For the diversity of devices for adaptation to an inhospitable
environment, the many species making up the non-succulent desert
vegetation provide an absorbing field for study. As we have seen, there
are two ways to survive the harsh desert climate; one is to avoid the
periods of excessive heat and drought (“escapers”); the other is to
adopt various protective devices (“evaders” and “resisters”).
Short-lived plants follow the first method; perennials, the second.
_Perennials_
Chief among the requirements for year-round survival in the desert is a
plant’s ability to control transpiration and thus maintain a balance
between water loss and water supply. In this struggle, the hours of
darkness are a great aid because in the cool of the night the air is
unable to take up as much moisture as it does under the influence of the
sun’s evaporating heat. Therefore, less exhaling and evaporating of
water occurs from plants, and both the rate and the amount of water loss
are reduced. This reduction in transpiration at night allows the plants
to recover from the severe drying effects of the day. One biologist may
have been close to the truth when he stated, “If the celestial machinery
should break down so that just one night were omitted in the midst of a
dry season, it would spell the doom of half the nonsucculent plants in
the desert.”
One of the common trees in the desert part of the monument is the
MESQUITE (mess-KEET). In general appearance it resembles a small, spiny
apple or peach tree with finely divided leaves. Its roots sometimes
penetrate to a depth of 40 or more feet, thus securing moisture at the
deeper, cooler soil levels, from a supply that remains nearly constant
throughout the year. This enables the tree to expose a rather large
expanse of leaf surface without losing more water than it can replace. A
number of mechanical devices help the tree reduce its water loss during
the driest part of the day (10 a.m. to 4 p.m.). Among these are its
ability to fold its leaves and close the stomata (breathing pores),
thereby greatly reducing the surface area exposed to exhaling and
evaporating influences. In April and May, mesquite trees are covered
with pale-yellow, catkinlike flowers which attract swarms of insects.
These flowers develop to stringbeanlike pods rich in sugar and important
as food for deer and other animals. In earlier days, the mesquite was
also a valuable source of food and firewood for Indians and pioneers.
[Illustration: Pricklypear blossom.]
[Illustration: Claret cup hedgehog.]
[Illustration: Fishhook cactus.]
[Illustration: Cholla in bloom.]
[Illustration: Staghorn cholla.]
Another desert tree abundant in the monument is the YELLOW PALOVERDE. It
is somewhat similar in size and general shape to the mesquite. Lacking
the deeply penetrating root system of the mesquite, the paloverde
(Spanish word meaning “green stick”) has no dependable moisture source;
but it has made unusual adaptations that enable it to retain as much as
possible of the water collected by its roots. In early spring the tree
leafs out in dense foliage, which is followed closely by a blanket of
yellow blossoms. At this season the paloverdes provide one of the most
spectacular displays of the desert, particularly along washes, where
they grow especially well. Blue paloverde, growing in the arroyos,
blooms well every year. Yellow, or foothill, paloverde, a separate
species, blooms only if the soil moisture is high following winter
rains.
With the coming of the hot, drying weather of late spring, the trees
need to reduce their moisture losses. They gradually drop their leaves
until, by early summer, each tree has become practically bare. The trees
do not enter a period of dormancy, but are able to remain active because
their green bark contains chlorophyll. Thus, the bark takes over some of
the food-manufacturing function normally performed by leaves, but
without the high rate of water loss.
Carrying the drought-evasion habits of the paloverde a step further, the
OCOTILLO (oh-koh-TEE-yoh) comes into full leaf following each rainy
spell during the warmer months. During the intervening dry periods it
sheds its foliage. The ocotillo, a common and conspicuous desert
dweller, is a shrub of striking appearance, with thorny, whiplike,
unbranching stems 8 to 12 feet long growing upward in a funnel-shaped
cluster. In spring, showy scarlet flower clusters appear at the tips of
the stems, making each plant a glowing splash of color.
[Illustration: Mesquite in bloom.]
