Environmental stresses of low
and unpredictable precipitation, low relative humidity with desiccating winds,
and high summer temperatures characterize climates of deserts and, coupled with
low nutrient availability, produce severe limitations of plant growth. Despite
such stresses, desert scrub communities often contain surprisingly large
amounts of plant biomass, and possess remarkable diversity of plant growth
forms.
The life form of a plant – whether
annual, perennial, herbaceous, woody, or succulent – and the characteristics or
its roots, stems, and leaves are presumed to be adaptations to the special
conditions within a desert. Diversity of life forms may be considerable (
Life forms may be classified
into 4 major categories that represent strategies of adaptation, according to
Solbrig and Orians, 1977.
1.
Drought-escaping plants – annuals which germinate and grow only when there is
sufficient moisture available to complete their life cycle. Only their seeds persist during times of drought.
Annuals.
2.
Drought-evading plants – non-succulent perennials which restrict their growth
activity to periods when moisture is available. Typically, they are drought-deciduous
shrubs which go dormant or die back during dry periods.
3.
Drought-enduring
plants – evergreen shrubs. Extensive
root systems coupled with various morphological and physiological adaptations
of their aerial parts enable these hardy xerophytes to maintain growth even in
times of extreme water stress. Creosote bush (Larrea tridentata)
4.
Drought-resisting plants – succulent
perennials. The water stored in their swollen leaves and stems is usually used
very sparingly. Cacti
The major adaptation of both
drought-escaping and drought-evading plants is an ability of accurately predict
the wet season and to restrict their major growth and reproductive activities
to the wet part of the year.
1.
The proportion of
annual species in desert floras is inversely related to the amount and
reliability of precipitation in a region.
2.
Life cycles of
these small, shallow-rooted plants commence when there is water available.
a.
Went noted that
no seedlings of any species germinated in the Sonoran Desert following a 10 mm
rainfall; extensive germination occurred only after a rainfall of 25 mm.
b.
This was
attributed to removal of inhibitors from the seed coats.
3.
Winter and summer
annuals are distinguished on the basis of optimum temperatures for seed
germination. This varies from 15-18 C for winter annuals, and 25-30 C for
summer annuals. These refer to temperature conditions prevailing at the time of
precipitation.
4.
Continued
survival of annuals requires that adequate seed reserves be maintained through
dry periods until conditions are once again suitable for germination.
5.
Few seeds rot in
the desert environment, but many are lost to seed predators, particularly
rodents. Kangaroo rats (Dipodomys
merriami) consume as much as 95% of the seed of Erodium cicutarium, an
annual of the
a.
The highest seed
densities are associated with wind shadows where sees are protected from
further movement by the wind; lowest values were found in open sites and dry
washes.
6.
Seeds of most
desert annuals have temperature or moisture controlled dormancy which may
prevent germination, but seed viability is initially high. Seeds may remain
viable for up to 10 years under artificial conditions.
7.
Mechanisms which
prevent seeds from germinating all at once can increase the chance of survival
of annual species. Seed heteroblasty, in which germination requirements differ
for seeds produced by the same plant, has been described for some annuals in
the Saharo-Arabian Deserts.
8.
Since they are
not restricted by water – they grow only when water is relatively abundant –
they can exploit the favorable light and temperature conditions of the desert.
They require none of the morphological adaptations that other strategists
require.
9.
Annuals’ “goal”
is to grow fast (large leaves, maximal photosynthesis, heavy transpiration),
flower, set seed, disperse seed, and persist through the dry period as a seed.
1.
Drought-deciduous
plants; they drop their leaves, and sometimes their stems during periods of
drought.
2.
Leaf production
from mid-Feb. to April, and a leaf canopy persists until drought conditions
become extreme in summer.
3.
Leaves may be
polymorphic.
a.
Leaves developed
in early spring when water is available and temperatures moderate, are large
and green. High photosynthetic rates.
b.
With the onset of
dry period, subsequent leaves produced are smaller, and often with a pubescent
covering. Lower photosynthetic rates, but lower heat load and transpiration
rates.
c.
Examples: Brittlebush (Encelia farinosa) white bursage (Ambrosia dumosa).
4.
Stem photosynthesis
a.
Stems lose less
water than leaves (reduced transpiration).
b.
During period of
low water stress, leaves are more productive than stems, but under periods of
water stress, the superiority of leaves is lost.
c.
Example –
cheeseweed (Hymenoclea salsola).
d.
A number of
plants are essentially aphyllous (without leaves), relying on stem
photosynthesis: Mormon tea (Ephedra spp.),
spiny menodora (Menodora spinescens),
turpentine bush (Thamnosma
1.
True xerophytes,
maintaining a canopy and positive net carbon gain (photosynthesis) throughout
the year.
2.
Example: creosote
bush (Larrea tridentata)
a.
Compound leaves
(2 leaflets) commonly live for 8-14 months, but a canopy of photosynthetically
active leaves is present throughout the year.
b.
Leaves of
creosote are oriented more or less vertically, parallel to the sun.
c.
Glandular
trichomes secrete a resin that covers the leaf surface. The resin limits
photosynthesis, but also drastically reduces transpiration.
3.
Phreatophytes: These
plants usually have extensive root systems which either spread through the
surface soils or penetrate several meters below the surface Example – mesquite (Prosopis).
a.
Branching usually
occurs in the capillary fringe above the water table in deep-rooted plants
(phreatophytes) that are able to tap permanent ground water; while short-lived
“rain roots” develop on woody surface roots in response to soil moistening.
1.
Succulence is the
most obvious characteristic of drought-resisting plants.
2.
Desert succulents
are generally shallow-rooted, allowing them to respond quickly to light
rainfalls.
3.
Stems are often
heavily waxed to reduced cuticular water loss.
4.
Leaves are often
reduced to spines, and this increases the volume to surface ratio. In barrel
cactus the ratio is 2.5, compared with 0.92 for a succulent leaf of an agave,
and 0.01 for many non-succulent leaves. The maximum ratio is achieved by the
spherical form of many cacti.
5.
Spines also help
to reduce heat load, and dissipate heat. Tissue temperatures below spines of
the cholla cactus (Opuntia bigelovii)
can be reduced by as much as 11 C.
6.
Desert succulent
are rarely killed by high temperatures, and several species of cacti and agave
can withstand temperatures over 60C (140F) for short periods. However, their
seedlings are especially sensitive to high-temperature injury, and
establishment is often prevented in open areas where soil temperatures can rise
to 80 C (176F).
7.
Seedlings of
saguaro and other cacti require the shade of a nurse plant, like palo verde, to
survive.
8.
9.
Water loss is very limited in desert
succulents, but photosynthetic rates and, consequently, growth rates are low
also. Annual growth rates in barrel cactus (Ferocactus
acanthodes) is less than 2 cm.
Other
adaptations
rate of water loss during periods of water stress,
and reduce head load when
stomata are closed.
.