Volatile chemicals are abundant in desert scrub and Mediterranean vegetation. The scented oils evaporating from the leaves of evergreen Mediterranean scrub (such as garrigue, Chapter 2) can make a hillside smell like one big pot pourri, and the sagebrush of the American southwest can also smell rather strong. It is generally thought that these compounds have a protective role in making the plants distasteful or indigestible to grazers. However, some ecologists have suggested that the plants use them for an additional purpose: being kept warmer and frost-free because of the "greenhouse" heat-trapping properties of these chemicals (which strongly absorb infra-red light). However, atmospheric physicists calculate that the "heat-trapping" effect of these chemicals is probably not strong enough to make any significant difference.
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