A number of desert shrubs fail to display as much ingenuity as the
paloverde. Some of these evade the dry season simply by going into a
state of dormancy. The WOLFBERRY bursts into full leaf soon after the
first winter rains and blossoms as early as January. Its small,
tomato-red, juicy fruits are sought by birds, which also find protective
cover for their nests and for overnight perches in the stiff, thorny
shrubs. In the past, the berrylike fruits were important to the Indians,
who ate them raw or made them into a sauce.
[Illustration: Yellow paloverde, Tucson Mountain Section.]
Commonest of the conspicuous desert non-succulent shrubs is the
wispy-looking but tough CREOSOTEBUSH, found principally on poor soils
and on the desert flats between mountain ranges. It is also sprinkled
throughout the paloverde-saguaro community in the monument. A new crop
of wax-coated, musty-smelling leaves, giving the plant the local (but
mistaken) name “grease-wood,” appears as early as January. The leaves
are followed by a profuse blooming of small yellow flowers and cottony
seed balls. During abnormally moist summers or in damp locations, the
leaves and flowers persist the year round; but usually the coming of dry
weather brings an end to the blossoming period. If the dry spell is
exceptionally long, the leaves turn brown, and the plants remain dormant
until awakened by the next winter’s rainfall. Pima Indians formerly
gathered a resinous material, known as lac, which accumulates on the
bark of its branches, and used it to mend pottery and fasten arrow
points. They also steeped the leaves to obtain an antiseptic medicine.
Ground squirrels commonly feed on the seeds.
[Illustration: Ironwood blossoms.]
[Illustration: Parry’s penstemon.]
A large shrub of open, sprawling growth usually found along desert
washes in company with mesquite is CATCLAW. Its name refers to the small
curved thorns that hide on its branches. In April and May, the small
trees are covered with fragrant, pale-yellow, catkinlike flower clusters
that attract swarms of insects. The seed pods were ground into meal by
the Indians and eaten as mush and cakes.
In lower elevations of the Tucson Mountain Section, the gray-blue
foliage of IRONWOOD is a common sight, but the species does not range
farther eastward. Its wisterialike lavender-and-white flowers blossom in
May and June. The nutritious seeds are harvested by rodents and formerly
were parched and eaten by Indians. The wood is so dense that it sinks in
water; Indians used it for making arrowheads and tool handles.
Ferns—commonly, plants of dank woods and other moist habitats—seem
entirely out of place in the desert; nevertheless, some members of the
fern family have overcome drought conditions. The GOLDFERN is common on
rocky ledges, where it persists by means of special drought-resistant
cells.
Among the smaller perennials are many that add to spring flower displays
when conditions of moisture and temperature are favorable. Perennials do
not need to mature their seeds before the coming of summer as do the
ephemerals; a majority start blossoming somewhat later in the spring,
and gaily flaunt their flowers long after the annuals have faded and
died. When the heat and drought of early summer begin to bear down, they
gradually die back, surviving the “long dry” by their persistent roots
and larger stems. One of the most noticeable and beautiful of this group
of small perennials fairly common in the monument is PARRY’S PENSTEMON.
It occurs in scattered clumps on well-drained slopes along the base of
Tanque Verde Ridge. The showy rose-magenta flowers and the glossy-green
leaves arise from erect stems that may grow 4 feet tall in favorable
seasons.
Among the first of the shrubby perennials to cover the rocky hillsides
with a blanket of winter and springtime bloom is the BRITTLEBUSH. Masses
of yellow sunflowerlike blossoms are borne on long stems that exude a
gum which was chewed by the Indians and was also burned as incense in
early mission churches.
A conspicuous perennial that survives the dry season as an underground
bulb is BLUEDICKS. Although it doesn’t occur in massed bloom, it does
add spots of color to the desert scene. Usually appearing from February
to May, bluedicks has violet flower clusters on long, slender, erect
stems. The bulbs were dug and eaten by Pima and Papago Indians.
