Wednesday, March 9, 2016

One More Tree

This is a continuation of my garden in the desert, Borrego Springs. Having covered all of my trees (except one) in previous posts, I now devote this one to the remaining tree in my desert garden, California Fan Palm (Wahsingtonia filifera).

Many people (out of pure ignorance) associate palm trees with southern California, based on the huge number of exotic palms that have been planted in places like San Diego and L.A. There is really only one palm that is native to California, and it is this one. And it is not found near the coast. It only occurs in canyons with perennial water on the east slope of the mountains leading into the desert and in a few isolated desert oases where a spring delivers water close to the surface.

To understand this palm we must start with its botanical classification. It is a monocot, like the grasses (including wheat and rice), agaves, yuccas, onions, rushes, reeds, sedges, brodiaeas, lilies, iris, orchids, tulips, corn, bananas, bamboo, and much more. There are several distinguishing characteristics of monocots compared with eudicots. First, they have a one-piece cotyledon (seed leaf), unlike the two-part cotyledons of eudicots. Secondly, they have parallel veins rather than branching network veins. Compare a palm frond to a maple leaf. The veins in the maple branch and re-branch all over the place, while the veins in a palm frond run straight and parallel to each other from the base to the tip. Thirdly, the monocots lack a cambium layer; their vascular bundles are scattered throughout the main stem or trunk of the plant. Eudicots have their vascular bundles in a thin layer right under the bark, which makes the visible rings in tree wood. About a quarter of all flowering plants in the world are monocots.

To illustrate the vascular bundles, here's a photo of a cross sections of a palm trunk. Note the total absence of rings or layers.

Because of this distributed pattern of vessels, palms are somewhat less vulnerable to fire compared with eudicot trees. The latter are killed if fire destroys the cambium layer just under the bark. But palms can get seriously burned and still survive by virtue of the bundles of tubes that are in the middle of the trunk. An example of this is the palm in the forground below that has been burned but is still quite healthy. However, palms are killed if the apical meristem is burned. That's the place at the top where all the fronds emerge from. If that gets cut off or burned, the palm is dead.

Palms are vulnerable to fire in large part due to the dried fronds which either hang onto the crown to form a skirt or fall to the ground, as below. This accumulation of dry, flammable material can easily transmit from the ground up to the crown. There is evidence that the native Cahuilla people intentionally burned the palm groves for various reasons.

 Palms also have very shallow roots, as shown in the photo below. This palm and many others were uprooted from the palm oasis in Borrego Palm Canyon (Anza-Borrego Desert State Park) by a major flood event about ten years ago.

Some of these trees had been weakened by the palm borer beetle which is a natural resident of all palm groves. These palms can tolerate a lot of damage from the beetles, but eventually they will succumb.

On the other hand, there are plenty of seedlings just outside the main grove.

 These palms are not at all drought tolerant. They are only found in the canyons that have water year round, such as Borrego Palm Canyon.

The palms tend to grow in dense groves. I have read no explanation for this. Certainly they are going to grow in the stream, so they will form a linear patch. Maybe the fruits and seeds just tend to stay close to home.

The largest of the three palms in my garden does not produce dates every year, but when it does produce it is impressive. The dates were eaten by the native people, and they are enjoyed today by coyotes, birds, rodents, and other wildlife. I've eaten them, and while they aren't the tastiest dates I've ever had, they were okay. The palm groves are used by many wildlife species including nesting birds, bats, snakes, lizards, frogs, ringtails and more.

There is a really good book about Washintonia filifera. It is titled Desert Palm Oasis, and the author is James W. Cornett who was Director of Natural Sciences at the Palm Springs Desert Museum. I have based a lot of the following on his book.

For a long time it was widely believed that California Fan Palms are a relict species that had survived from an earlier epoch. It was believed to have been much more common in the past, their numbers declining as the climate changed to become warmer and drier. This is certainly the case with Torrey Pines and a few other species, so it was not unlikely that California Fan Palms would be similar. However, Cornett found two problems with this hypothesis. One is that there is no indication of these palms in the fossil record, which doesn't make sense if it was more common in the past. Secondly, the range and population of W. filifera in California appears to be expanding.

Cornett's hypothesis is that W. filifera was not present in our area until the climate changed. It moved northward from Mexico into the desert canyons of California as conditions became warmer and drier. Cornett's studies of palm groves throughout southern California shows that the number of trees in the groves is increasing. Furthermore, groves are appearing in more northerly places where they did not exist previously, such as Death Valley and southern Nevada. Cornett's conclusion: "...the desert fan palm is a recently evolved, invasive species that is rapidly expanding both in range as well as in numbers."

When Cornett says "invasive" he is not suggesting that W. filifera is an unwanted weed. Rather, he is simply describing the plant's behavior as it spreads and occupies new territory. W. filifera is now a permanent resident of the southern California deserts, and it plays several important roles in the ecology of the deserts.

Several years ago I met a man who said he lived in our house back in the 50's and 60's, and he said he planted this palm. That would make it about fifty to sixty years old now, and still going strong. I wish I had more.

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