Biological Survey of the Ironwood Forest

1. c o n n e c t :
Biological Survey of Ironwood Forest National
Monument
Natural History of the Desert Ironwood Tree (Olneya tesota)
Photographs by Mark Dimmitt
Flowering ironwood tree, Saguaro
National Park
Ironwood in lush foliage, Ironwood Forest National
Monument
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 woody
legumes found in washes and hillside drainages in the Sonoran Desert. It ranks among the most ecologically and
economically important plant species in the region. Ironwood functions as a “nurse plant” and a “habitat-modifying
keystone 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 the
sole 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 paired
leaves, flower clusters on short shoots that extend from the middle of the stem (instead of the end of a branch), and
pods more rounded than the pods of peteria and brushpea.
Ironwood as a species may have evolved as the Sonoran Desert flora formed in
the middle Miocene (ca. 15 to 8 million years ago) (Van Devender 2000), but
most paleogeological records of Ironwood date from the mid- to late Holocene.
Dating of ironwood trees is difficult through standard tree-ring dating, but
annual trunk diameter growth rates (Turner 1963; Suzán 1994) and unpublished
Biological Survey of
Ironwood Forest
National Monument
General Vegetation
Vascular Flora
Cacti
Distribution and Status
of Saguaros and Trees
Natural History of the
Desert Ironwood Tree
(Olneya tesota)
Rare Plant Inventory
Exotic Plants
Assessment
Human Impacts
Lesser Long­Nosed Bat
Geologic Aspects of
Ironwood Forest
National Monument
Avra Valley Gallery
Aguirre Valley Images
Pan Quemado Gallery
Ragged Top Gallery
Roskruge Mountains
Gallery
Samaniego Hills Gallery
Sawtooth Mountains
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Waterman Mountains
Gallery
s e a r c h
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

2. Ancient ironwood stump
Map courtesy Bill Singleton, Pima
County (AZ) Administrative Office
Dense Arizona Upland forest on
granite near Ragged Top, Ironwood
Forest National Monument
radiocarbon dating estimates (Suzán 1994) suggest that some trees have
persisted for more than 800 years. The wood of the Ironwood is one of the
hardest and heaviest woods in the world (Búrquez 1999). It is remarkably
resistant to rotting, perhaps because its heartwood is rich in toxic chemicals that
make it essentially non-biodegradable (Dimmitt 2000a). Ironwood trunks can
persist for up to 1600 years (Dimmitt 2000a). (See further discussion of
ironwood longevity below.)
Geography
The geographic limits of ironwood distribution are closely matched
with the boundaries of the Sonoran Desert. Ironwood barely reaches
into adjacent Mohave desertscrub, coastal thornscrub south of
Guaymas, Sonora, and foothills thornscrub east of Hermosillo,
Sonora. It occurs in five states and territories within the Sonoran
Desert region: southwestern Arizona, southeastern California, eastern
Baja California, Baja California Sur, and Sonora, Mexico. Populations
occur from sea level to 1100 m (3280 ft) in elevation, where low
winter temperatures and catastrophic freezes limit its distribution.
Near its northern limit ironwood grows best on rocky benches and
slopes, above the valley bottoms that characteristically have cold air
pockets at night that would damage leaves and young branches
(Turner et al. 1995). While ironwood occurs in all six subdivisions of
the Sonoran Desert, it varies greatly in its density and relative
dominance among these regions. In the U.S., the highest ironwood
densities recorded per hectare are in Arizona Upland sites in Pima
County (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 survey
some of our 0.4-hectare census plots had much higher densities that would extrapolate to more than 300 ironwoods per
hectare. Ironwood densities are much lower in Mexico (mean density of 6.6 trees/ha = 2.4/acre across many plots in
coastal and central Sonora). Elevational range is also greater in the species’ northernmost limits in the Arizona Uplands
and Lower Colorado River Valley.
The densest stands of ironwoods and palo verdes occur where the soil
is derived from Precambrian Oracle granite. This granite is
characterized by large crystal size and it weathers into a coarse, very
porous soil that allows deep infiltration of water and air. This porous,
well-aerated soil permits tree roots to penetrate deeply to reach the
deep moisture. Soil explains much of the lushness of the tree growth
in the Silver Bell region. The reason for the greater diversity of plants
associated with ironwood trees here compared with other regions of
the 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-forming
tree with one thick trunk achieving heights up to 15 meters (49 ft; Shreve and Wiggins 1964, Solís-Garza 1993, Arizona
Register of Big Trees 2000). The largest known ironwood, located close to Gila Bend, measures 4.32 meters (14.2 ft)
West Silver Bell
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Tree ­ Olneya tesota
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3. Ironwood leaves usually turn yellow
and are shed in April before the
trees flower.
around its trunk, with a canopy height of 15 meters 49 ft), and a crown spread of 14 meters (46 ft; Arizona Register of
Big 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 of
small 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 (Parkinsonia
spp.) 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 are
abundant only four years per decade (Dimmitt 2000a). This variable
level of flowering and fruiting, along with differences in rainfall each
year, may cause a pattern of mass seed production and seedling
germination that occurs as occasional bursts. Suzán (1994) observed this
unpredictable pattern of germination, known as “discrete episodic
recruitment.” Flowering and fruiting require considerable diversion of
nutrients and energy from other parts of the plant. Branches that
produce flowers often drop their leaves during bud formation, and re-
leaf when summer rains begin. In some years flowering does not occur
at all (Dimmitt (2000b). Patterns of flowering and fruiting generally
occur in a south to north wave. Flowers and fruit occur as early as
March 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. These
flowers attract one generalist bee and two solitary specialist bees. After
pollination occurs, ironwoods produce slightly curved, knobby pods
that 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-Garza
and Espericueta 1997). Seed maturation coincides with the summer
rains, increasing the probability of immediate germination (Shreve and
Wiggins 1964). Maturation occurs within four to eight weeks of
pollination (late June through August) (Turner et al. 1995). The seeds
are small and light compared to those of blue palo verde or the
indehiscent 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 as
deterrents to herbivores, reducing palatability and digestibility.
Ironwood seeds mature at a time when little else is producing fruit in
the Arizona Upland region (Dimmitt 2000b), leading to a high
dependence of wildlife on the seeds. Many animals gather and store