Although neither conspicuous nor attractive, the common TRIANGLE BURSAGE
is an important part of the paloverde-saguaro community in the Tucson
Mountains. A low, rounded, white-barked shrub, bursage has small,
colorless flowers without petals. (Being wind-pollinated, the flowers do
not need to attract insects.)
One of the handsome shrubs abundant in the high desert along the base of
Tanque Verde Ridge is the JOJOBA (ho-HOH-bah), or deernut. Its thick,
leathery, evergreen leaves are especially noticeable in winter and
furnish excellent browse for deer. The flowers are small and yellowish,
but the nutlike fruits are large and edible, although bitter. They were
eaten raw or parched by the Indians, and were pulverized by early-day
settlers for use as a coffee substitute.
Among the attractive flowering shrubs are the INDIGOBUSHES, of which
there are several species adapted to the desert environment. The local,
low-growing indigobushes are especially ornamental when covered with
masses of deep-blue flowers in spring.
Another small shrub, noticeable from February to May because of its
large, tassel-like pink-to-red blossoms and its fernlike leaves is
FAIRY-DUSTER. Deer browse on its delicate foliage.
The PAPER FLOWER, growing in dome-shaped clumps covered with yellow
flowers, sometimes blooms throughout the entire year. The petals bleach
and dry and may remain on the plant weeks after the blossoms have faded.
Quick to attract attention because of their apparent lack of foliage,
the JOINTFIRS, of which there are several desert species, grow in clumps
of harsh, stringy, yellow-green, erect stems. The skin or outer bark of
the stems performs the usual functions of leaves, which on these plants
have been reduced to scales. Small, fragrant, yellow blossom clusters,
appearing at the stem joints in spring, are visited by insects attracted
to their nectar.
_Ephemerals_
Every spring, after a winter of normal rainfall, parts of the
southwestern deserts are carpeted with a lush blanket of fast-growing
annual herbs and wildflowers—the early spring ephemerals. The monument
does not get massive displays, however, since it is lacking in the
species that make the best show. But it does have many annuals that are
beautiful individually or in small groups. Many of these “quickies” do
not have the characteristics of desert plants; some of them, in fact,
are part of the common vegetation of other climes where moisture is
plentiful and summer temperatures are much less severe.
What are these “foreign” plants doing in the desert, and how do they
survive? With its often frostfree winter climate and its normal
December-to-March rains, the desert presents in early spring ideal
growing weather for annuals that are able to compress a generation into
several months. Several hundred species of plants have taken advantage
of this situation.
There is WILD CARROT, which is a summer plant in South Carolina and a
winter annual in California (where it is called “rattlesnake weed”). In
the desert, its seeds lie dormant in the soil through the long, hot
summer and the drying weather of autumn. Then, under the influence of
winter rains and the soil-warming effects of early spring sunshine, they
burst into rapid growth. One of a host of species, this early spring
ephemeral is enabled by these favorable conditions to flower and mature
its seed before the pall of summer heat and drought descends upon the
desert. With their task complete, the parents wither and die. Their
ripened seeds are scattered over the desert until winter rains enable
them to cover the desert with another multicolored but short-lived
carpet of foliage and bloom.
The one-season ephemerals do not limit themselves to the winter growing
period. From July to September, local thundershowers deluge parts of the
desert while other areas, not so fortunate, remain dry. Where rain has
fallen, another and entirely different group of plants, called the
summer ephemerals, find ideal conditions for growth and take their turn
at weaving a desert carpet. Their seeds have lain dormant over winter.
These summer “quickies” are plants that, farther south in Baja
California and Sonora, Mexico, flourish during the winter rainy season.
Saguaro National Monument is doubly fortunate in that it lies within a
section of desert having not only its own year-round vegetation, but
also summer wildflowers “borrowed,” for winter use, from its eastern and
western neighbors, and winter wildflowers for summer decorations from
its southern neighbors.
[Illustration: Phacelia, an ephemeral.]
