(Picture 1) A disbursed grove of mature ponderosa trees with no young trees growing among them. One single young tree is growing outside of the grove in direct sunlight. Photo by Robert Dryja

By ROBERT DRYJA
Los Alamos

Ponderosa trees do not tolerant shade. 

Young ponderosa seedlings close to adult ponderosa trees grow slowly or not all in the shade of adult trees.  A grove of ponderosa trees therefore spreads out with few or no young trees growing among them. Low intensity forest fires reinforce this arrangement by burning low growing trees and shrubs. (A low intensity fire does not burn the upper branches of tall mature trees.)

The long term combined result of shade and fire is a disbursed grove of trees that are the same size, (See Picture 1).

An alternative growth pattern occurs if a high intensity forest fire occurs. Only a few adult trees survive and are very spread apart. Sun light now can shine below the adult trees. Seeds falling directly from adult trees can successfully grow in the sunshine. The result is a field young trees that are growing close to one another, (See Picture 2). The young trees will become thinned out by low intensity forest fires.  

One grove of adult trees may survive a low intensity fire while an adjacent grove is burnt from a high intensity fire. The result is a field of young trees that suddenly stops next to a field of adult trees, (See Picture 3).

Other factors can affect how a grove of trees may grow. For example, the overall slope coming down a mountain side may be interspersed with broad, gentle valley-like areas. These valley areas may hold moisture and so be more favorable for the growth of trees. The trees may be twice as tall as trees growing further up or down the slope, (compare the height of the trees in Pictures 2 and 4). Soil sediment also may accumulate in a valley rather than washing down slope. The sediment further supports the growth of the trees.

(Picture 2) Young ponderosa trees are growing among a few widely disbursed adult trees that survived a high intensity forest fire. The young trees are about five feet tall. The fire occurred twenty-two years earlier. Photo by Robert Dryja

(Picture 3) Two groves of tall or short ponderosa trees are growing next to one another as a result of the low and high intensity fires that passed through. The shade under the tall trees also is discouraging the growth of young trees. Photo by Robert Dryja

(Picture 4) Larger new growth trees are growing in a broad valley area. The trunks lying on the ground reflect the thinning from a low intensity fire from twenty two years ago. Photo by Robert Dryja