c o n n e c t :

Biological Survey of Ironwood Forest NationalMonumentNatural History of the Desert Ironwood Tree (Olneya tesota)

Photographs by Mark Dimmitt

Flowering ironwood tree, SaguaroNational Park

Ironwood in lush foliage, Ironwood Forest NationalMonument

Synopsis of published literature Compiled by Tani Hubbard

The tree known in the U.S./Mexico borderlands as desert ironwood or palo fierro (Olneya tesota) is one of many woodylegumes found in washes and hillside drainages in the Sonoran Desert. It ranks among the most ecologically andeconomically important plant species in the region. Ironwood functions as a “nurse plant” and a “habitat-modifyingkeystone species” of benefit to many other species of flora and fauna. While Ironwood is not endangered or threatened,its populations dwindle annually over tens of thousands of square kilometers. Ironwood is nearly endemic to the Sonoran Desert (Turner et al. 1995). The species was first described in 1854 as thesole species of the genus Olneya by botanist Asa Gray and is still recognized as a monotypic genus (Lavin 1988).Ironwood is similar in morphology to only two other legume genera, peteria (Peteria sp.) and brushpea (Genistidium sp.).While all three genera have narrowly elliptical leaves and less than 12 ovules per pod, Olneya is distinguished by its pairedleaves, flower clusters on short shoots that extend from the middle of the stem (instead of the end of a branch), andpods more rounded than the pods of peteria and brushpea.

Ironwood as a species may have evolved as the Sonoran Desert flora formed inthe middle Miocene (ca. 15 to 8 million years ago) (Van Devender 2000), butmost paleogeological records of Ironwood date from the mid- to late Holocene.Dating of ironwood trees is difficult through standard tree-ring dating, butannual trunk diameter growth rates (Turner 1963; Suzán 1994) and unpublished

Biological Survey ofIronwood ForestNational Monument

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Distribution and Statusof Saguaros and Trees

Natural History of theDesert Ironwood Tree(Olneya tesota)

Rare Plant Inventory

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Geologic Aspects ofIronwood ForestNational Monument

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V I S I T A B O U T A R T S M E M B E R S H I P C O N S E R V A T I O N E D U C A T I O N S U P P O R T S H O P K I D S H O M E

Ancient ironwood stump

Map courtesy Bill Singleton, PimaCounty (AZ) Administrative Office

Dense Arizona Upland forest ongranite near Ragged Top, IronwoodForest National Monument

radiocarbon dating estimates (Suzán 1994) suggest that some trees havepersisted for more than 800 years. The wood of the Ironwood is one of thehardest and heaviest woods in the world (Búrquez 1999). It is remarkablyresistant to rotting, perhaps because its heartwood is rich in toxic chemicals thatmake it essentially non-biodegradable (Dimmitt 2000a). Ironwood trunks canpersist for up to 1600 years (Dimmitt 2000a). (See further discussion ofironwood longevity below.)

Geography

The geographic limits of ironwood distribution are closely matchedwith the boundaries of the Sonoran Desert. Ironwood barely reachesinto adjacent Mohave desertscrub, coastal thornscrub south ofGuaymas, Sonora, and foothills thornscrub east of Hermosillo,Sonora. It occurs in five states and territories within the SonoranDesert region: southwestern Arizona, southeastern California, easternBaja California, Baja California Sur, and Sonora, Mexico. Populationsoccur from sea level to 1100 m (3280 ft) in elevation, where lowwinter temperatures and catastrophic freezes limit its distribution.Near its northern limit ironwood grows best on rocky benches andslopes, above the valley bottoms that characteristically have cold airpockets at night that would damage leaves and young branches(Turner et al. 1995). While ironwood occurs in all six subdivisions ofthe Sonoran Desert, it varies greatly in its density and relativedominance among these regions. In the U.S., the highest ironwooddensities recorded per hectare are in Arizona Upland sites in PimaCounty (Ragged Top, 35 trees/ha = 14.2/acre; Cocoraque Butte[Roskruge Mountains] and Saguaro National Park West, 22 trees/ha = 8.9/acre) (ASDM 2000). In the IFNM surveysome of our 0.4-hectare census plots had much higher densities that would extrapolate to more than 300 ironwoods perhectare. Ironwood densities are much lower in Mexico (mean density of 6.6 trees/ha = 2.4/acre across many plots incoastal and central Sonora). Elevational range is also greater in the species’ northernmost limits in the Arizona Uplandsand Lower Colorado River Valley.

