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Yosemite Nature Notes 46(2) (1977)


forest shade
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The Influence of Shade on Yosemite’s Forests

by Steve Gold

To varying degrees, Yosemite’s forest pines are sun-loving and intolerant of shade. Because of this, we can determine from observation some things about the past history of these trees. Other Sierran trees, such as the true firs (Abies concolor, A. magnifica) or Mountain Hemlock (Tsuga mertensiana) are often much more shade tolerant, and the following generalizations may not invariably be applicable to them.

As a tree grows in height, the upper crown casts a deepening shade on the lower branches. This decreases photosynthentic efficiency and, eventually, photosynthetic activity may fall below the point where energy is produced in excess of that required for respiration. When this happens, the limb becomes an energy drain on the rest of the tree, and soon dies. When a limb is dead, it rapidly dries and becomes brittle. In this state, wind, snow, or falling limbs or trees can remove the branch.

As the tree continues to grow in diameter, the branch stub is covered with new wood. The result is a tree with the lower trunk, or bole, clear of branches, depending on the amount of shade it is subject to and the tree’s tolerance to lack of light.

Not only is shade caused by the tree’s own crown, but by other influences as well, such as the presence of other trees. Where the forest is dense, the shade is heavier, causing the trees to have higher crowns. If grown in a clearing, even a relatively intolerant tree may have branches almost to the ground.

Once a tree has shed its lower branches, they will not grow back. Therefore, if an intolerant tree spent most of its life in a dense forest which was subsequently cleared by logging, it would retain its clean bole despite increased light availability.

Trees with extensive clean boles growing in open areas are probably survivors of a disturbance such as logging. Several large sugar pines (Pines lambertiana) in the Crane Flat Campground area exhibit this characteristic — apparently they are survivors of the logging operations of the 1930’s.

Frequent fires keep forests thinned. Trees which grew under the more open conditions of the pre-fire-control era, but are later “protected” from fire, become subject to increased crowding and shade. Lower branches, able to obtain enough light to survive under the open conditions, would begin to die. On older trees, where these low branches may get quite large, they might persist on the tree for long periods, despite their being dead and brittle.

In forests which have become unnaturally crowded due to the absence of fire, often there are large trees, widely spaced, surrounded by a dense growth of smaller trees (often shade tolerant species, such as fir or Incense cedar). The large trees may have several large, dead lower branches, giving the forest an unhealthy appearance. Such conditions are widespread throughout the Sierra Nevada.

Shade functions in a natural forest to influence the shape of the trees. Disturbed forests can often be identified by inconsistencies in shade and the degree of clearing of the bole. Perhaps this means of identification can help us realize the extent of forest modification by human intervention, and spur us to take measures to reverse these changes.



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