The short-lived leafy plants of summer and winter are able to compress
their entire active life into 6 to 12 weeks when conditions are most
suitable. Thus, they can escape all the rigorous periods of the desert
climate by living for 8 or 9 months in the dormant seed stage. Some of
the spectacular and colorful flowers of the monument are among these
ephemerals that survive desert conditions by escaping them. It must be
remembered, however, that when drought conditions or abnormally cold
spring weather upset the norm—a not unusual occurrence—response of
ephemeral plants is greatly restricted. If suitable conditions do not
develop during the season for growth of a particular kind of ephemeral,
its seeds will simply wait a year or more until conditions are
favorable.
[Illustration: White tackstem.]
How do the seeds of ephemerals “know” when it is time to germinate?
Experiments have revealed that the seeds of annuals will germinate only
when enough water percolates through the soil to dissolve away a “growth
inhibitor” in their coats. A single light rain will not do the job. In
this way premature sprouting into a too-dry environment is prevented.
The winter annuals, furthermore, will only germinate when soils are
cool, and the summer annuals when soils are warm. These finely tuned
adaptations thus allow plantlife to take full advantage of favorable
seasons in the desert.
Early spring ephemerals climax their show in March. From late February
to mid-April they are completing their growth and putting forth the
precious seeds that will assure survival for the next generation. At the
head of this parade of flowers in the monument is a purple-blossomed
immigrant from the Mediterranean, the now thoroughly naturalized
FILAREE. In addition to the small purple flowers, which may appear as
early as January, the conspicuous “tailed” fruits almost always attract
attention. When dry, they are tightly twisted, corkscrewlike; when damp
they uncoil, forcing the needle-tipped seeds into the soil.
[Illustration: Desert-marigold.]
INDIAN WHEAT is one of the first plants to lay a green carpet over the
sandy desert floor in spring. The tan, individual flower heads are
conspicuous, but their numerous, close-growing spikes form a thick,
luxuriant, pilelike ground cover. The countless tiny seeds are eagerly
sought each spring by coveys of Gambel’s quail, and are also harvested
by Pima and Papago Indians.
DESERT CHICORY is somewhat like the common yellow dandelion but is
longer stemmed and less coarse. Its white or butter-yellow blossoms make
it one of the noticeable spring annuals in the desert. It rarely grows
in pure stands but appears in conspicuous clumps among other short-lived
plants.
Somewhat similar in appearance to desert chicory is WHITE TACKSTEM, one
of the handsomest of the spring quickies. It is usually found on dry,
rocky hillsides and has white or rose-colored flowers. Its name is
derived from the presence of small glands which protrude in the manner
of tiny tacks partially driven into the stems.
Following abnormally wet winters, FIDDLENECK covers patches of sandy or
gravelly soil with a dense growth of bristly erect plants. These bear
tight clusters of small yellow-orange blossoms arranged along a curling
flower stem resembling the scroll end of a violin, hence the name. This
plant, favored by the same growing conditions as creosotebush,
frequently forms a dense though short-lived growth around the bases of
those shrubs.
Associated with fiddleneck and creosotebush, SCORPIONWEED adds its
violet-purple blooms to the spring flower display following winters of
above-normal precipitation. The name is derived from the curling habit
of the blossom heads, which may remind the observer of the flexed tail
of a scorpion. Touching the plant may cause skin irritation in
susceptible individuals. Unfortunately, scorpionweed is also widely
known as wild-heliotrope, thus contributing to the confusion engendered
by duplication of popular names. The plant properly called
WILD-HELIOTROPE is similar in general appearance, but the flowers are
white to pale purple and their odor is more pleasing than that of the
scorpionweed. Wild-heliotrope, or “quailplant,” is another of the early
spring ephemerals, but under favorable conditions, where soils are
moist, it may continue to live and bloom throughout the year.
[Illustration: Bladder-pod.]
