The Hum of the Sun and the Fault Line

by Clement Kent

December 1995

As many of our Hort members know, I am interested in ecological issues as they apply to gardening. It being a season when gardening is confined to windowsills in Toronto, I thought you'd be interested in some lessons about the web of life gleaned in warmer climes, where the hum of the sun along the fault lines illustrated again to me the sometimes obscure workings of the balance of nature.

My wife and I recently flew over Ontario (Ca.) to Palm Springs. Although one can spend one's time in the Imperial Valley entirely in swimming pools and on the golf courses, we elected to travel out to sites in the surrounding desert for our daily amusement.

Any vacant lot in Palm Springs shows you the true nature of this land. Sandy, gravelly soil lies mostly exposed to the sun and to the dry, dry air, clothed only in the occasional hardy shrub or mesquite bush. Most of the valley near Palm Springs is windy (vast farms of peculiar Triffid-like windmills squat in the gap between mountains where the Interstate highway and railroad pass through) and incredibly hot in mid-summer. Temperatures of 128 F in the day, going down to 1 1 8 at night, conspire with the wind and very low humidity to rob the surface soil of all moisture.

Even cacti are rare in this hot, dry desert. But just where the desert meets the mountain slope lies the famous Palm Canyon, in which runs a cool stream lined with majestic native fan palms - Washingtonia filifera. These rare survivors from the glacial epoch are wrapped in the dead branches of previous years' fronds, which stay on the trees for many years and may act as insulation for the trunks.

The Washingtonia palm has "shallow" roots which go down "only" 8-10 feet - compared to the 50 feet the roots of a mesquite or creosote bush may delve for water. The palms can live only where a con- tinuous supply of fresh water, at or near the surface, bathes the roots. Ten millennia ago the valley was cooler and rainier and the Salton Sea made beaches on the mountain slopes. Then the palms lined the beaches and the moist stream valleys. As the land dried up in the warmer interglacial the trees retreated until they are now restricted to tiny populations in a few rare oases. Palm Canyon is on such oasis. But there are others hidden away in the desert nearby, such as in the Coachella nature preserve.

As you drive to Coachella it is hard to imagine you will find anything alive, much less palms. Palm Canyon's stream is fed in part by water that is sieved from the sky by high mountains, but Coachella has only bone dry hills surrounded by miles of desert. Yet against these hills runs a straggling line of palms, while the land nearby is dry and sere. As you round a curve you spot a cluster of dozens of palms in one spot - the oasis.

As you walk from the parking lot into the oasis you pass from pure desert to lush jungle in the space of fifty feet. Under the palms the air is cooler and birds can be heard high in the branches. Many kinds of plants and animals are found only in the palm oases, nowhere else in the world. In the midst of the grove a faint trickling noise is heard, then Hallelujah! you see the spring emerging from the sand and flowing down into the wash.

At the point where the damp sand of the oasis meets the dry sand of the desert the soil is covered by crusts of salts, white, yellow, and umber. The sun evaporates the water here faster than it can be replenished and the salt crusts build up until sandstorms blow them away. From the midst of the crusty surface rise the stems of alkali-loving plant of the desert, such as the alkali goldenbush, Haplopappus. These tough plants grow in soil saturated with salts that would instantly kill a petunia or pansy.

Further down, the stream runs into a few pools in which live desert pupfish, an extremely rare survivor from the time when water was common. These are now an endangered species and some have been transplanted into the palm oasis to aid their survival.

Next to the pupfish pool there is perhaps 30-40 feet of marsh. Here the soil is not the sandy gravel of the desert. It is a fine-textured material called "earthquake gouge" formed by the crushing and abrasion along the San Andreas fault, which runs right through the oasis. The nearest Canadian analogy might be the glacial "flour" deposited by streams running down out of the Rockies, also formed by crushing and grinding. In the marsh, growing in the gouge, are grasses 7-8 feet tall. Their lush foliage was spotted with small clear lumps of some material. Was it salt secreted from the leaves?