4. Ironwood tree in fruit.
This ironwood tree founded a
minicommunity. The young palo
verde in the foreground and
the large saguaro (as well as
smaller plants not visible)
established beneath its canopy.
Now several small saguaros and
shrubs are growing in the shelter
of the palo verde. The adjacent
ground is comparatively open.
Tucson Mountains, AZ.
Saguaro seedling beneath a palo
verde tree.
ironwood seeds in caches to be eaten later. Roughly half of all new
germinated seedlings found for plants such as jojoba (Simmondsia
chinensis) and palo verde occur in tight clusters near rodent burrows
(McAuliffe 1990). While studies have not been conducted for
ironwood, it is likely that ironwood seedlings germinate from rodent
caches.
Ironwood grows extremely slowly, perhaps due to its low rates of
photosynthesis that keep it from wasting soil moisture. These slow
rates of biomass accumulation contribute to the remarkable density of
its heartwood. Like other desert legume trees, ironwood trees conserve
water during the high daytime temperatures and during dry seasons.
Ironwoods lose less water through their leaves than other woody
perennial plants. During long droughts, the trees slough off leaves, limbs and rootlets to reduce their water needs. The
water use efficiency of ironwood ranks with some of the most drought tolerant Sonoran Desert plants, such as
creosotebush (Larrea), bursage (Ambrosia), and wolfberry (Lycium spp.) (Szarek 1979). Considering ironwood’s
conservative growth rate, small leaves, diffuse canopies, and preference for arid and hyper-arid xeroriparian soils, it is
not surprising that they exhibit relatively low levels of annual net primary productivity (55 g. dry weight/m2/yr. = 15.6
oz/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 the
roles 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 influences
on the distribution and abundance of associated species (Mills et al. 1993). A
chain of influences generated by ironwoods on associated understory plants
affects their dispersal, germination, establishment, and rates of growth as
well as reproduction. These ecological dynamics are termed “nurse plant
ecology.” Other large trees co-occur with ironwoods along washes, but
ironwoods may be the only tall branching woody plants on the valley floors
or bajada slopes (Vander Wall 1980). Their relative influence on plant and
wildlife diversity is proportionally greater in plains and rocky slope habitats
above ephemeral and intermittent watercourses. Along watercourses,
ironwood is but one of many nurse plants available. In addition, the size and
foliar density of an ironwood are strong factors influencing their relative
value as nurses. Medium-sized mature ironwoods harbor a greater diversity
of understory plants than either ironwood saplings or the largest, ancient
shade-forming ironwoods (Tewksbury and Petrovich 1994, Suzán et al.
1996). Some mid-sized trees, however, do not necessarily serve as nurses for
many plants, especially if grazing is heavy.
Mesquite and palo verde
also 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 subregions
of the Sonoran Desert. As nurse plants, ironwoods provide safe sites
for seed dispersal, protect seedlings from extreme cold and freezes,
protect saplings from extreme heat and damaging radiation, and
function as prey refugia. Also, like other legumes, they alter the soil
composition beneath their canopies, enriching the soil with nutrients

