By Donald Edward McHenry, Junior Naturalist

THE MOST CHARACTERISTIC plant association found in the transition climatic life zone* in Grand Canyon is that represented by the tall, sturdy Western Yellow or Ponderosa Pines. To walk in the forest beneath the towering orange-yellow trunks of these trees is an experience recalling the stately pillars of ancient cathedrals, so open and free from undergrowth is the forest floor.

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*Climate similar to that of North Dakota and Montana.

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As if to add to the charm of this sylvan sanctuary, isolated groups of Gambel Oaks, Quercus grambelii, are found judiciously scattered here and there among these pines. These oaks seem always to be found in company with the yellow pines at Grand Canyon. They grow in colonies of a few to an average of about thirty individuals, and range in height from a foot or so to about fifteen feet.

We can hardly refer to Gambel Oaks as individuals because of their peculiar communal life. Rarely does one find one of these trees growing alone. The colonial habit which they have developed is the result of an interesting if not unusual type of root system. Instead of producing the tap root commonly found in members of its family, the Gambel Oak develops an elaborate horizontal underground root system in some ways similar in growth habit to the rhizome of a fern. This horizontal root system grows in several directions from the "parent" tree of the colony and from these "runners" new aerial shoots arise at intervals. In this manner all the individuals of a colony continue to live attached to a common horizontal root system. Beneath each tree a limited secondary root system develops. One would conclude that, provided with sufficient tools and man power, an entire colony of Gambel Oaks could be dug up and transplanted en toto.

So characteristic is this method of colonial propagation that some persons have erroneously assumed that these trees produce no acorns, but reproduce themselves entirely through their unique root systems. A little reflection soon reveals the fallacy of such a conclusion. On the one hand individual colonies are too widely separated to be accounted for in this manner. Then, too, acorns can easily be gathered from the Gambel Oaks during the fruiting season.

These oaks, together with the associated Western Yellow Pines, are found growing in somewhat restricted areas in the Grand Canyon National Park on the South Rim and in a narrow belt just along the edge of the North Rim. They give these parts of the forest the pleasing park-like appear so typical of our transition zone areas.

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