Plant Life in Arid Lands

S. P. McLaughlin

Fall, 2004

Contact Information:

Offices:

Natural Products Center, Room 100. 743-4522

Office of Arid Lands Studies, Room 107A1. 621-8577

Herbarium, Herring 103A. 621-7243

E-mail: spmcl@ag.arizona.edu

Homepage: http://Ag.Arizona.Edu/~spmcl/mclaughlin.html

Part 1. GENERAL PRINCIPLES

I. Classification of Plant Species Based on Availability of Water.

A. Types.

Hydrophytes. Aquatic species or plants requiring saturated soils for most of their growing season.

Mesophytes. Plants adapted to well-aerated soils with an adequate water supply during their entire growing season.

Xerophytes: Plants able to survive extended periods of low soil moisture.

B. Distribution.

Xerophytes [generally] occur only in arid and semiarid lands lands (why?, and what are the exceptions?). But not all plants found in dry areas are xerophytes. Hydrophytes occur in riparian and other aquatic habitats within the arid zones; mesophytes may also be common. The latter are generally plants with short life spans.

II. Classification of Plant Species from Arid and Semiarid Lands Based on their General Adaptations.

"Drought evaders." Generally riparian species. I.e., species from habitats with relatively high water tables.

"Drought avoiders." Generally ephermals ("annuals"), geophytes, or other herbaceous perennials which grow, flower, and set seed during the short wet season. Desert ephemerals are strongly indeterminate; that is their size at maturity and seed production are highly dependent on available resources, primarily water.

"Drought endurers." True xerophytes.

A large proportion of the flora of arid regions with a predictable wet season(s) are ephemerals. The floras of dry lands with winter precipitation differ greatly from those with summer precipitation.

III. The Physiology of Water Stress.

A. Gas exchange principles.

Plants take up carbon as carbon dioxide from the atmosphere. CO₂ diffuses passively into leaves through epidermal structures call stomata. Stomata are pores created by a pair of guard cells. When these pores are open, CO₂ can enter the leaf. At the same time, however, water vapor exits from the leaves through these pores. This loss of water as water vapor is called transpiration.

Carbon gain (CO₂ uptake) and water loss (by transpiration) are thus unavoidably linked.

Water stress causes the stomatal pores to close. With the stomata closed, plants cannot capture CO₂ and thus cannot grow and reproduce.

B. Consequences of prolonged stomatal closure.

1. Light stress.

Photosynthesis involves both "light reactions" and "dark reactions." The light reactions involve the capture of light energy by pigments, which is used to produce key metabolites (ATP, NADPH) and hydrolyze water. Without the final electron acceptor, carbon, these reactive molecules and free oxygen radicals can accumulate in the cell and damage cell constituents. This is often referred to as photo-oxidation.

[Dark reactions involve the capture of CO₂ by certain key photosynthetic enzymes (discussed below), the reduction of organic acids, and synthesis of carbohydrates.]

2. Temperature stress.

Prolonged stomatal closure alters the energy balance of the leaf. We can expressed this energy balance as a simple equation:

I - R - T = A + P + LE + dH

where:

I = Intercepted (or incident) radiation

R = Reflected radiation

T = Transmitted radiation

A = Convective energy loss

P = Photosynthesis (ca. 5.3% of I under ideal conditions)

LE = Latent heat exchange

dH = Change in sensible heat (storage or loss of heat as measured by temperature changes)

The left side of the equation represents the amount of light energy gained by the leaf; the right side is the corresponding energy loss.

Latent heat exchange refers to the energy expended in converting water from its liquid to gaseous state (latent heat of vaporization = 2442 J g^-1); LE accounts for most of the energy loss by leaves under normal conditions.

If plants cannot lose energy through transpiration when stomata are closed, then dH must increase. In other words, more energy is stored within the leaf as heat, and leaf temperature rises. In a mesophyte, leaf temperatures would reach damaging or lethal levels with only relatively short periods of stomatal closure. Survival of leaves of mesophytes for weeks or even months of low soil moisture and restricted transpiration is simply not possible.

3. Adaptations to aridity.

Look at the energy budget equation again. If LE is restricted, and there are physiological limits to how much dH can increase, then the model tells us that one or more of five things must happen if leaves are to remain at sublethal temperatures; which of these are real solutions (adaptations) employed by xeropohytes?

a. Decrease I?

b. Increase R?

c. Increase T?

d. Increase A?

e. Increase P?

