213]. Where natural flow regimes are more intact and tamarisk is suppressed by native vegetation, Arizona sycamore and other native trees are more likely to dominate postfire landscapes [213].

Fire management in southwestern riparian areas may include fuel reduction and prescribed fire treatments that promote or maintain native vegetation [213] and reduce or eradicate invasives [168,212]. Although simulated fuel reduction treatments showed potential to mitigate fire severity in some mixed-deciduous woodlands and forests under projected climate change in the Huachuca Mountains, projected basal area and mortality rates in riparian zones were unchanged by simulated fuel treatments, and a reduction in basal area and spatial extent of riparian species occurred with or without fire and regardless of treatments [140]. Prescribed fire may be most effective when used in combination with other restoration, such as restoring natural waterway flows [213]. However, little information was available on using prescribed fire in riparian plant communities of the Southwest [12]. The effects of fire, including prescribed burning, have been better studied in southwestern grasslands and savannas than in adjacent riparian woodlands and forests [15,65,79,171], so more information is needed on the use of prescribed fire and postfire restoration in riparian areas. Fire may be severe, and lethal to overstory deciduous trees, in riparian woodlands that have not experienced fire for many decades.

Bock and Bock (1990) do not recommend prescribed fire in late-successional riparian woodlands of the Southwest because "fire is difficult to manage and potentially very destructive" in those habitats [14]. Refined fuel and fire behavior models are needed for riparian ecosystems of the Southwest, including models for areas in which tamarisk has become dominant. Webb et al. (2019) review information available for using prescribed fire in southwestern riparian areas, with and without invasive species [213].

OTHER MANAGEMENT CONSIDERATIONS

OTHER STATUS
Arizona sycamore and Fremont cottonwood-Arizona sycamore woodlands and Arizona sycamore-Arizona alder/mule-fat forests of the Southwest are ranked Imperiled (G2) [136]. Other information on state- and province-level protection status of plants in the United States and Canada is available at NatureServe.

IMPORTANCE TO WILDLIFE AND LIVESTOCK
A myriad of wildlife species use riparian habitats [53,54,90,118,128,210,211,213]. Riparian areas comprise a small part of total land area in the southwestern United States and northern Mexico, but they are extremely important to biotic communities [213]. Riparian areas are among the most diverse and productive ecosystems in desert bioregions, supporting floras and faunas that are biologically richer than adjacent uplands [28,39,43,213]. Riparian areas and washes provide water, cover, and breeding habitat for many species of wildlife, including obligate riparian species [77,128]. For wildlife that also use uplands, riparian areas provide refuge from the harsher, surrounding environments [128]. Wildlife that use riparian ecosystems for breeding, migration, and wintering habitat include large ungulate [53,213], small mammal, bird [3,53,90,154,189,211], reptile, amphibian, and arthropod species [29,32,53,154,213]. Deciduous woodlands with Arizona sycamore provide birds and small mammals with nesting sites and foraging opportunities that are often absent in upland plant communities [16,39,174]. Many invertebrate species live in and consume Arizona sycamore litter [111]. Riparian ecosystems provide habitat for many threatened or endangered species. Of 40 Federally listed Threatened or Endangered wildlife species in New Mexico—which includes several endemic species—at least 70% require aquatic and/or riparian habitat to feed, reproduce, and/or carry out their life cycles [200,213]. Riparian areas and washes also provide water, cover, and breeding habitat for livestock [31,77,128].

Southwestern riparian woodlands with Arizona sycamore provide breeding, wintering, and migration corridor habit for a variety of bird species [14,28,39,73,82,189,191,192,196]. In the Huachuca Mountains of southeastern Arizona, Fremont cottonwood-Arizona sycamore woodlands had the highest bird species richness among 25 riparian communities [189]. Many bird species selectively nest in Arizona sycamore (see Cover Value). Because its wood decays easily [1,59], Arizona sycamore is often selected by primary cavity nesters such as the acorn woodpecker [189] and secondary cavity nesters such as Lucy's warblers [173] and whiskered screech-owls [73] (fig. 4). On the Buenos Aires National Wildlife Refuge, Arizona, richness of passerine species increased with Arizona sycamore density [147], and 41% of all nests observed in a mixed oak-Arizona walnut-Arizona sycamore and Arizona sycamore-netleaf hackberry communities were in Arizona sycamore trees [146]. See these sources for lists of bird species that use riparian habitats with Arizona sycamore as breeding/nesting habitat: [39,90,147,189].

Figure 4—A whiskered screech-owl sheltering in an Arizona sycamore cavity. Image by Brian Henderson, used with permission.