The densest stands of ironwoods and palo verdes occur where the soilis derived from Precambrian Oracle granite. This granite ischaracterized by large crystal size and it weathers into a coarse, veryporous soil that allows deep infiltration of water and air. This porous,well-aerated soil permits tree roots to penetrate deeply to reach thedeep moisture. Soil explains much of the lushness of the tree growthin the Silver Bell region. The reason for the greater diversity of plantsassociated with ironwood trees here compared with other regions ofthe Sonoran Desert is not known.

Characteristics, Phenology, and Physiology

Ironwood may take the shape of either a multi-trunked shrub no more than two meters in height, or a canopy-formingtree with one thick trunk achieving heights up to 15 meters (49 ft; Shreve and Wiggins 1964, Solís-Garza 1993, ArizonaRegister of Big Trees 2000). The largest known ironwood, located close to Gila Bend, measures 4.32 meters (14.2 ft)

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Ironwood leaves usually turn yellowand are shed in April before thetrees flower.

around its trunk, with a canopy height of 15 meters 49 ft), and a crown spread of 14 meters (46 ft; Arizona Register ofBig Trees 2000). The ironwood “leaf” is doubly divided into 4 to 12 pairs of narrowly elliptic leaflets called pinnae.Each leaf consists of two to four “fingers” with paired leaflets down the sides of each. This compound leaf has a pair ofsmall curved spines at its attachment to the branch (Dimmitt 2000a). These leaflets have a bluish-green cast to them,producing a mottled canopy quite unlike the yellow-green of mesquites (Prosopis spp.) and palo verdes (Parkinsoniaspp.) growing in the same region. Ironwood’s clusters of flowers bloom on the end of short shoots along the branches(Lavin 1988).

Ironwood trees flowering at the Arizona­Sonora Desert Museum(Tucson Mountains)

In Tucson, ironwood flowers and fruit occur in most years, but areabundant only four years per decade (Dimmitt 2000a). This variablelevel of flowering and fruiting, along with differences in rainfall eachyear, may cause a pattern of mass seed production and seedlinggermination that occurs as occasional bursts. Suzán (1994) observed thisunpredictable pattern of germination, known as “discrete episodicrecruitment.” Flowering and fruiting require considerable diversion ofnutrients and energy from other parts of the plant. Branches thatproduce flowers often drop their leaves during bud formation, and re-leaf when summer rains begin. In some years flowering does not occurat all (Dimmitt (2000b). Patterns of flowering and fruiting generallyoccur in a south to north wave. Flowers and fruit occur as early asMarch in the southern states of Sonora and Baja California, Mexico, than in Arizona and California to the north.

The flowering period in each locality lasts only 10-18 days. Theseflowers attract one generalist bee and two solitary specialist bees. Afterpollination occurs, ironwoods produce slightly curved, knobby podsthat reach lengths of 3-6 cm (1.2-2.4 inches) and widths of 8-9 mm(ca. 0.4 inch). These pods contain one to eight ovoid, shiny, coffee-colored, and extremely hard-shelled (at maturity) seeds (Solís-Garzaand Espericueta 1997). Seed maturation coincides with the summerrains, increasing the probability of immediate germination (Shreve andWiggins 1964). Maturation occurs within four to eight weeks ofpollination (late June through August) (Turner et al. 1995). The seedsare small and light compared to those of blue palo verde or theindehiscent pods of mesquite (4,440-4,480 seeds per kg. Dry weight)

(Kraugman 1948). They are high in protein and soluble fiber, but they also contain bitter chemicals that serve asdeterrents to herbivores, reducing palatability and digestibility.