4. Consequences of adaptations to aridity.

Given the insight provided by the energy budget, and the solutions to the problem of maintaining suitable temperatures discussed in class, what are some of the implications of xerophytic adaptations? More specifically, what are the constraints and growth rates of xerophytes when growing conditions are good, i.e., adequate soil moisture and mild temperatures?

IV. Photosynthesis.

A. Pathways. There are three photosynthetic pathways employed by vascular plants; two of these appear to confer advantages to plants in dry environments.

1. C3. This is the basic means by which all plants capture carbon. The key enzyme (the most abundant protein in nature) is RUBISCO - ribulose bisphosphate carboxylase-oxygenase. RUBSICO can combine the 5-carbon sugar ribulose with CO₂ to produce a highly unstable 6-carbon compound with immediately breaks down to yield two molecules of a 3-carbon compound (3-PGA), the first stable product of the dark reactions.

However, RUBISCO can also use oxygen as a substrate (hence the "oxygenase" part of its name). When RUBISCO combines ribulose with oxygen the process is call "photorespiration" which results in a net loss of carbon to the plant. The ratio of the carboxylase:oxygenase activities depends on (a) the concentrations of CO₂ and O₂ within the leaf, and (b) temperature. Oxygenase activity increases with increasing temperature; thus C3 photosynthesis becomes less efficient at higher temperatures.

2. C4. Some plants have an second system of capturing CO₂ in addition to (not instead of) of the C3 pathway. In the C4 pathway, CO₂ is captured by combining it with the 3-carbon compound phosphoenol pyruvate (PEP) using the enzyme PEP-carboxylase. This enzyme has no oxygenase activity. The first stable product is malic acid, a 4-carbon compound. However, plants have no efficient mechanism for directly converting malic acid into carbohydrates.

In C4 plants the C3 and C4 pathways are separated physically. C3 activity is restricted to cells in the middle of the leaf surrounding the vascular tissue-the bundle sheath cells. The C4 pathway is found in the matrix of mesophyll cells, which account for the bulk of the leaf mass. Malic acid is transported to the bundle sheath, where it is decarboxylated to produce a 3-carbon compound and CO₂. The "purpose" of this is to raise the concentration of CO₂ relative to O₂ within the bundle sheath, effectively eliminating the oxygenase activity within the bundle sheath regardless of the temperature. The CO₂ is reassimilated in the bundle sheath cells by the C3 pathway, which produces the carbohydrates used to fuel plant growth and metabolism.

3. CAM. CAM refers to "crassulacean acid metabolism," because it was first described in species belonging to the Crassulaceae family. CAM plants also have both C4 and C3 activity, but the pathways are separated temporally. The unique adaptation of CAM plants is that they only open their stomata at night. CO₂ is captured as malic acid by PEP-carboxylase during the night when the stomata are open. CAM plants are usually quite succulent; the large vacuoles of the cells of CAM plants permits them to store malic acid produced during the night. The pH of leaf (or stem) tissues of CAM plants measurably decreases from dusk to dawn. During the day, CAM plants close their stomata, malic acid is decarboxylated within the plant and the CO₂ is recaptured by RUBISCO in the C3 pathways.

Why is opening the stomata only at night advantageous? [See the next section.]

V. Water-use Efficiency.

It is a relatively common fallacy that plants adapted to arid lands use water comparatively efficiently. In fact, just the opposite generally is true.

Water-use efficiency can be defined in various ways, always as some measure of plant carbon gain or growth over some measure of water consumed. At the cellular, instantaneous level water use-efficiency is simply the ratio of photosynthesis (A) to transpiration (T):

WUE = A/T

[Note: I am using the symbols conventionally employed by plant physiologists; "A" and "T" have entirely different meanings in this context than in the energy budget model described above.]

For an entire crop of plants over the growing season, we could define water-use efficiency as the ratio of yield (Y) to total evapotranspiration (ET):

WUE = Y/ET

The key to understanding WUE is to focus on the denominator, transpiration or evapotranspiration. Transpiration is a rate of flow of water from the leaf surface. The magnitude of that flow depends on the driving force and the resistance. Many xerophytes do have mechanisms to increase resistance to water vapor transport, thereby decreasing transpiration. Such mechanisms, however, also increase resistance to CO₂ flow into the leaves.

