Too Wet!

A surprising limiting factor in some parts of the desert is too much moisture.

Wait, you say, I thought deserts were too dry?

Because there’s usually so little rain, nobody is really prepared for much. So even what might be considered a light rain elsewhere can be enough to flood out desert burrows, nests, hollows, and lowlands. If you are a creature who can move fast, or a plant that can hold its breath, you will get through it.

But even in the desert, rain is not always light. During the summer monsoon months, and a few other times of year, there can be heavy downpours.

And adding to the effect, there’s that old caliche we talked about before. This is the impervious layer of minerals and clay that forms over time a short distance down under the desert surface, and acts as a water-trap. The water can’t sink in–so where it can, it slides off downhill.

With a light rain, you can suddenly get a flash flood. With a heavy, or extended rain, you can get massive dangerous flooding, the kind that kills animals, large as well as small, and washes away trees and cars and buildings.

In the middle of many human habitations in the desert, you will find long empty wild spaces, often called “washes.” These are where the flash floods wash through, carrying away whatever is in their path.

It’s strange, when you think about it, how moisture could be a problem, and a limiting factor, in both directions in the desert.

Or maybe not. Because a lot of living things want conditions that are not too much, or too little. Instead, they want things to be generally within an agreeable range. So when you look at a limiting factor, it can be fun to check if there is another one lurking at the opposite side. Yet another mystery.

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A Dry Heat

The joke is, in the Sonoran desert, even though it’s 114, it’s a dry heat.

The idea being that with very low humidity, amazingly, under 5 percent at times, the heat is easier to bear.

114, an average high for a few days each summer, is still bad enough, but given how few trees and other shelter there is in the desert, it’s almost always a sunny heat during the daytime, which adds another 7 to 10 degrees.

All plants and animals and people that live in the desert need to be built or adapt in some way to survive these extreme temperatures.

The hottest time of year is a few days before the summer Solstice on June 21, and a few weeks after that. Then the average daily temperature starts going down again, very very gradually.

A few weeks after the solstice is also the time when the summer monsoons really get going. The standard definition of monsoon season is the change in the wind, but in the desert, the important thing is that the monsoon usually brings moisture and finally rain from the Gulf of California and the Gulf of Mexico.

And of course, when the humidity goes up and the rains start, it’s still over 100 degrees in the desert. And it’s no longer a dry heat.

Ocotillos in bloom

It is mid-April and the ocotillos are in bloom in the Sonoran desert, a great sight to see.

These plants can leaf out several times a year after rains, but they only bloom once a year, the blossoms providing food for a variety of insects and birds.

Limiting factors in the desert

In some ways the term “limiting factor” is almost a definition of deserts, because deserts lack one or more essential ingredients for the survival of most kinds of plants and animals.

First, a desert is, by definition, a place with very little available water, either because there is little rainfall, or because it evaporates so fast. It may also be that most of the precipitation is unavailable because it is frozen most of the year, as in the far north or south, or in some high altitude regions.

Some areas may get bursts of moisture, even flooding, but it is episodic and not dependably available most of the year.

Temperature is another limiting factor. Many deserts get too hot for most organisms. Some deserts near the north and south poles are too cold. And yet others have wide swings in temperature.

Wind is another. There are few trees or shrubs to stop the wind. Desert mountains, river-courses or other sheltered areas may harbor small oases in their wind and sun shadows.

One unusual limiting factor is lack of sunlight. This can occur in deserts near the poles.

Especially interesting are the adaptations that plants and animals have developed to deal with these limiting factors. Many of them are what we talk about here.

Leafing again: the strange case of the ocotillo

One of the odder native residents of the Sonoran desert is the ocotillo bush. It typically consists of several spindly branches, vertically striped and spiny, up to 20 feet tall, that are leafless for long periods, only to sprout thick lines of small leaves within a few days after a rain.

Ocotillo branches with few leaves

Its desert adaptation of dropping leaves and playing possum is so complete that ocotillo plants for sale, in their leafless phase, with their branches tied in a bundle and roots bare, look thoroughly dead.

Ocotillo branches with many leaves

In the spring, they produce numerous red tubular blossoms at the ends of the branches that attract insects and birds of many species. They make great landscape plants because of their sparse sculptural look, and extremely low water requirements, and striking seasonal changes.

Why does the cactus look like that?

The first thing you notice about a cactus is generally the spines. Some have short, nasty little almost invisible fishhooks, some inches-long needles. Then there’s the form of the plant. Most cacti are plump, rounded, often stubby. They tend to be grayish-green in color. And they usually do not have obvious leaves.

Why?

Whenever you come across questions like this, think about the ecosystem where the organism lives. Cacti live in dry climates with a lot of sun, and few other plants, especially leafy ones.

So keeping cool is an issue. A light neutral color does not absorb a huge amount of the sun’s heat. There are very few plants with dark bark, or skins, or leaves in the desert.

Retaining moisture is perhaps the most important issue for a desert plant. The blunt, compact shape of cacti, their thick skins, and lack of leaves are all moisture-conserving features.

And lastly, toothsome, moist leaves would attract all kinds of herbivores. So the lack of leaves discourages plant-eaters, and the big spines and tough hide protect any soft, moist tissue a cactus does have.

Strangeness in the desert soil: Caliche

Here’s one way to encounter caliche. Some friends pulled over one night in the New Mexico desert and camped in their truck. Happily for the desert, there was rain in the night. Unhappily for the humans, the next morning they found themselves surrounded by a huge pond of extremely slippery ankle-deep goo. Only after a long dirty struggle were they able to push their truck back onto the road. Caliche is what allowed a mudbath to form in the middle of the desert.

Another way to discover caliche is to dig a hole almost anywhere in the desert. Soon, inches or feet deep, you’ll hit a layer of what seems to be cement, even in places where you know there’s never been any construction.

What you’ve found is caliche, a white layer of soil, rich in calcium carbonate, clay, and other minerals that have been driven down into the ground by the hard infrequent desert rains, until the minerals form an often impervious layer of soil, also known as hardpan.

Wikipedia can tell you more.

When rain falls on an area with dense caliche, rather than sinking into the soil, the water runs off, if it can find a channel, sometimes causing the surprising desert flash floods, because so little water is absorbed into the ground. Or it puddles up, creating a sea of mud.

Digging a simple hole can require a pickaxe. Since the calcium carbonate can be alkaline enough to harm plant roots, planting in an area with caliche means that you may have to add conditioners as well as nutrients to the soil– and make sure there is drainage, even if that means digging a drainage hole through the caliche.

The Arizona Master Gardener, a great source of information, gives details of how to deal with it.

Dry rain

Even the simplest things about the desert can be strange.

Rain, for instance.

When you need rain the most, where you need it most, seems to be the place you find it least.

And sometimes it comes unusually close, but never arrives.

An eerie and frustrating example is the dry rain that can occur in the desert west, called “virga.”

In the distance, you will see the clouds form, then curtains of rain finally begin to fall. But the rain disappears in the middle of the sky, between cloud and land.

Leaving you high and dry. Or low and dry.

Virga actually does happen in other regions, and can involve snow as well as rain.

It’s a sight to see, watching the rain come down, but never land on land.