Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 161

region of northwestern Sonora there are places where gravels of volcanic cinder are
dissected by drainage channels or wind deposits of fine silt. In wet years purple mat ( Nama
demissum ) grows abundantly on the gravel and the related sand bells ( Nama hispidum ) on
the silt. I have seen the two species within inches of each other where these soil types meet,
but not one plant of either species could be found on the other soil. There are specialists
in loose sand such as dune evening primrose ( Oenothera deltoides ) and sand verbena, and
others are restricted to rocky soils such as most caterpillar weeds ( Phacelia spp.). This
phenomenon of occupying different physical locations is spatial niche separation.
Another diversity-promoting phenomenon is temporal niche separation: The mix of spe-
cies at the same location changes from year to year. Seeds of the various species have
different germination requirements. The time of the season (which determines tempera-
ture) and quantity of the first, germination-triggering, rain determines which species will
dominate, or even be present at all in that year. Of the three commonest annuals of south-
ern Arizona listed earlier, any one may occur in a nearly pure stand on a given hillside in
different years, and occasionally all three are nearly equally abundant. This interpretation
of the cause of these year-to-year variations is a hypothesis based on decades of empiri-
cal observation. Much more research is needed to discover the ecological requirements
of most species of desert annuals. And of course the Sonoran Desert's two rainy seasons
provide two major temporal niches. Summer and winter annuals almost never overlap.
The dramatic wildflower shows are only a small part of the ecological story of desert
annuals. For each conspicuous species there are a score of others that either have less
colorful flowers or don't grow in large numbers. Every time the desert has a wet fall or
winter, it will turn green with annuals, but not always ablaze with other colors. One of
the most common winter annuals is desert plantain ( Plantago insularis ). It grows only a few
inches (centimeters) tall and bears spikes of tiny greenish flowers, but billions of plants
cover many square miles in good years. The tiny seeds are covered with a soluble fiber
that forms a sticky mucilage when wet by rain; this aids germination by retaining water
around the seed and sticking it to the soil. A related species from India is the commercial
source of psyllium fiber (e.g., Metamucil ® ). The buckwheat family (Polygonaceae) is well-
represented. There are more than a score of skeleton weeds ( Eriogonum spp.) and half as
many spiny buckwheats ( Chorizanthe spp.), most of which go unnoticed except by botanists
(see species accounts). Fiddlenecks ( Amsinckia spp., Boraginaceae ) may grow in solid masses
over many acres, but the tiny yellow flowers don't significantly modify the dominant
green of the foliage. These more modest species produce more biomass than the showy
wildflowers in most years, and thus form the foundation of a great food pyramid.
Some perennials also evade drought much as annuals do by having underground parts
that send up leaves and flowers only during wet years. Coyote gourd ( Cucurbita digitata )
and perennial devil's claw ( Proboscidea althaeifolia ) have fleshy roots that remain dormant
in dry years. Desert larkspur ( Delphinium parryi ) is a perennial that has woody rootstocks
but also grows only in wetter years. Desert mariposa ( Calochortus kennedyi ) and desert
lily ( Hesperocallis undulata ) have bulbs that may remain dormant for several years until a
deep soaking rain awakens them. Our desert wildflower displays are in jeopardy from
invasive exotic plants. Species such as Russian thistle ( Salsola tragus ), mustards, especially
Sahara mustard ( Brassica tournefortii ), red stem filaree ( Erodium cicutarium ), and Lehmann's
lovegrass ( Eragrostis lehmanniana ) are more aggressive than most of the native annuals and
are crowding them out in many areas where they have become established. Some are still
increasing their geographic ranges with every wet winter. Disturbed sites such as sand
dunes, washes (naturally disturbed by wind and water, respectively), roadsides, and live-
stock-grazed lands are particularly vulnerable to invasion by these aliens.