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Adaptations of Plants To Aridity

Most desert species have found remarkable ways to survive by evading drought (Robichaux 1999). Desert succulents, such as cacti or rock plants (Lithops) for example, survive dry spells by accumulating moisture in their fleshy tissues. They have an extensive system of shallow roots that allows them to capture soil water only a few hours after it has rained. Their photosynthesis is modified to exchange gases and fix CO during the night, when evaporative demand is low, and to accumulate the fixed carbon in the form of malic acid, which is later used by the plant as a building block of more complex organic molecules (this photosynthetic pathway is called "Crassulacean Acid Metabolism" or CAM).

Additionally, many cacti and other stem-succulent plants of hot deserts present columnar growth, with leafless, vertically-erect, green trunks that maximize light interception during the early and late hours of the day, but avoid the midday sun, when excessive heat may damage, or even kill, the plant tissues. Thus, erect columnar plants may avoid drought by (a) accumulating water, (b) exchanging gases at night, and (c) morphologically avoiding midday exposure to solar radiation (Zavala-Hurtado and others 1998). Instead of having developed succulent stems adapted for water storage, other CAM plants such as aloes and agaves have developed ground-level rosettes of succulent leaves. Because of the funnel-shape of their leafwhorls, these plants seem to be adapted to collect dew from morning fogs in desert mountains, hillslopes, and ocean coasts, accumulating the water thus harvested in their fleshy leaves and central bud (Martorell and Ezcurra 00 ).

Woody desert trees, such as acacias, cannot store much water in their trunks but many of them evade drought by shedding their leaves as the dry season sets in, entering into a sort of drought-induced latency. Many of these desert species also have deep taproots that explore deep underground water layers. Other trees have convergently evolved a mixture of these strategies: they can store water in gigantic trunks and have a smooth bark that can do some cactus-like photosynthesis during dry periods; but, when it rains they produce abundant green leaves and shift their metabolism towards that of normal-leaved plants. This group is formed by trees with famously "bizarre" trunks, such as the African baobab (Adansonia), the Baja-Californian Boojum-tree (Fouquieria columnaris) and elephanttrees (Bursera and Pachycormus), and the South African commiphoras (Commiphora), bottle-trees (Pachypodium), kokerbooms (Aloe dichotoma, Figure 1.1 ) and botterbooms (Tylecodon).

A third group of plants, the "true xerophytes" or true desert plants, have simply adapted their morphology and their metabolism to survive extremely long droughts. These species have remarkably low osmotic potentials in their tissue, which means that they can still extract moisture from the soil when most other plants cannot do so. True xerophytes, such as the creosote bush (Larrea), are mostly shrubs with small, leathery leaves that are protected from excessive evaporation by a dense cover of hairs or a thick varnish of epidermal resin. Their adaptive advantage lies in their capacity to extract a fraction of soil water that is not available to other life-forms. However, because their leaves are so small and protected from transpiration, their gas-exchange metabolism is very inefficient during rain pulses when moisture is abundant. In consequence, these species are extremely slow growers, but extremely efficient water users and very hardy.

Finally, one of the most effective drought-survival adaptations for many species is the evolution of an ephemeral life-cycle. A short life and the capacity to leave behind resistant forms of propagation is perhaps one of the most important evolutionary responses in most deserts, found not only in plants but also in many invertebrates. Desert ephemerals are amazingly rapid growers capable of reproducing at a remarkably high rate during good seasons, leaving behind myriad resistance forms that persist during adverse periods. Their population numbers simply track environmental bonanzas; their way to evade critical periods is to die-off, leaving behind immense numbers of propagules (seeds or bulbs in the case of plants, eggs in the case of insects) that will restart the life cycle when conditions improve. These opportunist species play an immensely important role in the ecological web of deserts: myriad organisms, like ants, rodents, and birds, survive the dry spells by harvesting and consuming the seeds left behind by the short-lived ephemeral plants. Granivory (the consumption of seeds) and not herbivory (the consumption of leaves) is at the base of the food chain in most deserts, as those few plants that maintain leaves during dry spells usually endow them with toxic compounds or protect them with spines. The onset of rainy periods brings to the desert a reproduction frenzy of desert ephemerals, and a subsequent seed-pulse that drives the entire food web for years.



From the information above it can be seen the survival strategies of desert plants are classifiable along a gradient ranging between two extreme categories: (a) adaptation for quick use of ephemerally abundant resources, or (b) adaptation for the efficient use of poor but more permanent resources (Shmida 1985). The first category, typically exhibited by desert ephemerals, represents a "maximum variance" behaviour that consists essentially in tracking environmental variation, while the second category, exhibited by true xerophytes and cacti, is a "minimum variance" behaviour that consists in adapting to the worst possible conditions. Drought-deciduous perennials and grasses represent a compromise between these two extreme behaviours. Attributes necessary for the quick use of water include rapid growth (often at the cost of low water-use efficiency) and abundant seed production. Attributes for survival with little water include high water-use efficiency, slow growth, and passive cooling. Drought deciduousness, as an intermediate strategy, requires the capacity to shed leaves and to quickly recover them when moisture conditions improve.

The survival strategies of desert plants present some of the most striking cases in nature of evolutionary convergence: plants from widely different families and from divergent evolutionary origins have developed, in the different deserts, life-forms so similar that it is sometimes difficult to tell them apart. Such is the case of the succulent cactoid growth form, evolved in Africa from the families Euphorbiaceae, Asclepediaceae, and Aizoaceae, and in the Americas from the family Cactaceae. Similarly, bottle trees in Africa evolved from the families Apocynaceae, Aloeaceae, and Crassulaceae, while in the New World they belong to the Fouquieriaceae, Anacardiaceae, and Burseraceae.

 
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