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Trees of Yosemite (1932, 1948) by Mary Curry Tresidder


Willow Family

Salicaceae

The willow family is represented in the Yosemite National Park by trees of two genera.

The first of these is the genus Salix, the willow proper. It has a number of species in the Park, but most of them are shrubs rather than trees. The willows may easily be distinguished as such, but to identify the species, which run through many variations, is often a matter for microscopic study. We have therefore ’ described the genus, with brief reference to the species commonly found in our region.

The other branch of the family is the poplar, genus Populus, to which the Black Cottonwood and the Quaking Aspen belong. Scientists tell us that the poplar is a tree of very ancient origin; from fossil remains they say that in the days of the Cretaceous period in Greenland, perhaps fifty millions of years ago, the poplar was the only one of the deciduous trees to grow among the darker evergreens. There are indications that they were to be found in this country and in Europe during the Miocene portion of the Tertiary era, twenty million years or so ago.

The trees of the willow family are deciduous; they have alternate simple leaves, with a leaf-stem at whose base is a pair of stipules—small, scale-like or leaf-like appendages—which often fall away long before the leaf. They have their flowers in long, narrow spikes, or catkins, with the staminate and pistillate flowers occurring on different plants. The fruit is a capsule containing many seeds, each with a tuft of hairs at the base, an aid to wide distribution by the wind. The fruit usually matures in the spring.

The bark is very bitter, particularly so in the genus Salix. Trees of the willow family are of rapid growth; they sprout freely both from the root-crown and from cut-off stumps, and grow readily from cuttings.

Salix has rather narrow leaves, with a short leaf-stem; the winter buds are covered by a single scale; the catkins are usually erect, and appear with or before the leaves. The leaf-scars on the willow have three small dots. Populus has a broader leaf and a longer stem; the buds are covered by several scales; the catkins in our species are pendulous, and appear before the leaves. They grow into larger trees than the other genus; this is particularly true of the cottonwood.

The sand-bars at the upper end of Mirror Lake give an excellent demonstration of the willows’ effect in consolidating the banks of streams and the margins of lakes. As the current of the stream is slowed down on entering the lake, the silt it carries is dropped. The willows soon take hold in the soil thus brought to their threshold, and the delta becomes larger year by year.


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