The driving force behind transpiration is the vapor pressure deficit (VPD), the gradient of water vapor concentration from inside the leaf to the outside air. Within the leaf the air is in contact with saturated cell walls and is therefore also saturated with water. Saturation vapor pressure of any parcel of air is an exponentially increasing function of temperature. The temperature of leaves of plants in arid lands is at or above ambient temperature, therefore the vapor-pressure within the leaf is typically very high. Outside the leaf, the air may be very dry. Diffusion rates along this concentration gradient are thus steep, and transpiration rates are correspondingly rapid.

Thus WUE in arid land plants is generally low because the VPD they experience is very high, resulting in high transpiration rates.

In general, WUE is highest in CAM plants, intermediate in C4 plants, and lowest in C3 plants. WUE efficiency in CAM plants is high because they transpire at night when temperatures are lower and humidity is higher, i.e. when the VPD is lower.

C4 plants have a somewhat higher assimilation rate (A), i.e., photosynthesis. Thus the higher numerator accounts for the higher WUE ratio in C4 plants compared to C3 plants.

Given all this, it would not be unreasonable to expect that arid lands would be dominated by plants with the C4 and CAM pathways. While it is true that proportion of succulents (CAM plants) and C4 species does increase with aridity, the most common pathway in deserts is still C3!

There are very few woody C4 plants. Many shrubby species of Atriplex (saltbushs) are C4, but there are few other examples. All legumes are C3. Thus the commonest woody plants of the Sonoran Desert, for example, are C3 species: creosote bush, bursages, brittlebush, mesquite, paloverde, and ironwood. Agaves, many yuccas, and cacti are CAM plants.

The Sonoran Desert has a biseasonal precipitation regime, and thus its flora includes both winter and summer ephemerals. All of the winter ephemerals are C3; many of the summer ephemerals are C4. Among crop plants, cool season grains (wheat, barley, oats), legumes, tubers, and fruit trees are all C3 species. Corn, sorghum, millet-all important grain crops in dry regions-are C4. Grain amaranth is also a C4 plant.

In semiarid rangelands, most of the grasses and sedges that grow in the spring in response to winter precipitation are C3 species. Those that grow in the summer and fall in response to warm season precipitation are more likely to be C4 species. The proportion of C4 grass species on semiarid rangelands increases with decreasing latitude and altitude.

Part 2. VEGETATION AND FLORA

I. Vegetation vs. Flora

Vegetation refers to the structure and function of the plant cover of a particular area of interest. Flora refers to the taxonomic composition of the plant cover-the families, genera, and species that are present on a particular area of interest. Flora is described by an inventory of the taxa present.

Vegetation structure can characterized and classified in reference to several descriptors:

A. Dominant species. Which species-usually woody perennials-account for the majority of the density, cover, or biomass; smaller, herbaceous species generally have a higher frequency.

1. Density. Numbers of individuals per unit area.

2. Cover. Amount of ground (usually expressed as a percentage) surface covered by vegetation. Leaf-area index is a more specific ratio, that of the total leaf surface per unit of ground surface.

3. Biomass. Dry weight of plant material per unit area.

4. Frequency. Number of sample units (quadrats or plots) in which a particular species occurs.

Remote-sensing techniques generally measure instantaneous plant cover; they are much less useful for measuring density, frequency, biomass, or species composition.

B. Physiognomy. What is the type of the vegetation, i.e., what are the life-forms of the dominant species. Such terms as evergreen forest, deciduous forest, woodland, savanna, grassland, chaparral, and desert scrub refer to the physiognomy of the vegetation.

Vegetation function refers to such ecosystem-level processes as productivity and nutrient cycling. Productivity (e.g., kg ha^-1 y^-1) is the rate of biomass (e.g., Mg ha^-1) accumulation. Net annual primary productivity (NAPP) refers to the amount of biomass accumulated during a year. Nutrient cycling processes include many transformations of essential nutrient elements (N, P, K, S, Ca, Mg, Fe, etc.), both physical and biological. Most of the important biological processes are microbial.

II. Vegetation of Arid Lands.

Plant growth is limited by available resources, particularly water. Vegetation of arid lands thus generally displays:

1. Low cover, density, and biomass of dominant plants.

2. Seasonal variation in plant cover in response to annual rainfall patterns.

3. Interannual and interdecadal variation in cover in response to long-term variation in climate.

4. Low NAPP. On a global scale, NAPP is most closely correlated with actual evapotranspiration (AE), the amount of moisture evaporated from soil and leaf surfaces integrated over the year. AE is high were both temperature and moisture are high (i.e., in the humid tropics. AE, and hence NAPP, are low when either precipitation or temperatures are low.

