Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 165

e.g., willows ( Salix sp.), sycamores ( Platanu s spp.) oaks, and grasses are all wind-pollinated.
Conifers and grasses are the dominant plants in the two biomes that bear their names.
Grasses occur in most biomes and comprise the sixth largest family of plants with about
9000 species worldwide.
8.6.1 Seed Dispersal
Seeds generally need to be transported some distance from the parent plant in order to
find a suitable site for establishment. Some plants have wind-dispersed seeds, which are
occasionally blown many miles from their origins. This means of dispersal is common
among pioneer plants —plants that are adapted to colonizing disturbed habitats. Because
of their superior ability to invade newly disturbed ground, pioneer plants comprise many
of our agricultural and garden weeds. Moreover, most annual crops are domesticated pio-
neer plants.
Many plants utilize animals to disperse their seeds in another complex coevolution-
ary process. Small, brightly colored fruits such as hackberry and boxthorn are offered
as food for birds that swallow them whole. Other fruits such as those of hedgehog cacti
( Echinocereus sp.) are large and the bird feeds on them repeatedly. Some bird fruits are
sticky such as mistletoe berries; a few stick to the bird's bill until wiped off on a branch
while others are successfully swallowed. The seeds of bird fruits are typically small and
hard; they pass through birds' guts undamaged and may be deposited many miles from
the parent plant.
Mammal-dispersed fruits tend to be larger, aromatic, not colorful (most nonprimate
mammals have poor color vision), and usually have larger seeds than found in fruits
birds feed on. The animal often transports the fruits a short distance (compared to the
flying distances of many birds) to a safer place before eating the pulp and dropping at
least some of the seeds. The seeds of coyote gourds ( Cucurbita spp.) may be dispersed in
this manner. Coyotes swallow the whole fruits of palm trees; they digest the thin pulp
and excrete the hard seeds intact. Since seeds contain energy stores to nourish the germi-
nating embryo, seeds themselves are also nutritious food for mammals and birds. Some
plants offer their seeds without juicy pulp to attract mammals. Pocket mice and antelope
squirrels gather the abundant seeds of foothill palo verdes and bury them as food caches
for the dry season. The animals don't eat all that they bury, so some seeds remain in the
ground and germinate when the rains come. (Birds that specialize in eating seeds, as
opposed to fruits containing seeds, crush and digest the seeds and therefore do not dis-
perse viable propagules.)
Even in the desert some seeds are water-dispersed. Blue palo-verde ( Cercidium floridum) )
grows mostly along washes. Flash floods disperse the very hard, waterproof seeds down-
stream, scarifying (abrading the surface) them in the process. In the absence of scarification
these seeds must weather in the ground for a few years before the seed coats become per-
meable and enable germination.
The timing of seed maturation is crucial for many plants. The less time seeds are present
before they sprout, the greater is their chance of survival. The tropically derived plants
in our region germinate with the summer rains. These species usually flower in spring
and their fruits ripen shortly before the arrival of the summer rainy season. Palo verde
and saguaro are examples. Other plants produce large quantities of seeds and rely on
camouflage or burial in the soil to conceal some of them from hungry animals. Brittlebush,
for example, flowers and seeds in spring, but the seeds germinate with fall rains. Annuals
do the same.