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Winter And Summer Rainfall Patterns

Two important weather systems bring precipitation to the world's deserts. The horizontal transport by wind of moist air from the sea into the land (known as advective transport) during winter, when the land becomes colder than the sea, causes atmospheric condensation over the cold continents and generates winter rains. Because this particular pattern of summer droughts and winter rains dominates the coasts around the Mediterranean Sea, the areas of the world that show this type of seasonal variation are called "Mediterranean" regions. In summer, in contrast, a different weather system is the main driver of rainfall pulses in arid lands: as the continents become hot they generate low-pressure centres with rapidly ascending warm air; and as the air ascends vertically (a phenomenon known as convective transport) the atmospheric mass cools rapidly and condenses large amounts of air moisture, which pour down in the form of summer thunderstorms. In many parts of the world this rainfall pattern is known as the "monsoon."

Most of the large deserts of the world lie between these two weather patterns: the large midlatitudinal deserts share their boundary with winterrain ecosystems in their higher latitudes, and with monsoon regions near their tropical reaches. The Sahara Desert, for example, is dominated by winter rains in its northern Mediterranean limit, and by summer rains in its more tropical Sahelian border. The Sonoran Desert, in North America, receives mostly winter rains in its northwestern reaches, near the Mojave, and is fed by the Mexican summer monsoon in its tropical southern reaches (Dimmit 2000). In South America, the Monte Desert receives mostly winter rains in its cold Patagonian austral limit but is dominated by summer thunderstorms near the Tropic of Capricorn (Ezcurra and others 1991).

Plants in winter-rain deserts have a rather narrow window of opportunity for growth. During winter, moisture accumulates in the soil but the weather is often too cold for plant growth. During summer, temperature may be adequate but the soil is parched and waterless. Only during spring, when temperatures start to rise and moisture is still present in the ground, can plants flourish. For perennial plants, retaining the leaves from the previous year gives the shrubs an early start and a competitive edge during the short spring. But in order to keep the foliage on until the next year the leaves have to survive the dry desert summer, when no rains occur and the soil is bone-dry. To avoid water loss during the hot season, many evergreen desert shrubs have developed small leaves, or leaves with a thick, leathery epidermis, and few, small stomata (the pores through which leaves breathe and fix carbon dioxide). Thus, evergreen shrubs with small and/or tough, leathery leaves are the dominant life form in winter-rain deserts.

Tough leaves, however, are not the only mechanism through which plants can efficiently use the short desert spring season. The winterrain deserts of the world are also amazingly rich in short-lived spring ephemerals. These plants survive the scorching summer in the form of seeds, bulbs, or tubers, and quickly sprout during the narrow window of opportunity that the desert spring provides. In contrast with the evergreen shrubs, their leaves are soft and tender, and their stomata are large, allowing the plants to grow very fast when conditions are favourable. One of their most astonishing traits is the lack of any evident adaptations to aridity other than fast growth. Because desert ephemerals complete their life cycles during short periods of abundance, natural selection has favoured fast growers and even faster reproducers.

In monsoon deserts, in contrast, rainfall pulses coincide with adequate temperatures for plant growth. Because moisture becomes available at a time in which plants can grow fast, leaf retention is rare and drought-deciduousness is a dominant trait in shrubs and trees. In these deserts, perennial plants often show extensive networks of shallow roots, and many plants compete to extract water from the soil immediately after the rain has fallen. Giant fleshy trunks or cactoid succulent stems, adapted to accumulate water, are thus common in summer-rain deserts.

© UNEP 2006