The capacity of arid lands to produce forage depends on both the quantity of vegetation (NAPP), and its quality for grazing animals. Plants in all types of vegetation have evolved various mechanisms to reduce herbivory. Herbivores, in turn, have evolved mechanisms to cope with plant defenses. It can be argued that the indigenous herbivores are those best suited to make efficient use of the indigenous vegetation.

III. Flora of Arid Lands.

A. Biodiversity.

Biodiversity is most commonly defined as species richness, which refers to the number of species found in a particular area. The total number of vascular plant species found on the Earth is not known, but probably is around 250,000 to 300,000.

Detailed floristic inventories exist for Europe and North America only; the size and composition of the flora in most parts of the Earth is still incompletely known.

Based on best available estimates, the highest species richness is found in the humid tropics, while arid regions have comparatively low species richness. [Why does that matter?]

Within a floristic province, species richness is strongly related to surface area. The form of the relationship (the "species-area curve") is generally a power curve:

S = c A^Z

or

log S = log c + z log A

A plot of the estimated size of floras for the different nations of the Earth shows a great deal of scatter. The value of the constant (c) is lower for arid regions, such that for a given size area arid regions have a much smaller number of species.

B. Floristic Plant Geography.

The following is taken from Takhtajan (1986, Floristic regions of the world, University of California Press, Berkeley), the first comprehensive, world-wide treatment of floristic geography.

Takhtajan provides a hierarchical treatment of the world's floras. The principal units in Takhtajan's hierarchy are Kingdom, Region, and Province. This system is based in theory on the degree of endemism among taxa of various categories:

Kingdoms. Have endemic families and a very high percentage of endemic genera and species. Takhtajan recognizes six kingdoms. Floras from different kingdoms have very few species in common-mostly just weedy species.

Regions. Have a high percentage of endemic genera and species, and usually a few small endemic families. Regional floras differ in their characteristic families. There are 35 regions in Takhtajan's system.

Provinces. Have several, usually small endemic genera and approximately 25 % endemic species. There are 153 floristic provinces in Takhtajan's system.

Following is a brief account of the floristic distribution of the world's arid and semiarid zones:

I. Holarctic Kingdom.

Region 6. Mediterranean Region. Nine provinces. Numerous endemic genera in the Brassicaceae, Fabaceae, Apiaceae, Lamiaceae, Asteraceae. This is considered a young flora characterized by increasing adaptation to aridity. Dominant species are mostly evergreen, sclerophyllus shrubs. Similar vegetation (but different floras) occurs in all areas of the world with similiar climates, i.e., cool, wet winters and hot, dry summers.

Region 7. Saharo-Arabian Region. Two provinces. Included are the Saharan Desert and all but the southwestern and eastern portions of the Arabian Peninsula. The total flora of the region is only about 1500 species with few endemic genera. The entire region is desert and semidesert.

Region 8. Irano-Turanian Region. Twelve provinces. This is a vast region of cool, winter-rainfall deserts, arid steppes, and oak and juniper woodlands (very similar to the southwestern United States), including the Iranian and Gobi Deserts. There are numerous species of Artemisia and Astragalus with many Chenopodiaceae. Endemic species and genera are concentrated in the Caryophyllaceae, Chenopodiaceae, Brassicaceae, Apiaceae, Boraginaceae, Lamiaceae, and Asteraceae. Many genera and species are shared with the Mediterranean Region. This region has been the source of many domesticated plants.

Region 9. Madrean Region. Takhtajan recognizes just four provinces but many of the subprovinces probably deserve the rank of province. Includes cold desert (Great Basin Province), sclerophyllus shrublands and annual grasslands (California Province), warm deserts (Mojave, Sonoran, and Chihuahuan deserts), and woodlands. Endemic genera are concentrated in the Papaveraceae, Cactaceae, Brassicaceae, Fabaceae, Rosaceae, Hydrophyllaceae, Polemoniaceae, Scrophulariaceae, and Asteraceae.

II. Paleotropical Kingdom.

Region 12. Sudan-Zambezian Region. Four subregions with nine provinces. Vegetation includes woodlands, savannas, grasslands, and deserts (includig the Thar Desert of Pakistan and India in the Omano-Sindian Subregion. Much of the savannas and thorn scrub is characteristized by species of Acacia, Aloe, Boswellia, Commiphora, Crotalaria, Euphorbia, and Indigofera.