5. The ironwood (right­center) is several
times older than the same­size
foothill palo verde to the left. The
ironwood also has a denser canopy
(next paragraph).
This exposed juvenile
saguaro has been eaten
by a jackrabbit (see scat
pellets around base). If it
were growing under a
larger nurse plant than
this shrub, it would
probably have escaped
predation.
such as nitrogen.
An ironwood canopy typically has been functioning as a safe site for
seed dispersal for three to four times longer than mesquite or palo
verde canopies of the same volume. The long life span of ironwood
trees and the stability of the microenvironments they create increase
the probability that seeds might be dispersed to these “safe sites” for
germination and establishment (Tewksbury and Petrovich 1984). Due
to the fact that ironwoods tend to be the tallest trees in desertscrub
and xeroriparian vegetation (Vander Wall 1980), they function as the
primary roosts in their landscape for both local breeding and migrating
birds. Ironwoods and their nurseries make the structure of vegetation
much more diverse providing birds with more nesting opportunities.
Birds in turn generate a literal “rain” of seeds and whole fruit. Partially
digested fruit from this “rain,” or from defecation of other animals,
are torn apart by animals seeking to gain sustenance while selecting
out 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) determined
the winter microenvironments under mature ironwood trees may be 4 C (6.6° F) warmer than adjacent open
environments and 1 C (2.8° F) warmer than under other vegetation. Studies of cactus seedling vulnerability demonstrate
that 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-free
areas (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 sunburn
and 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 decrease
water stress in mature plants (Suzán 1994).
In addition to serving as a buffer from abiotic stresses, ironwood buffers nursery
plants from some, but not all, biotic stresses impacting their survival and
reproduction. McAuliffe (1984a) demonstrated spiny, or thorny nurse plants can
dramatically reduce predation on cactus seedlings by large and small herbivores,
such as ungulates, rabbits, and rodents. The spiny, low-sweeping branches of the
ironwood provide an effective prey refugium for vulnerable seedlings intertwined
within its foliage. However, seedlings not fully sheltered by down-sweeping
branches can suffer higher levels of predation due to the resting, nesting, or
burrowing of desert tortoises, rabbits, jackrabbits, and packrats under ironwoods
(McAuliffe 1984a).
Legumes such as ironwood and mesquite influence the soil composition beneath
their canopies in several ways (Garcia-Moya and McKell 1970). Ironwoods “fix”
nitrogen through symbiotic relationships with Rhizobia bacteria (Felker and Clark
1981). They also “pump” nitrogen and other nutrients up from their deepest root
zones. Ironwoods incorporate these nutrients into their foliage, over time enriching
topsoil composition as their fallen leaves gradually accumulate and decomposes
beneath 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 bacteria
and fungi that aid in the establishment of understory plants, providing them with
moisture and nutrients not available in barren interspaces. The differences in the