Many raptors use woodlands with Arizona sycamore for hunting, resting, and/or nesting habitat. The ferruginous pygmy-owl uses Arizona walnut-Arizona sycamore-velvet ash riparian woodlands habitats in southern and central Arizona, where the owl is at the northern end of its distribution [41]. Bald eagles [87], Cooper's hawk [154], peregrine falcons [70], flammulated owls [73], Mexican spotted owls [120], western screech-owls [120], and whiskered screech-owls [73,120] also inhabit communities in which Arizona sycamore is dominant or important.

Arizona sycamore communities are important habitat for mammals [196]. Plains bison in the Chihuahuan Desert use a mosaic of habitats that include Arizona sycamore-Arizona walnut, oak-pine woodland, and plains grassland communities [113]. The Mexican gray wolf hunts in woodlands with Arizona sycamore [198]. Small mammals using riparian habitats of the Southwest include the desert gray shrew [44], Chiricahua fox [104,141] and Arizona gray [46,47,154] squirrels. In the Huachucha Mountains, Arizona gray squirrels strongly selected Arizona sycamore for nesting (8.4% expected use based on availability, 50% actual use) [46]. See Szaro (1991) for a list of mammals that use deciduous woodlands with Arizona sycamore [196].

Many herptiles also use Arizona sycamore habitats. Deciduous riparian woodlands are particularly important to tree-foraging reptiles [196] (e.g., tree skinks). Herptiles that use riparian communities with Arizona sycamore include the black-necked garter snake, lowland leopard frog [207], northern leopard frog, and canyon treefrog [118], and Sonoran mud turtle [88].

Several animal species that use deciduous woodlands with Arizona sycamore have federal and/or state protection status. The Federally endangered southwestern willow flycatcher and Federally threatened yellow-billed cuckoo [203] use Fremont cottonwood-Arizona sycamore-Arizona walnut woodlands habitats, and nest in mature Arizona sycamores [172,173,180]. Federally threatened Mexican spotted owls [203] inhabit canyon forests with Arizona sycamore [120]. In central Arizona, bald eagles (Arizona-New Mexico watch list [5], federally protected [199]) preferentially nest in cliffs above Arizona sycamore-green ash-Arizona alder communities [87]. Peregrine falcons (Arizona-New Mexico watch list [5], federally delisted [203]) also nest above and hunt in deciduous woodlands with Arizona sycamore [70]. The Federally endangered Mexican gray wolf [203] also uses Arizona sycamore habitats [198]. The Federally threatened Gila trout [203] uses aquatic habitats adjacent to deciduous woodlands with Arizona sycamore [25].

Palatability and Nutritional Value
Cattle and other livestock browse Arizona sycamore sprouts and seedlings, although they may not be preferred. In a Fremont cottonwood-Arizona sycamore woodland in Santa Cruz County, Arizona, cattle selected Fremont cottonwood sprouts and seedlings over those of Arizona sycamore [85]. Chiricahua fox squirrels eat Arizona sycamore leaves sparingly [104]. The fruits are a minor food for Arizona gray squirrels [47]. The flowers are not attractive to or eaten by insects, other invertebrates, or vertebrates [98].

See LeRoy (2014) for information on the nutritional value of Arizona sycamore litter [111].

Cover Value
Arizona sycamore branches are used as roosting perches [163], and the bark provides foraging opportunities for gleaning and probing foragers. On the Sierra Ancha Experimental Forest, the bridled titmouse and Hutton's vireo fed on Arizona sycamore bark [154]. Bird species that selectively nest in Arizona sycamore include the bridled titmouse [154], buff-breasted flycatcher [122], Cassin's kingbird [154], elegant trogan [89], greater pewee [42], painted redstart [121], and western kingbird [154]. Ground nesters that select Arizona sycamore habitats include the Montezuma quail [208] and Gould’s wild turkey [154].

High-quality fish habitat depends on suitable water temperatures for spawning and survival [53,86]. Arizona sycamore provides shade cover for fish, including the Federally Endangered [203] Apache and Gila trout [25,53].

VALUE FOR REHABILITATION OR RESTORATION
Arizona sycamore is planted for erosion control [103,115] and restoration of riparian areas [20,56,62]. It is recommended for planting and maintenance in riparian areas to help prevent invasion by tamarisk [56].

Arizona sycamore is propagated from seeds [153,205], stem cuttings [62,144,153,205], and mound layering [60,62], although a greenhouse study found Arizona sycamore was difficult to establish from either stem cuttings or tissue culture [145]. See these sources for information on propagation: [18,61,144,153].