Growing in sandy locations and quite noticeable because of its large,
yellow, showy, long-stemmed flower heads, the DESERT-MARIGOLD helps to
open the spring blossoming season. Where moisture conditions are
favorable, these plants may continue to bloom throughout the summer and
well into autumn. Sometimes, during the hottest, driest time of the
year, desert-marigolds are among the very few blossoms brightening the
desert floor. Their bleached, papery petals persist for days after the
flowers have faded, giving the plant the name paper-daisy.
One of the few species that makes a carpet of color in the monument is
the tiny BLADDER-POD. This low-growing annual of the mustard family
begins to cover open stretches of desert with a yellow blanket in late
February or early March following wet winters. Bladder-pod is usually
found in pure stands surrounding islands of cholla, creosotebush, and
paloverde. It also mingles with other spring ephemerals, where it is
promptly submerged by the ranker, taller-growing, more conspicuous
annuals.
Illustrating one of the interesting phases in evolutionary variations
among plants, the LUPINES are represented by several species which are
able to survive and prosper in the desert. Some of the lupines are
annuals of the quickie type; others are perennials with a life cycle of
several years. Some of these longer-lived species join the ephemerals in
the spring flower show, while others are more leisurely in approaching
their blossoming time.
[Illustration: The Desert Scrub Community occupies the lower parts
of the monument, from about 2,200 to 4,000 feet.]
[Illustration: The Grassland Transition begins at about 3,500 feet
(overlapping in altitude with the Desert Scrub) and extends to 4,500
feet.]
major plant communities
of saguaro
[Illustration: SAGUARO NATIONAL MONUMENT]
Tucson Mountain Section
Biotic Communities
Desert Scrub
Grassland
Transition
In any region where a great range of altitude exists, the vegetation
grows in a continuum of overlapping but recognizable zones. Climate
being the major controlling factor of this zonal distribution, the
plants of each band grow higher on south-facing slopes than on cooler,
moister north slopes. In Saguaro the major plant communities roughly
correspond to altitudinal bands—with the desert at the lower, drier
levels, and the transition through grasses and shrubland to forest
community occurring with increasing altitude (and increasing rainfall).
Timberline—the boundary between high mountain forests and the alpine
meadows and barrens—does not exist in Saguaro, where trees clothe the
highest peaks.
The fauna of each community is made up of animals adapted to the
climatic conditions and the available food, cover, and water. In
Saguaro, some animals, such as the desert kangaroo rat and the cactus
wren, are restricted by their life requirements to a narrow belt;
others, such as the gray fox and great horned owl, are more adaptable
and live in all the major plant communities.
[Illustration: SAGUARO NATIONAL MONUMENT]
Rincon Mountain Section
Biotic Communities
Desert Scrub
Grassland Transition
Oak Woodland
Oak-Pine Woodland
Ponderosa Pine Forest
Douglas-fir Forest
[Illustration: The Open Oak Woodland has scattered trees associated
with many plants of the Grassland Transition.]
[Illustration: The Oak-Pine Woodland, from 4,500 to 7,000 feet,
occupying a slightly cooler, wetter environment than those below, is
broken by open glades and grassy hillsides.]
[Illustration: The Ponderosa Pine Forest, unlike the Oak-Pine “pygmy
woodland,” has tall trees growing in clear, open stands. It covers
much of the Rincon Mountains above 6,000 feet.]
[Illustration: The Douglas-fir Forest, limited to the higher
northern and northeastern slopes of Mica Mountain and the north side
of Rincon Peak, often contains aspens and white firs.]
[Illustration: Spanish dagger, with Rincon Peak in the background.]
plants of the hills
and mountains
Most visitors to the monument see only a small part of it—the lowland
cactus forests. But above the desert in the high back country of the
Rincon Mountains is another world. These highlands are accessible only
on foot or horseback; and they should remain so, for they are now in the
last major roadless range in southern Arizona, and as such their
wilderness value appreciates yearly. Interesting scenic trails reach the
high places of the Tucson Mountains, too, but to see the greatest
changes in plantlife you must climb the higher Rincons.