Download Desert Biogeography Labkey

Desert Biogeography Lab Key
Adaptations of Sonoran Desert Plants to Heat/Aridity
The Sonoran Desert falls into a category of deserts referred to as Tropical-Subtropical
Desertlands. These areas all lie near the Tropic of Cancer or Capricorn, and owe their
aridity to warm dry subsiding airflow associated with subtropical high pressure. In
addition, these deserts have westerly airflow that has steered maritime air masses over
mountains or cold ocean currents before arriving depleted of moisture. The Sonoran
Desert contains many plant species which have evolved from members of more humid
tropical biomes to the south. These plants have a variety of adaptations that enable them
to live in conditions far more arid than their progenitor species. This lab is designed to be
completed at the demonstration garden at the Glendale Public Library.
What are the three main adaptive strategies used by desert plants to survive aridity?
1. Succulence
2. Drought tolerance
3. Drought evasion
Succulence
What are three ways in which succulent plants are adapted to arid conditions?
1. Succulents are capable of storing water in fleshy leaves, stems, or roots.
The water is stored by binding with extracellular mucilage (green snotlike substance).
2. Succulents have extensive shallow root systems so that they can take
advantage of even small rainfall events.
3. Most succulent species have waxy cuticles to seal in moisture.
What is the most common group of succulents in the Sonoran Desert?
Cactus family (Cactaceae)
How many cacti species are there in the world? In the Sonoran Desert?
Approximately 1800 species in the world, all of which but one is native to the
Americas.
Approximately 300 species occur in the Sonoran Desert.
What are some of the characteristics that identify a cactus?
1. It must have a cactus flower. All plant species are assigned into families
based on their flower characteristics. Cactus flowers have numerous
tepals (combined sepals and petals) that intergrade with each other, and
have numerous stamens.
2. Cactus tend to have spines (modified leaves) that arise in clusters from
small gray fleshy mounds known as areoles.
What are the three large columnar cacti of Arizona and how can you identify
them?
1. Saguaro (Carnegiea gigantea) – 40 to 60 feet tall, at maturity with one to
numerous arms branching from the main stem from 4 to 8 feet above the
ground.
2. Organpipe (Stenocereus thurberi) – 9 to 11 feet in height with numerous
unbranched stems all arising in a cluster from the ground. More frost
sensitive than saguaro, limited to far southern Arizona.
3. Senita (Lophocereus schotti) – 10 to 13 feet in height and generally
resembling an organpipe. However, senita can be distinguished by having
far fewer ribs than the organpipe. More frost sensitive than saguaro also,
limited to southern Arizona.
What are the other major genera of cacti in Arizona and what do they look like?
1. Opuntia – are recognized as being segmented cacti.
Prickly pear have flattened disc-like segments
Cholla have cylindrical segments.
2. Ferocactus – known as the barrel cacti, members of this genus are short,
relatively round, and unbranched.
Agaves and Yuccas belong to what plant family?
Agave family (Agavaceae). Related to lilies and were previously included in
the lily family. Species tend to have white flowers with three petals.
What type of succulents are they?
Leaf succulents.
Drought Tolerance
How are drought tolerant plants adapted to arid conditions?
Drought tolerant plants are not capable of storing water during times of low
rainfall. Drought tolerant plants are capable of slowing down all of their vital
life processes so that they are able to live on less water. These plants may let
their leaves, twigs, or entire branches die to save water. These plants are also
capable of extracting water from soil much drier than other plants are
capable of. Drought tolerant plants also tend to have very small leaves to cut
down on evaporation
What is the major drought tolerant shrub in Arizona?
Creosote bush (Zygophyllaceae). It is capable of living on less water than any
other known plant.
What family of plant has large numbers of drought tolerant genera and species?
The pea and bean family (Leguminaceae/Fabaceae)
What are these genera and how can one recognize them?
Palo verde genus (Cercidium) – smooth green bark
Mesquite genus (Prosopis) – shaggy brown bark
Ironwood genus (Olneya) – smooth gray bark with large thorns
Acacia genus (Acacia) – tiny, tiny leaves
Fairyduster genus (Calliandra) – medium-sized shrub with acacia-like leaves
Drought Evasion
How do drought evaders tolerate arid conditions?
Drought evaders only live brief lives and therefore do not have to live
through long dry periods. Their seeds germinate and grow quickly after a
rain, they flower, set seed, and then die all within a few months.
What types of plants are drought evaders?
Technically they are known as annuals, but we know them as either weeds or
wildflowers.
When during the year do these plants germinate and grow?
Arizona annuals are referred to as either winter or summer annuals. Winter
annuals germinate following winter rains and bloom in the spring. Summer
annuals germinate following summer rains and tend to bloom in the late
summer and fall.
What part of Arizona has many of these plants?
The drier a desert is, the higher percentage of its plants follow the drought evasion
strategy. Therefore, southwestern Arizona has the highest percentage of its plants
taking this strategy, although not necessarily the greatest total number.