Ironwood seeds mature at a time when little else is producing fruit inthe Arizona Upland region (Dimmitt 2000b), leading to a highdependence of wildlife on the seeds. Many animals gather and store

Ironwood tree in fruit.

This ironwood tree founded aminicommunity. The young paloverde in the foreground andthe large saguaro (as well assmaller plants not visible)established beneath its canopy.Now several small saguaros andshrubs are growing in the shelterof the palo verde. The adjacentground is comparatively open.Tucson Mountains, AZ.

Saguaro seedling beneath a paloverde tree.

ironwood seeds in caches to be eaten later. Roughly half of all newgerminated seedlings found for plants such as jojoba (Simmondsiachinensis) and palo verde occur in tight clusters near rodent burrows(McAuliffe 1990). While studies have not been conducted forironwood, it is likely that ironwood seedlings germinate from rodentcaches.

Ironwood grows extremely slowly, perhaps due to its low rates ofphotosynthesis that keep it from wasting soil moisture. These slowrates of biomass accumulation contribute to the remarkable density ofits heartwood. Like other desert legume trees, ironwood trees conservewater during the high daytime temperatures and during dry seasons.Ironwoods lose less water through their leaves than other woodyperennial plants. During long droughts, the trees slough off leaves, limbs and rootlets to reduce their water needs. Thewater use efficiency of ironwood ranks with some of the most drought tolerant Sonoran Desert plants, such ascreosotebush (Larrea), bursage (Ambrosia), and wolfberry (Lycium spp.) (Szarek 1979). Considering ironwood’sconservative growth rate, small leaves, diffuse canopies, and preference for arid and hyper-arid xeroriparian soils, it isnot surprising that they exhibit relatively low levels of annual net primary productivity (55 g. dry weight/m2/yr. = 15.6oz/yd2/yr) as well as low gross productivity (7.42 kg = 16.3 lb. Dry weight/tree/yr) (Szarek 1979).

Ecological Importance

The ecological importance of the ironwood tree comes largely through theroles it plays for over 500 other species in the Sonoran Desert (Arizona-Sonora Desert Museum 2000). Ironwood trees function as a habitat-modifying keystone species, that is, a species that exhibits strong influenceson the distribution and abundance of associated species (Mills et al. 1993). Achain of influences generated by ironwoods on associated understory plantsaffects their dispersal, germination, establishment, and rates of growth aswell as reproduction. These ecological dynamics are termed “nurse plantecology.” Other large trees co-occur with ironwoods along washes, butironwoods may be the only tall branching woody plants on the valley floorsor bajada slopes (Vander Wall 1980). Their relative influence on plant andwildlife diversity is proportionally greater in plains and rocky slope habitatsabove ephemeral and intermittent watercourses. Along watercourses,ironwood is but one of many nurse plants available. In addition, the size andfoliar density of an ironwood are strong factors influencing their relativevalue as nurses. Medium-sized mature ironwoods harbor a greater diversityof understory plants than either ironwood saplings or the largest, ancientshade-forming ironwoods (Tewksbury and Petrovich 1994, Suzán et al.1996). Some mid-sized trees, however, do not necessarily serve as nurses formany plants, especially if grazing is heavy.

Mesquite and palo verdealso serve as nurse plants,

however, each tree caters to slightly different sets of plants in its“nursery.” Ironwood is the dominant nurse plant in some subregionsof the Sonoran Desert. As nurse plants, ironwoods provide safe sitesfor seed dispersal, protect seedlings from extreme cold and freezes,protect saplings from extreme heat and damaging radiation, andfunction as prey refugia. Also, like other legumes, they alter the soilcomposition beneath their canopies, enriching the soil with nutrients

The ironwood (right­center) is severaltimes older than the same­sizefoothill palo verde to the left. Theironwood also has a denser canopy(next paragraph).

