Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 160

FIGURE 8.6
Desert marigold ( B. multiradiata ). Desert marigold also displays the plant adaptation of having grayish green
foliage and many small trichomes on the leaf surfaces.
The annual habit is a very successful strategy for warm-arid climates. There are no
annual plants in the Polar Regions or the wet tropics. In the polar zones the growing season
is too short to complete a life cycle. In both habitats the intense competition for suitable
growing sites favors longevity (once you've got it, you should hang onto it). Annuals
become common only in communities that have dry seasons, where the perennials are
widely spaced because they must command a large soil area to survive the drier years.
In the occasional wetter years both open space and moisture are available to be exploited
by plants that can do so rapidly. The more arid the habitat, the greater the proportion
of annual species in North America that you will see in the landscape. (The percentage
decreases in the extremely arid parts of the Saharan-Arabian region.) Half of the Sonoran
Desert's flora is comprised of annual species. In the driest habitats such as the sandy flats
near Yuma, Arizona up to 90% of the plants are annuals.
Winter annuals provide most of the color for our famous wildflower shows. Woody
perennials and succulents can be individually beautiful, but their adaptive strategies
require them to be widely spaced so they usually don't create masses of color. A couple
of exceptions are brittlebush when it occurs in pure stands, and extensive woodlands
of foothill palo verde ( Cercidium microphyllum ). The most common of the showy winter
annuals that contribute to these displays in southern Arizona are Mexican gold poppy
( Eschscholtzia mexicana ), lupine ( Lupinus sparsiflorus and Lupinus arizonicus ), and owl clover
( Orthocarpus purpurascens ).
One of the contributing factors to the great number of annual species is niche separation.
(A niche is an organism's ecological role, e.g., sand verbena [ Abronia villosa ] is a butterfly-
pollinated winter annual of sandy soils.) Most species have definite preferences for
particular soil textures, and perhaps soil chemistry as well. For example, in the Pinacate