Visiting Arizona--July in the Desert Near Tucson

I recently attended the Botanical Society of America annual meeting, this year in Tucson, Arizona. Of course we botanists, gathered from not only across America but from around the world, spilled out of the conference center to see the surrounding desert, botanical gardens, plant breeding centers, and more.

The daily high temperatures were over 100º F and some nights the low was above 80º F. I expected it to be like being in an oven, and it was. But I also expected the desert to be brown, like the photo below of the desert in Baja California in April

Dry hillside, Baja California (April)

--but the desert was green! (See below)

Tucson hillside, July

The Tucson area averages 12" of rain a year (compare San Francisco 24", Chicago 36", New York 45" a year, map ). If you divide 12" across the year, each month would get 1" of rain, which is very little. But it doesn't fall evenly. Winter has storms and monthly averages of up to an inch, but it almost never rains at all from April to June. Think about plants surviving with no water for three months! July brings the monsoons, storms, and the average rainfall for July and August is 2", a lot in Arizona. (see chart, note the lines are 1/3"). Summer rains are relatively predictable and lots of plants use that water to flower. It is not the dramatic carpet of color seen in March this year (link, link)--those are winter annuals flowering. The summer flowers are perennials, including big cacti, shrubs such as acacias, and smaller herbaceous perennials.

Not only that, there were leaves on plants that are normally leafless.

It was hot, but it was fascinating.

A pea-family (Fabaceae) shrub with bright yellow flowers, probably desert senna, Senna covesii.

The small shrubby plant with scarlet flowers is ratany, genus Krameria, although I do not know which species. Ratanies are the only genus in a plant family of 18 species, the Krameriaceae, native from Arizona and Texas to southern Chile. See also In Defense of Plant's story about them link.

ratany, Krameria

There were leaves on ocotillo (Fouquieria splendens). Its stems are photosynthetic and it drops its leaves in dry periods. That nice green leafy look conceals nasty spines up and down the stems. Ocotillo is another unusual southwestern plant. It is classified in the Fouquieriaceae, a plant family with one genus, Fouquieria and 11 species; the family's total range is from the southwestern U.S. to southern Mexico. 

ocotillo, Fouquieria splendens

The green parts of cacti are modified stems, not leaves, which adapts them to dry conditions. A very few cacti ever have leaves, and those only briefly. Here is a cholla (genus Cylindropuntia, cactus family Cactaceae) and if you look carefully at the branches, there are tubular green leaves along the stem. That inch or so of rain was enough for this plant to grow leaves. Compared to stems, leaves photosynthesize better, bringing more energy to the plant, but they lose more water, too. The cholla is growing while the growing is good.

cholla with leaves 

closer view of leaves

Here is a look down the hill. Lots of green leaves on the shrubs.

So don't be fooled by Arizona's high temperatures. The plants are active in July and August. Lots to see.  Leaves, flowers, and pollinators, day and night. Most bees and butterflies flew off before I could take a photo. Although I didn't get the focus right, below are two flies visiting thee flowers of rough menodora, Menodora scabra, olive family Oleaceae. 

rough menodora, Menodora scabra, and flower-visiting flies

It is a very tough climate and yet there are diverse native plants and animals.  

Comments and corrections welcome.

Kathy Keeler, A Wandering Botanist

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