Green Thumb: Plant adaptations in an arid landscape

No matter where you come from, the first time you see the beauty of the Sonoran Desert, is a moment you won’t soon forget.

The landscape here is strikingly different from anywhere else in the world.

When you think about it, though, what is it that makes it so different from anywhere else? Sure, there are the obvious things like giant saguaro cacti, but take those away and you still have an unusual environment. The desert just flat out looks different.

Plants grow where they do for a simple fact: it is where they are adapted to grow.

Lush green trees and pines that aren’t from arid environments could die if you planted them in the desert without special care. Similarly, a saguaro cactus would die pretty quick if you planted it in Seattle. The broad-leaved plants growing in the north are adapted to that climate just as the giant saguaro is adapted to the Sonoran Desert.

This brings us back to why the Sonoran Desert looks the way is does. The plants here are adapted to aridity. It is this adaptation that makes our environment look so much different from much of the rest of the world.

How a plant looks and behaves is basically a product of the environmental influences placed on it throughout its evolution. In the desert, the biggest limiting factor placed on plants is aridity. Sure, there is frost in some parts but the sheer lack of water is the driving force of evolution here.

You can break down the methods plants use to survive in an arid environment into three basic techniques: Succulence, Avoidance, and Tolerance.

Succulence

The obvious one and the one that arguable adds the most to the charisma of the desert is Succulence. The truth is not all cacti grow in deserts and not all deserts have cacti; however,the succulent nature of a cactus is an adaptation to arid conditions.

Now, cacti aren’t the only succulent plants — far from it, actually. Succulent plants store water in their leaves, stems, and roots. Cacti, agaves, aloes, many euphorbia, Ice Plants, and elephant trees are all succulents; they have developed a way to store water in their tissues. They take up water very quickly when it is available, more then they need immediately in fact, and store it away for when they do need it during drier periods. This “behavior” is a direct adaptation to living in an arid environment.

Avoidance

Another favorite adaptation to drought for gardeners is Avoidance. Some plants simply avoid activity when the weather isn’t favorable. These are annual plants. The reason we have beautiful wildflower shows only every few years is because the plants are living in seed form until there is enough water to sustain them. These plants have evolved to simply not exist when the conditions are unfavorable.

Annual plants complete their entire life cycle (germination, flowering, and seed production) in one season. When the conditions are right, when there is enough rain, they germinate and quickly flower and produce seeds to ensure a future population. The drought-avoidance strategy is so effective that many of the plants in arid regions are annual plants. In fact in the Sonoran Desert roughly half of the total plants are annual species and in the driest of habitats the annuals can comprise up to 90 percent of the total flora.

Tolerance

Finally, we get to the last strategy, Drought Tolerance. Plants in this category have developed ways to survive in the desert without drying out and without storing water in their tissues. These plants are flat out tough.

Here in the Sonoran desert we are talking about Palo Verde Trees, Creosote bushes, and Brittlebush to name a few.

These plants have developed structures to withstand the extremely arid times. Palo Verde Trees for instance have very small leaves. Most water loss in plants is attributed to transpiration from the leaves. The bigger the leaf, the more water loss. Also, Palo Verdes tend to be leafless most of the year. To further prevent water loss, they shed their leaves. Because they have green stems, they are able to continue to photosynthesize and produce energy even without leaves.

Plants that don’t usually drop their leaves, such as creosote, have a waxy coating on the surface of the leaves to hold water in.

The root systems of these plants also differ from plants that aren’t adapted to arid conditions. Drought-tolerant plants have extensive roots systems, sometimes extending up to twice the diameter of the canopy.

Although most of the roots of drought tolerant trees are still within the top 3 feet of soil, they do have some roots that can reach extreme depths. Because water that is deeper in the ground remains in the soil longer, plants with deeper roots systems can use the water over a longer period of time.

Another feature of drought tolerant plants can be seen in Brittlebush. We’ve all noticed that the green here in the desert is different from the greens of the forest or the tropics. It’s more of a pale green or a greenish-gray. Brittlebush is a perfect example.

If you’ve ever felt a Brittlebush leaf, you know that it is soft and almost velvety. What you’re feeling are tiny little hairs that cover the surface of the leaf. These hairs are a way to prevent too much heat buildup on the leaf and to prevent moisture loss. The tiny hairs shade the surface of the leaf and on a hot windy day the hairs prevent the wind from getting to the surface and desiccating the plant..

The plants here in the Sonoran Desert are tough. They can survive the hottest and driest years, and look great doing it.

The techniques they use to survive here are what make them so special and they are what give the Sonoran Desert landscape its unique beauty.

As gardeners, it’s important for us to understand the plants in our areas. If we know the conditions our plants are adapted to we can better understand how to care for them.

When designing a garden, we can utilize our knowledge of the form and structure of the plants to create a more interesting garden. Most importantly, if we utilize plants that are native to our region we can conserve resources and have a garden that exists more harmoniously with the native environment.

On top of that, you can enjoy the interesting beauty of the desert in your own back yard. After all, if you remember how you felt the first time you saw the desert imagine having that feeling every time you go out into your yard.

Green Thumb articles are written by Green Valley Gardeners with assistance from seminar speakers.

Details

Daniel Armenta is a horticulturist with the Arizona-Sonoran Desert Museum in the Botany Department.

He will speak at the Green Valley Gardeners seminar Thursday, Oct. 9. Remember that coffee is ready at 9 a.m. and his talk will begin at 9:30.