Even though the geological record indicates that arid conditions existed for a long time (since the Devonian), the current modern desert flora might have originated in the Miocene, expanded in the Pliocene (after restrictions during moist periods in the Cretaceous and Tertiary), and reached its current distribution only during the Pleistocene. Specifically, the deserts in the North American Southwest are relatively young. Overall richness and uniqueness of desert floras reflect size, age, and isolation of desert areas, with larger deserts typically hosting larger numbers of endemic species. Smaller desert regions and edges of larger regions are often characterized by species that evolved in adjacent more mesic areas and partially adapted to arid conditions. A good illustration is the high incidence of Mediterranean plants in desert areas bordering regions with semiarid Mediterranean climates in all parts of the world. In general, desert floras tend to have high affinity to bordering semiarid climate zones, such as Mediterranean climate-type regions and semiarid grassland. Taxonomical studies of many species groups revealed that desert species have evolved (recently) from nondesert species. Biogeographically, strong floristic links exist between old deserts in North Africa, Middle East, and Asia. Floristic similarities among desert regions stretching from North Africa to Central Asia are particularly obvious since no wide barriers of ocean or humid vegetation exist to restrict plant migration; these floristic similarities are present despite strong climatic contrasts ranging from hot environments in North Africa to the much colder, arid Central Asia deserts. Apparent links between the North American Great Basin and Central Asian deserts might be explained by plant migration across the Beringian land bridge. Clear affinities between the deserts in both Americas can be explained by the Panamanian land bridge. In this respect, the distribution of Larrea shrubs is remarkable. The two recognized species — Larrea divaricata in South America and L. tridentata in North America - are taxonomically and phenotypi-cally very close. It appears that the genus Larrea evolved in South America and migrated only tens of thousands of years ago (bird assisted?) to North America where it quickly became the dominant shrub in all warmer desert areas. Corresponding to the isolation of the Australian continent, the flora of the Australian desert is very different from all other deserts of the world.
Dominant plant life forms in deserts reflect water stress conditions typical for deserts (for a treatment of drought adaptations see the following section). While trees are relatively rare and restricted to more-mesic microsites, a wide range of plant life forms can be found that include many short-lived and seasonal active plants (e.g., annual or ephemeral plants and bulbous plants/ geophytes). The dominant life forms that visually shape
the plant formations are perennial woody plants (mostly shrubs) and fleshy succulent plants (cacti and others). Large succulent species can be dominant in some of the hot desert regions (e.g., the saguaro cactus in the Sonoran Desert). A few plant families are predominant in desert areas. The aster family (Asteraceae) is the most diverse plant family in deserts overall; it is especially numerous in Australia, southern Africa, the Middle East, and North America. Some deserts can be dominated by grass species (Poaceae). Some plant families have their global center of diversity in deserts and most likely evolved here. Notable examples are the chenopods (Chenopodiaceae) that are diverse in arid and semiarid regions of Australia, North America, and from the Sahara to Central Asia. The New World cacti (Cactaceae) are another example of a group of species rich in deserts but relatively sparse in other biomes.
Deserts are home to some of physiognomically extremely unusual plant types. Worth mentioning in this respect are plant characters as the Joshua trees of the Mojave Desert (Yucca brevifolia), the famous Welwitchia of the Namib (Welwitchia mirabilis), and the boojum tree (Figure 10) of the Sonoran Desert in Baja California (Fouquieria columnaris). Exactly why and how deserts host these exceptional plant types is not clearly understood and such 'Dr. Seussification' of the desert flora deserves systematic study.
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