Region 13. Karoo-Namib Region. Four provinces. Includes arid and semiarid southwestern Africa; floras characterized by many endemic species in the Aizoaceae, Asclepiadaceae, Crassulaceae, Liliaceae, Scrophulariaceae, and Poaceae. The ancient gymnosperm Welwitschia occurs here; there are many succulent Euphorbia. There is a great diversity of life forms (similar to the Sonoran Desert in Baja California) with high species endemism. The region includes the Kalahari Desert with numerous thorny plants and the Karoo Desert with many xerophytic dwarf shrubs in the Asteraceae.

Region 15. Madagascan Region. Six provinces. This region has an exceptionally high degree of endemism with many endemic families and genera, including many unusual xerophytes, such as the Didiereaceae (ocotillo-like) and Pachypodium (Apocynaceae).

Region 16. Indian Region. Four provinces, with the most arid areas concentrated in the Deccan Province. This area has dry deciduous forests and thorn forests.

III. Neotropical Kingdom.

Region 23. Carribean Region. Three provinces; semiarid thornscrubs and savannas occur in the Central American Province.

Region 26. Brazilian Region. Five provinces; semiarid regions are most common in the Caatinga Province of northeastern Brazil, and the Chacoan Province of northern Argentina, western Paraguay, and southeastern Brazil. The vegetation consists of thorny shrubs, cactus parklands, and different types of savannas in the Caatinga, and arid tropical forests and woodlands in the Chaco.

Region 27. Andean Region. Two provinces. There are three areas of desert vegetation: (1) The Prepuna, an area of dry slopes in northwestern Argentina between 2000 and 3400 m, with vegetation dominated by xerophytic shrubs and cacti; (2) the Puna, a high plateau (3400-3800 m) in northwestern Argentina, western Bolivia, and southern Peru, with a cold, dry climate with low, xerophytic shrubland and grassy steppes, including some cacti despite the high elevation; and (3) the Pacific Coastal Desert of northern Chile and southern Peru, including the Atacoma Desert, with a small flora characterized by extreme xerophytic adaptations.

IV. Cape Kingdom.

Region 28. Cape Region. A single province. This region has a Mediterranean climate with sclerophyllous vegetation ("fynbos"), very high endemism (73 % of the species), and exceptionally high species richness. There are approximately 8550 native species, roughly the same number as in the entire western United States.

V. Australian Kingdom.

Region 30. Southwest Australian Region. A single province, also with a Mediterranean climate, sclerophyllus shrublands, Eucalyptus woodlands, and sand heaths. Approximately 87 % of the nearly 3000 species are endemic. The endemic xerophytic flora includes many species of Casuarinaceae, Epacridaceae, Droseraceae, Myrtaceae, Proteaceae, Rhamnaceae, Verbenaceae, Goodeniaceae, Asteraceae, Dasypogonaceae, Xanthorrhoeaceae, and Haemodoraceae.

Region 31. Central Australian Region. A single province. This region includes the Great Sandy Desert, the Gibson Desert, and Great Victoria Desert. Many endemic genera in the Asteraceae (20), Chenopodiaceae (15), and Brassicaceae (12). Vegetation includes deserts, Acacia, Eucalyptus, and Casuarina woodlands and savannas, spinifex grasslands (Plectrachne, Triodia, Zygochloa). Halophytes are common, especially species of Atriplex and Maireana.

VI. Holantarctic Kingdom.

Region 33. Chile-Patagonian Region. Five provinces. Included within this region are semiarid Mediterranean shrublands of central Chile, the Monte Desert of northern Argentina, and the Patagonian Steppe of southern Argentina. The Monte, which has several species of creosotebush, Larrea, is a semidesert area similar in physiognomy to the Sonoran Desert. The Patagonian Steppe is a cold temperate area of low shrubs, cushion plants, and xerophytic bunchgrasses. The genera of shrubs are all South American, but grass genera include Stipa, Poa, Festuca, Bromus, Hordeum and Agropyron s.l., all of which are common in similar climates (e.g., the Great Basin) in North America. Arid areas of Argentina date from the uplift of the Andes in the Pliocene and Pleistocene, i.e., they are relatively recent in origin.

URL: http://ag.arizona.edu/~spmcl/ARL565/ARL565.Notes.Fall04.htm