A City in Mid Desert

Saguaro National Park has been described as a "Wilderness with a City at its Center."

Two park districts bookend the city of Tucson, and we recently visited Saguaro East--the Rincon Mountain District.

After a stop at the Visitor Center, we traveled the eight-mile Cactus Forest Drive. The drive was one-way with numerous, but very narrow, pullovers.

The day was overcast, and those weather conditions in mid-winter did not bode well for finding colorful displays in the desert.

So, our search for color shifted from vistas to small areas of interest. The Rincon Mountains provided the background for this prickly pear cactus and

this staghorn cholla.

"The most conspicuous adaptation of the cactus family to the harsh conditions of the desert is found in the spines, which are modified leaves. In addition to protecting the plant from hungry or thirsty animals, spines provide shade during hot summer days and warmth on cold winter nights. Spines also help prevent water loss due to dry winds.

"On cholla cacti (left and below), which are often called jumping cactus, the spines also play a major role in reproduction. Cholla are segmented cacti. The plants are made up of many segments, which are loosely attached to the preceding segment. Additionally, each spine is covered with a thin sheath, which separates from the spine quite easily. When an animal accidentally brushes against the cactus, the sharp spines stick into its skin and the segment breaks off the parent plant. Eventually, the spines slip from their sheaths and the segment falls to the ground. If soil conditions are right, the segment may take root and grow into a new plant (nps.gov/sagu/planyourvisit/upload/Cacti%20of%20Saguaro%20National%20Park).

In our search for color, we had to be content with the smallest of displays, such as this plant below

and the soon-to-be-flowering fishook barrel cactus.

"If you take a close look at either a saguaro or a barrel cactus, you will notice a series of distinct accordion-like pleats on the outside of the plant. These pleats allow the plant to expand while it is absorbing rainwater, and to shrink when using its stores of water. Without the pleats, damage would certainly occur to the plant’s skin. Even so, a saguaro may take in more water than its pleats will allow.

"When this happens, the skin splits into an open wound. If this split does not heal quickly, bacteria may get into the warm, moist tissue of the plant and possibly kill the plant. As with most plants, cacti make their food through a process called photosynthesis. Unlike most plants that only take in carbon dioxide (CO2) during the day, cacti utilize a complex form of CO2 fixation known as Crassulacean Acid Metabolism, or CAM.

"This method of taking in CO2 reduces the amount of water lost to the atmosphere because the stomata (pores) are open only at night when temperatures are lower and humidities are higher. The plant changes the CO2 into four-carbon compounds, which are largely malic acid, and stores it overnight. The following day, with sunlight as its energy source, the plant completes its cycle of photosynthesis.

"As a result of CAM, the liquids within many cacti are very acidic. Contrary to popular myth, you are not able to get potable water from a saguaro" (nps.gov/sagu/planyourvisit/upload).

The relationship between the saguaro and "nurse plants" was very interesting. Seeds of the saguaro are eaten by a rodent under a palo verde or mesquite tree (for protection from predators). The seeds of the saguaro fruit must go through the digestive tract of an animal to break down the outer coating of the seed in order for germination to be possible. When the rodent later expels the seed, usually in the same area, and conditions such as temperature and rainfall are right, the seed can then begin the process of forming a living plant.

After germination, the "nurse plant" then begins the process of protecting the saguaro from direct sunlight, hard winds and fierce rains during the summer monsoon storms, and from being trampled by large animals like cattle and horses.

Unfortunately for the nurse plant, as the saguaro grows larger, it needs more and more water and nutrients, and over the decades the nurse plant does not get enough water to live.