6. Ironwood Forest National Monument
viewed from Saguaro National Park
to the west. Agriculture and
urbanization in the intervening Avra
Valley could isolate the two,
fragmenting the habitat into smaller
parcels that will not support some
wide­ranging species such as
mountain lions. The Waterman
Mountains are on the middle
horizon, with the Silver Bell Mountains
and Ragged Top to the right. The
Roskruge Mountains are just visible
on the far left horizon, and the taller
Santa Rosa Mountains on the
Tohono O'odham Nation to their
right.
mycorrhizal fungi and soil composition under ironwoods and mesquites allow them to favor different sets of understory
plants creating heterogeneity through “patch dynamics.” Ironwoods tend to slightly increase alkalinity and moisture
availability, hardly effect soil texture, but significantly increase root, bacteria, and fungi densities where mesquites
decrease soil alkalinity and increase clay content and moisture availability.
Threats
Ironwood habitat faces threats from habitat fragmentation due to
urbanization and conversion of natural habitat to agricultural lands.
The population explosion in the Sonoran Desert over the past 50 years
has also led to increasing recreational impacts in ironwood habitat.
There are also preliminary indications that both woodcutting and
buffelgrass competition can decrease understory species richness and
diversity (Suzán 1994, Búrquez and Quintana 1994). Ironwood cutting
can result in greater damage to understory plants (Nabhan and Suzán
1994, Suzán et al. 1999). Nurslings exposed by woodcutting have a
greater probability of damage from radiation, breakage from
trampling, and death due to browsing (Nabhan and Suzán 1994). Solís-
Garza and Espericueta (1997) have confirmed that virtually no
ironwood regeneration had occurred to date in areas where
commercial woodcutting has been permitted in Sonora. Buffelgrass is
highly invasive, decreases plant species richness and diversity in native
plant communities, and increases fire frequency. Fires in communities
invaded by buffelgrass tend to be hot burning and destroy ironwood
and other trees, shrubs, and cacti.
While ironwood is not considered endangered because of its large
range, 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 sustaining
other species and populations of the Sonoran Desert. If ironwoods
were eliminated from Sonoran Desert habitats, there would be a
decrease in the density of associated plants and subsequently in
associated local faunal communities. Ironwoods must be protected
both to maintain the diversity and lushness of the Sonoran Desert communities they inhabit and to maintain the
regeneration dynamics of rare plant populations that grow under its canopies. Ironwoods are truly a hallmark of the
desert landscape living well beyond other desert plant species. The ironwood is both a constant witness to a changing
environment and an active participant in the maintenance of generations of lush Sonoran Desert plant and animal
communities.
Ecology of Ironwood Trees in Ironwood Forest National Monument
by Mark Dimmitt
All of the trees in the background
are ironwoods. Most of them are
growing in small drainages on the
finely­dissected bajada and valley
floor. Little Maria Mountains
northwest of Blythe, California.
This large wash in the Chuckwalla Valley west of
Blythe, California is vegetated with ironwood trees (the
darker ones), some blue palo verdes (Parkinsonia
florida), and a few desert willows (Chilopsis linearis)
and smoke trees (Psorothamnus spinosus). The McCoy
Mountains are on the horizon.

7. In most of the Sonoran Desert ironwood trees grow mainly on valley floors and are restricted to washes in the driest
habitats (Turner et al. 1995; images above). Arizona Upland is the highest elevation, wettest, and coldest of the six
subdivisions of the Sonoran Desert (Shreve 1964). Ironwoods behave differently in this zone; they live on bajadas
above 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. Their
eastern 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 of
Tucson.
Ironwood trees grow on the upper
bajadas and lower slopes of the Tucson
Mountains (above) and the eastern
ranges of Ironwood Forest National
Monument. This is atypical habitat for
the species.
Ironwoods typically grow on valley floors, as here in
the Avra Valley east of the Samaniego Hills in IFNM.
There are few ironwood groves in Avra Valley,
however, because of its higher elevation and more
frequent frosts that kill this tree of tropical origin.
From east to west Ironwood Forest National Monument trends to lower, warmer, more arid terrain. The transition
from AZU to LCV is apparent between the Silver Bell and West Silver Bell Mountains (maps below). It is quite obvious
by the time one reaches the Sawtooth Mountains in the far northwest part of the National Monument; there the slopes
are thinly vegetated with trees and saguaros and trees on the valley floors are mostly restricted to washes. Along this
transect ironwood trees change their habitat from their anomalous location on upper bajadas in the east and move
down into their more characteristic habitat of valley floor washes in the west. However, they are not ubiquitous in
washes. Ironwood trees are abundant in some washes, while adjacent ones only half a mile away are dominated by
mesquite or blue palo verde and nearly or completely devoid of ironwoods.
Distribution of desert ironwood trees within
Ironwood Forest National Monument. The
size of the green circles indicates the
relative abundance of ironwood trees from
rare to common. The red triangles are sites
surveyed. In the east ironwood trees are
common on lower slopes and bajadas and
not on valley floors. In the West Silverbell
Mountains they are absent from most
slopes but common on the lower bajadas,
and the Sawtooth Mountains area is almost
devoid of ironwoods in any habitat.
In the eastern part of IFNM ironwood trees
occur on bajadas and lower mountain slopes
in Arizona Upland and are not restricted to
washes (blue lines). They are excluded from
most of Avra Valley by winter cold.

8. Large washes on low valley floors
are the typical habitat of ironwood
trees. Tiro Wash, West Silver Bell
Mountains, Ironwood Forest National
Mon.
Ironwood tree distribution in the
northeastern part of IFNM is similar to that in
the southern part. There are a few groves of
large trees in parts of Avra Valley,
especially east of the Samaniego Hills.
In the midwestern part of IFNM ironwood trees
more often occur in their typical habitat ­ large
washes on valley floors and lower bajadas.
Valley floors are lower elevation and thus
warmer. They are nearly absent from the slopes
and bajadas of the arid West Silver Bell
Mountains. Aridity would be the logical reason,
except that less drought­tolerant foothill palo
verdes are common on these slopes. There are
good groves on some bajadas such as the
south side of granitic Solo Peak (the rest of the
West Silver Bells are other volcanics).
Ironwood tree distribution extends all the way to the western margin
of the Sonoran Desert near Palm Springs, California. But we found no
ironwood trees in the Sawtooth Mountains and adjacent valley floors
within the IFNM boundary. (Two trees grow at the edge of a quarry
just outside the boundary.) Apparently the drainages in this region do
not 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 and
much shorter than the saguaros that usually grow with them. A large proportion of the individuals in these habitats have
old, dead trunks that have resprouted from the crowns. Some have evidently died to the ground (topkilled) at least twice
in 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).

