Deserts and chaparral

In a subtropical desert the soil surface can pass 60 °C in the day and fall close to 0 °C at night.

Where the deserts are

Subtropical deserts lie between about 15° and 30° north and south of the equator, centred on the Tropics of Cancer and Capricorn. OpenStax describes them as very dry: in some years more water evaporates than falls. Annual precipitation is under 30 cm, it varies little from month to month, and it cannot be predicted well. In parts of central Australia, the Outback, and in northern Africa, a year can bring as little as 2 cm.

The air above a desert holds very little water, and water vapour acts like a blanket that slows the loss of heat. Without it, the ground heats fast in the sun and loses heat fast at night. OpenStax gives soil surface temperatures above 60 °C in the day and close to 0 °C at night in hot deserts. Not every desert is hot. In cold deserts temperatures can reach 25 °C and drop below −30 °C, a swing of more than 55 degrees across the year.

How desert life saves water

Very dry deserts have few perennial plants that live from year to year. Instead many plants are annuals. Their seeds can lie dormant for long periods, then sprout when rain finally comes, grow quickly, flower, set seed and die before the ground dries out again. Other plants hold on through the dry times with deep roots, few or tiny leaves, and stems that store water. The ocotillo of the Sonoran Desert grows leaves only after rain and then sheds them. Desert animals avoid the heat of the day by being active at night and by burrowing underground, where the temperature changes much less.

Chaparral: shrubs that need fire

The chaparral, also called scrub forest, grows in California, around the Mediterranean Sea and along the southern coast of Australia. It gets 65 cm to 75 cm of rain a year, mostly in winter, and the summers are very dry. Many chaparral plants are dormant in summer. The vegetation is dominated by shrubs adapted to periodic fires. Some produce seeds that will only germinate after a hot fire, and the ash left behind is rich in nutrients such as phosphorus and potassium that fertilise the soil for the new growth.

For people living there, this means fire is part of the landscape rather than a rare accident. Managing chaparral and the towns beside it is a question of when and how fires burn, not whether they burn at all.

Two answers to a short water supply

Deserts and chaparral both force plants to deal with dry spells, and the solutions look alike. Some plants wait: desert annuals sit as seeds until rain, and many chaparral plants go dormant through the dry summer. Others store or save water with deep roots and small, tough leaves. OpenStax links the low animal diversity of deserts to the low and unpredictable rain, since plants are the base of every food web there. The difference is reliability. Chaparral rain is scarce in summer but returns every winter; desert rain may skip a whole year.