This exposed juvenilesaguaro has been eatenby a jackrabbit (see scatpellets around base). If itwere growing under alarger nurse plant thanthis shrub, it wouldprobably have escapedpredation.

such as nitrogen.

An ironwood canopy typically has been functioning as a safe site forseed dispersal for three to four times longer than mesquite or paloverde canopies of the same volume. The long life span of ironwoodtrees and the stability of the microenvironments they create increasethe probability that seeds might be dispersed to these “safe sites” forgermination and establishment (Tewksbury and Petrovich 1984). Dueto the fact that ironwoods tend to be the tallest trees in desertscruband xeroriparian vegetation (Vander Wall 1980), they function as theprimary roosts in their landscape for both local breeding and migratingbirds. Ironwoods and their nurseries make the structure of vegetationmuch more diverse providing birds with more nesting opportunities.Birds in turn generate a literal “rain” of seeds and whole fruit. Partiallydigested fruit from this “rain,” or from defecation of other animals,are torn apart by animals seeking to gain sustenance while selectingout toxic or distasteful portions of the fruit. Seeds also flow into the

areas underneath ironwoods during storms and floods where they are trapped by exposed tree roots, or by the stems,roosts, and litter of understory herbs, vines and shrubs.

Ironwood canopies provide microenvironments buffered from freezes for understory plants. Suzán (1994) determinedthe winter microenvironments under mature ironwood trees may be 4 C (6.6° F) warmer than adjacent openenvironments and 1 C (2.8° F) warmer than under other vegetation. Studies of cactus seedling vulnerability demonstratethat without the protective cover of desert legumes, the distributional ranges of saguaro (Carnegiea gigantea), organ pipe(Stenocereus thurberi) and senita (Lophocereus schottii) would retreat many kilometers to more southerly, frost-freeareas (Nobel 1980). In addition to frost protection, nurse plant canopies provide relief from heat and radiation stress.They reduce the exposure that leads to tissue damage and destruction of understory seedlings and saplings (Suzán 1994,Tewksbury et al. 1998). When stripped of ironwood’s protective cover above them, some cacti actually suffer sunburnand die (Nabhan and Suzán 1994). Where open soil temperatures can reach 65 C (148°; F), the 15 Centigrade degrees(27 Fahrenheit degrees) cooler temperatures under ironwood canopies increase seedling survival rates and decreasewater stress in mature plants (Suzán 1994).

In addition to serving as a buffer from abiotic stresses, ironwood buffers nurseryplants from some, but not all, biotic stresses impacting their survival andreproduction. McAuliffe (1984a) demonstrated spiny, or thorny nurse plants candramatically reduce predation on cactus seedlings by large and small herbivores,such as ungulates, rabbits, and rodents. The spiny, low-sweeping branches of theironwood provide an effective prey refugium for vulnerable seedlings intertwinedwithin its foliage. However, seedlings not fully sheltered by down-sweepingbranches can suffer higher levels of predation due to the resting, nesting, orburrowing of desert tortoises, rabbits, jackrabbits, and packrats under ironwoods(McAuliffe 1984a).

Legumes such as ironwood and mesquite influence the soil composition beneaththeir canopies in several ways (Garcia-Moya and McKell 1970). Ironwoods “fix”nitrogen through symbiotic relationships with Rhizobia bacteria (Felker and Clark1981). They also “pump” nitrogen and other nutrients up from their deepest rootzones. Ironwoods incorporate these nutrients into their foliage, over time enrichingtopsoil composition as their fallen leaves gradually accumulate and decomposesbeneath their canopies. Ironwoods and mesquites also act as traps for the nutrient-rich organic debris carried by flash floods (Nabhan 1993). The “resource islands”around ironwood and mesquite trunks support high densities of symbiotic bacteriaand fungi that aid in the establishment of understory plants, providing them withmoisture and nutrients not available in barren interspaces. The differences in the