9. Ironwood trees that grow in deep
soil in large washes are probably not
very old. This one is east of Indio,
California.
The heartwood of Olneya tesota is
almost nonbiodegradable. The
minerals deposited in the
heartwood that make it too dense
to float in water are toxic to most
saprophytes and termites. This
ironwood tree stump is weathering
away by physical processes. It
probably died two or more centuries
ago. Chuckwalla Mountains,
This ironwood in the Roskruge Mountains (Ironwood
Forest National Monument) appears to have died
to the ground and crown­sprouted at least twice.
This ironwood tree in the Chuckwalla
Valley, California shows evidence of
four generations of topkill and
resprouting.
Comparison of ironwoods with foothill palo verdes in the same
location provides further evidence of the longevity of ironwood trees.
A significant percentage of foothill palo verdes died during the
droughts of the mid 1990s and 2001, while we found almost no
ironwood trees that had died or were topkilled recently. The droughts
that caused the observed topkill in the ironwoods must have been
more severe than what we have recorded during the last century, and
the same drought would likely have killed most of the mature foothill
palo verdes (Bowers and Turner 2001). Because mature foothill palo
verdes in the nearby Tucson Mountains are 125 to 175 years old
(Turner et al., 2003), the mature palo verdes currently living in the
Monument are presumably at least 100 years old. Therefore the
younger ironwood stumps visible at present must have been topkilled
before today’s mature palo verdes established, and those that have topkilled and resprouted twice or more are probably
several 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 killed
ironwood trees to the ground is the one of 1891-1904; some call it the
most severe drought documented in southern Arizona (Turner et al.
2003). But others consider the mid century drought of 1942 to 1977 to
be the worst since 1700 A.D.; it eliminated most pinyons and junipers
established before 1850 in much of New Mexico (Swetnam and
Betancourt 1998). That drought also thinned out many desert
landscapes (Turner et al. 2003). A more recent drought in the 1990s
killed 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 no
mortality of ironwood trees in undisturbed areas of the Tucson
Mountains during that period (Dimmitt pers. obs.), nor does the
literature reviewed mention dieback of ironwood trees during the past
several decades.
The primary cause of death of ironwood trees is unknown. If drought
is a significant mortality factor, then these events must be spaced

10. California.
The largest ironwood known in
Ironwood Forest National
Monument.
centuries apart. There is a root rot caused by a fungus in the genus
Ganoderma (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 the
foothills of the Tortolita Mountains. We did not observe these symptoms in any trees in IFNM, nor did we find any
publications 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 be
extremely low because it is rare to see more than a few dead trees in a population. A
disturbing 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 of
them are now dead. This mortality has not been reported in the scientific literature. It may be
long­term drought, though groundwater pumping has also been suggested. Another
possibility is that the smaller drainages were diverted into larger ones that had culverts under
the old highway (now replaced by Interstate 10 a few miles to the south and renamed Ford
Dry Lake Rd. and Corn Spring Rd.). The summer storm (right) came much too late to save the
tree in the foreground, though the large wash in the background has healthy trees that have
leafed out from an earlier storm.
The largest ironwood tree we found in the Monument is 11 meters tall
and 13 m wide (36 X 43 feet) in Avra Valley east of the Samaniego
Hills. This area is characterized by sparse but large ironwood trees; the
plot had ten trees over 7 m (23 ft) tall.
It is worth noting that Ironwood Forest National Monument was not
created and named after the ironwood tree because it has the largest
trees or densest forests. The Monument’s claim to fame is that in this
area ironwood trees have more ecological associates than anywhere
else this phenomenon was measured (ASDM 2000).

11. Three generations of a keystone
species. The old ironwood stump
that died several centuries ago may
be a grandparent of the 200­year­
old tree in the background, which
may in turn be a parent of the two­
week­old seedling in the foreground.
Bajada of Cottonwood Canyon,
south of Joshua Tree National Park,
California.
References
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