As I looked more closely I saw that many of the leaves were covered with colonies of emerald green aphids. Might the clear lumps be aphid honeydew? But the aphids were so numerous I feared for the grasses. What might control the aphids?

I looked more carefully among the aphid colonies, and thanks to my cherry trees in Toronto I understood what I saw. You may recall the story of how each spring my cherries are attacked by aphids, which I don't spray. Instead, several kinds of insect predators kill off the infestation within 2 weeks of its reaching its peak.

Here in the palm oases I saw the larvae of one such predator busily at work among the aphids. Small maggots crawled on the leaves. hoovering up aphids and turning them into flies. Once I saw the larvae, I stopped, stood and listened.

The desert can be a very quiet place, when the Marines aren't flying practice missions right overhead. In the quiet I heard a low humming, the hum of sun and fault line. It was the hum of the wings of the adult hoverflies whose maggots were eating the aphids. Once I knew what to look for I could see them all around me, perched on palm leaves sunning themselves. I heard a second hum, more occasionally but louder, which was a small dragonfly zooming through to scoop up the occasional winged adult aphid flying above the reeds.

Both the hoverflies and the dragonflies are tied to the oasis with bonds of water. There is no food for the hoverfly larvae in the desert 10 months of the year, so they survive during the drought months only in the oasis. The dragonflies can and do fly out many miles from the oasis to feed on other insects, but the dragonfly eggs are laid in water where the nymphs pass their lives until emerging as winged adults.

Thus, we were in the presence of a simple part of a special ecosystem. The sun and the earthquake gouge and the water fed the grasses which fed the aphids which fed the hoverflies and dragonflies which in their turn are eaten by spiders and birds. Even within so small a scope as a 10,000 year old oasis the balance of nature is preserved.

But where does the water come from? The answer will please all those who believe in maxims about clouds and silver linings. The water comes from the earthquake! What rain does fall on the desert in large part falls through the coarse, gravelly soils which usually have little organic matter to trap the water. Eventually the water reaches impermeable rock or clay layers, sometimes at great depth, and then flows underground.

However, when these great underground flows meet the fractured rock along the San Andreas and other faults, they are sometimes barred from further progress. The fault may have lifted the impermeable rock layer tens or even hundreds of feet on the other side of the fault line. The water pools up, and in some places bubbles up to the surface along the fault. There it nourishes an oasis for a short distance until it disappears again back underground.

In the interpretive centre of the park we saw an aerial photograph. The line of the San Andreas fault could be picked out quite clearly. Here and there upon it were black dots - palm oases. In one of these we later visited, Cottonwood Springs in the Joshua Tree National Park, the springs had been declining over a hundred years, from a strong flow of thousands of gallons per day in the last century to just a trickle in the sixties. The oasis seemed doomed.

Then came the destructive San Fernando earthquake of 1971. Although the epicentre was far away, the fault shifted enough in the desert to increase the flow of surface water back to healthy levels.

When we saw Cottonwood Springs in early December, the leaves of the cottonwood trees were just turning yellow. Where they fell into the wash they became so dry that they crackled like firecrackers under foot. A beautiful Zauschneria californica plant grew just at the edge of the water, its brilliant red trumpets providing sustenance for the local hummingbirds. Further up the wash, more hummingbird flowers were blooming - the waxy flower clusters of a few ocotillo plants threw a tongue of fire into the desert. A jackrabbit browsed on the fresh green grass under the palms, and sipped cautiously from the stream. All this courtesy of the San Andreas fault!

So, gentle reader, the next time you take out your hose to water the lawn, or your sprayer to zap the aphids, think about the delicate and unexpected webs of life that hold together the balance of nature. Think of the palm oasis humming with wings powered by sun and earthquake, and then think of what obscure threads may bind together our own Ontario ecosystems.