Ironwood Forest National Monumentviewed from Saguaro National Parkto the west. Agriculture andurbanization in the intervening AvraValley could isolate the two,fragmenting the habitat into smallerparcels that will not support somewide­ranging species such asmountain lions. The WatermanMountains are on the middlehorizon, with the Silver Bell Mountainsand Ragged Top to the right. TheRoskruge Mountains are just visibleon the far left horizon, and the tallerSanta Rosa Mountains on theTohono O'odham Nation to theirright.

mycorrhizal fungi and soil composition under ironwoods and mesquites allow them to favor different sets of understoryplants creating heterogeneity through “patch dynamics.” Ironwoods tend to slightly increase alkalinity and moistureavailability, hardly effect soil texture, but significantly increase root, bacteria, and fungi densities where mesquitesdecrease soil alkalinity and increase clay content and moisture availability.

Threats

Ironwood habitat faces threats from habitat fragmentation due tourbanization and conversion of natural habitat to agricultural lands.The population explosion in the Sonoran Desert over the past 50 yearshas also led to increasing recreational impacts in ironwood habitat.There are also preliminary indications that both woodcutting andbuffelgrass competition can decrease understory species richness anddiversity (Suzán 1994, Búrquez and Quintana 1994). Ironwood cuttingcan result in greater damage to understory plants (Nabhan and Suzán1994, Suzán et al. 1999). Nurslings exposed by woodcutting have agreater probability of damage from radiation, breakage fromtrampling, and death due to browsing (Nabhan and Suzán 1994). Solís-Garza and Espericueta (1997) have confirmed that virtually noironwood regeneration had occurred to date in areas wherecommercial woodcutting has been permitted in Sonora. Buffelgrass ishighly invasive, decreases plant species richness and diversity in nativeplant communities, and increases fire frequency. Fires in communitiesinvaded by buffelgrass tend to be hot burning and destroy ironwoodand other trees, shrubs, and cacti.While ironwood is not considered endangered because of its largerange, it is easily overexploited because of certain life history traits,primarily its slow growth rates and low levels of seedling establishment(Suzán 1994). Ironwood populations play a vital role in sustainingother species and populations of the Sonoran Desert. If ironwoodswere eliminated from Sonoran Desert habitats, there would be adecrease in the density of associated plants and subsequently inassociated local faunal communities. Ironwoods must be protected

both to maintain the diversity and lushness of the Sonoran Desert communities they inhabit and to maintain theregeneration dynamics of rare plant populations that grow under its canopies. Ironwoods are truly a hallmark of thedesert landscape living well beyond other desert plant species. The ironwood is both a constant witness to a changingenvironment and an active participant in the maintenance of generations of lush Sonoran Desert plant and animalcommunities.

Ecology of Ironwood Trees in Ironwood Forest National Monument

by Mark Dimmitt

All of the trees in the backgroundare ironwoods. Most of them aregrowing in small drainages on thefinely­dissected bajada and valleyfloor. Little Maria Mountainsnorthwest of Blythe, California.

This large wash in the Chuckwalla Valley west ofBlythe, California is vegetated with ironwood trees (thedarker ones), some blue palo verdes (Parkinsoniaflorida), and a few desert willows (Chilopsis linearis)and smoke trees (Psorothamnus spinosus). The McCoyMountains are on the horizon.

In most of the Sonoran Desert ironwood trees grow mainly on valley floors and are restricted to washes in the driesthabitats (Turner et al. 1995; images above). Arizona Upland is the highest elevation, wettest, and coldest of the sixsubdivisions of the Sonoran Desert (Shreve 1964). Ironwoods behave differently in this zone; they live on bajadasabove the cold valley floors (image below left). On the eastern side of IFNM which is mostly Arizona Upland,ironwoods are abundant on most of the bajadas of the Roskruge, Silver Bell, Waterman, and Ragged Top ranges.Occasionally they are even common on rocky slopes such as on Cocoraque Butte in the Roskruge Mountains. Theireastern range limit is in the Tucson Mountains and the extreme southwestern foothills of the Santa Catalina Mountains.They extend into Avra Valley in some drainages, but almost not at all into the colder Santa Cruz Valley in the vicinity ofTucson.

Ironwood trees grow on the upperbajadas and lower slopes of the TucsonMountains (above) and the easternranges of Ironwood Forest NationalMonument. This is atypical habitat forthe species.

Ironwoods typically grow on valley floors, as here inthe Avra Valley east of the Samaniego Hills in IFNM.There are few ironwood groves in Avra Valley,however, because of its higher elevation and morefrequent frosts that kill this tree of tropical origin.

From east to west Ironwood Forest National Monument trends to lower, warmer, more arid terrain. The transitionfrom AZU to LCV is apparent between the Silver Bell and West Silver Bell Mountains (maps below). It is quite obviousby the time one reaches the Sawtooth Mountains in the far northwest part of the National Monument; there the slopesare thinly vegetated with trees and saguaros and trees on the valley floors are mostly restricted to washes. Along thistransect ironwood trees change their habitat from their anomalous location on upper bajadas in the east and movedown into their more characteristic habitat of valley floor washes in the west. However, they are not ubiquitous inwashes. Ironwood trees are abundant in some washes, while adjacent ones only half a mile away are dominated bymesquite or blue palo verde and nearly or completely devoid of ironwoods.

Distribution of desert ironwood trees withinIronwood Forest National Monument. Thesize of the green circles indicates therelative abundance of ironwood trees fromrare to common. The red triangles are sitessurveyed. In the east ironwood trees arecommon on lower slopes and bajadas andnot on valley floors. In the West SilverbellMountains they are absent from mostslopes but common on the lower bajadas,and the Sawtooth Mountains area is almostdevoid of ironwoods in any habitat.

In the eastern part of IFNM ironwood treesoccur on bajadas and lower mountain slopesin Arizona Upland and are not restricted towashes (blue lines). They are excluded frommost of Avra Valley by winter cold.

Large washes on low valley floorsare the typical habitat of ironwoodtrees. Tiro Wash, West Silver BellMountains, Ironwood Forest NationalMon.

Ironwood tree distribution in thenortheastern part of IFNM is similar to that inthe southern part. There are a few groves oflarge trees in parts of Avra Valley,especially east of the Samaniego Hills.

In the midwestern part of IFNM ironwood treesmore often occur in their typical habitat ­ largewashes on valley floors and lower bajadas.Valley floors are lower elevation and thuswarmer. They are nearly absent from the slopesand bajadas of the arid West Silver BellMountains. Aridity would be the logical reason,except that less drought­tolerant foothill paloverdes are common on these slopes. There aregood groves on some bajadas such as thesouth side of granitic Solo Peak (the rest of theWest Silver Bells are other volcanics).

Ironwood tree distribution extends all the way to the western marginof the Sonoran Desert near Palm Springs, California. But we found noironwood trees in the Sawtooth Mountains and adjacent valley floorswithin the IFNM boundary. (Two trees grow at the edge of a quarryjust outside the boundary.) Apparently the drainages in this region donot collect sufficient runoff to support them.

Ironwood longevity

On most of the rocky bajadas ironwoods are small trees or large shrubs seldom more than four meters (13 ft) tall andmuch shorter than the saguaros that usually grow with them. A large proportion of the individuals in these habitats haveold, dead trunks that have resprouted from the crowns. Some have evidently died to the ground (topkilled) at least twicein the past. So even though most of the trees on these rocky sites are rather short and look like saplings at first sight,many are in fact ancient (see images below).

Ironwood trees that grow in deepsoil in large washes are probably notvery old. This one is east of Indio,California.

The heartwood of Olneya tesota isalmost nonbiodegradable. Theminerals deposited in theheartwood that make it too denseto float in water are toxic to mostsaprophytes and termites. Thisironwood tree stump is weatheringaway by physical processes. Itprobably died two or more centuriesago. Chuckwalla Mountains,

This ironwood in the Roskruge Mountains (IronwoodForest National Monument) appears to have diedto the ground and crown­sprouted at least twice.

This ironwood tree in the ChuckwallaValley, California shows evidence offour generations of topkill andresprouting.

Comparison of ironwoods with foothill palo verdes in the samelocation provides further evidence of the longevity of ironwood trees.A significant percentage of foothill palo verdes died during thedroughts of the mid 1990s and 2001, while we found almost noironwood trees that had died or were topkilled recently. The droughtsthat caused the observed topkill in the ironwoods must have beenmore severe than what we have recorded during the last century, andthe same drought would likely have killed most of the mature foothillpalo verdes (Bowers and Turner 2001). Because mature foothill paloverdes in the nearby Tucson Mountains are 125 to 175 years old(Turner et al., 2003), the mature palo verdes currently living in theMonument are presumably at least 100 years old. Therefore theyounger ironwood stumps visible at present must have been topkilledbefore today’s mature palo verdes established, and those that have topkilled and resprouted twice or more are probablyseveral centuries old. Conversely, the large, vigorous specimens in well-watered washes may be only 100-200 years old.

The most likely candidate for the last catastrophic drought that killedironwood trees to the ground is the one of 1891-1904; some call it themost severe drought documented in southern Arizona (Turner et al.2003). But others consider the mid century drought of 1942 to 1977 tobe the worst since 1700 A.D.; it eliminated most pinyons and junipersestablished before 1850 in much of New Mexico (Swetnam andBetancourt 1998). That drought also thinned out many desertlandscapes (Turner et al. 2003). A more recent drought in the 1990skilled significant numbers of palo verdes (Bowers and Turner 2001).We can still identify the carcasses of these palo verdes at the Arizona-Sonora Desert Museum in the Tucson Mountains. We observed nomortality of ironwood trees in undisturbed areas of the TucsonMountains during that period (Dimmitt pers. obs.), nor does theliterature reviewed mention dieback of ironwood trees during the pastseveral decades.

The primary cause of death of ironwood trees is unknown. If droughtis a significant mortality factor, then these events must be spaced

California.

The largest ironwood known inIronwood Forest NationalMonument.

centuries apart. There is a root rot caused by a fungus in the genusGanoderma (Olsen 1999, Hine 1999). We have observed its effects of

slow yellowing and diminution of foliage over several years at ASDM and at Bach’s Greenhouse Cactus Nursery in thefoothills of the Tortolita Mountains. We did not observe these symptoms in any trees in IFNM, nor did we find anypublications documenting its occurrence in wild trees. The disease seems to be associated with disturbance.

Considering how long dead ironwood trees persist in the landscape, mortality must beextremely low because it is rare to see more than a few dead trees in a population. Adisturbing exception is these old ironwood trees in the Chuckwalla Valley (west of Blythe, CA)that have been declining for at least 30 years. In some areas more than three­quarters ofthem are now dead. This mortality has not been reported in the scientific literature. It may belong­term drought, though groundwater pumping has also been suggested. Anotherpossibility is that the smaller drainages were diverted into larger ones that had culverts underthe old highway (now replaced by Interstate 10 a few miles to the south and renamed FordDry Lake Rd. and Corn Spring Rd.). The summer storm (right) came much too late to save thetree in the foreground, though the large wash in the background has healthy trees that haveleafed out from an earlier storm.

The largest ironwood tree we found in the Monument is 11 meters talland 13 m wide (36 X 43 feet) in Avra Valley east of the SamaniegoHills. This area is characterized by sparse but large ironwood trees; theplot had ten trees over 7 m (23 ft) tall.

It is worth noting that Ironwood Forest National Monument was notcreated and named after the ironwood tree because it has the largesttrees or densest forests. The Monument’s claim to fame is that in thisarea ironwood trees have more ecological associates than anywhereelse this phenomenon was measured (ASDM 2000).

Three generations of a keystonespecies. The old ironwood stumpthat died several centuries ago maybe a grandparent of the 200­year­old tree in the background, whichmay in turn be a parent of the two­week­old seedling in the foreground.Bajada of Cottonwood Canyon,south of Joshua Tree National Park,California.

References

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