Part 35 (2/2)

[Ill.u.s.tration: Fig. 64. Tolerant Maple. The trees are too slender to stand alone. _U. S. Forest Service._]

[Ill.u.s.tration: Fig. 65. Intolerant Aspen, a ”nurse” of Tolerant Spruce._ U. S. Forest Service._]

This matter of tolerance has also much to do with the branching of trees. The leaves on the lower branches of an intolerant tree will not thrive, with the result that those branches die and later drop off.

This is called ”cleaning,” or natural pruning. