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.
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).
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.
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.
rate of water loss during periods of water stress, and reduce head load when
stomata are closed.