OTHER USES
Riparian areas with Arizona sycamore provide many ecosystem services including recreational opportunities, harboring pollinators, providing livestock forage [170,213], and enhancing water quality [213]. Riparian areas reduce the delivery of sediments to stream channels, maintain streambanks stability, and filter excess nutrients that occur in runoff water [24,177]. Riparian vegetation reduces nonpoint source pollution in waterways by filtering sediments, chemicals, and nutrients from overflow [24,177,201]. The roots of streamside plants help prevent excessive erosion during floods [201] and promote groundwater infiltration [71], and vegetation surrounding active channels helps slow water velocity during floods [201]. Riparian trees and shrubs shade surface water, reducing water temperatures [201] and lost to evaporation [213].

Arizona sycamore is planted as an ornamental [2,205]. It is an important shade tree with high aesthetic value. The gray and white, variegated bark that results from shedding—and the reddish-brown leaves that may be retained into late winter—provide color to winter landscapes [22].

Sycamore wood is dense [205] and moderately strong, but it decays easily. See Alden (1995) for descriptions of sycamore wood [1].

ADDITIONAL MANAGMENT CONSIDERATIONS
The value of southwestern riparian woodlands for both biotic communities and human livelihoods is well established [53,54,94,135,173,183,204], and maintaining and/or restoring riparian areas of the Southwest is a critical management issue [53,54,124,173]. Management goals for southwestern riparian areas may include restoring floodplains, safeguarding high-quality riparian areas, reducing fuels, preventing spread of fire, and preventing establishment and spread of nonnative invasive plant species [168,212].

Many riparian communities of the Southwest are highly imperiled due to anthropogenic disturbances [170]. Few naturally functioning riparian areas remain in the region, and those that do are threatened with climate change [128,213]. Within the last 100 to 150 years, humans have modified many riparian areas extensively by irrigating, pumping groundwater, building dams and diversions, clearing floodplains, cutting wood, using off-highway vehicles [76], grazing livestock [30], and spreading nonnative invasive plants. These modifications have resulted in significant changes in the physical environment and biota [68,128,129,184,213]. For example, livestock generally use riparian areas more than adjacent, less productive upland communities, compacting soils and reducing recruitment of palatable deciduous seedlings and saplings into the overstory [50,128]. Livestock grazing may be responsible for the reduction or extirpation of Arizona sycamore in parts of southwestern New Mexico and southeastern Arizona (Spellenberg, personal communication cited in [17]).

Altered hydrological regimes are often associated with establishment and spread of nonnative plants. Altered hydrology and sediment deposition resulting from river impoundment may reduce or eliminate germination and recruitment of native deciduous trees while promoting germination and recruitment of tamarisk [213]. Consequently, tamarisk species have established in many riparian areas of the Southwest [6,128,157,185,188,219], including communities in which Arizona sycamore was formerly dominant or important [27,56,117,119] (see Fire Management Considerations). Maintaining an ecologically healthy riparian community can help stop establishment and spread of tamarisk and other invasive plant species [4]. Dense shade in closed-canopy communities with Arizona sycamore may inhibit establishment of tamarisk [56].

Flow regimes that fall within the natural range of variability for undammed waterways can have multiple benefits, including:

• increasing surface and groundwater availability for native vegetation [33,134,212],
• promoting flood scour, which clears surface fuels and discourages recruitment of understory plant species [112,212],
• reallocating sediments and nutrients that help support native tree species [33,112,212], and
• promoting geomorphological stream and floodplain structure that promotes native tree species, including germination sites [134,135,212].

MANAGEMENT UNDER A CHANGING CLIMATE
Southwestern riparian ecosystems are vulnerable to decline under hotter and drier conditions [38,183,212,213]. Climate change and variation—and associated impacts on hydrology, channel morphology, and riparian vegetation—may alter fire regimes, increasing the likelihood of more frequent or severe wildfires [15,65,175,213]. Prolonged drought can increase fire risk by decreasing water availability and reducing fuel moisture [143,212,213].

Changes in water flow are expected with climate change, including lower water tables and increased frequency and severity of extreme weather that will likely result in more frequent and severe floods as well as more intense droughts [66,79,164,171,173,212]. Warming in montane areas may increase the number of severe floods downstream by increasing the rate of snowmelt into downstream waterways such as the Middle Rio Grande [212]. Monsoons may occur later in the year and with greater severity [173]. Droughts are likely to be increasingly severe and prolonged, resulting in further reductions in discharge volume [173].

APPENDIX

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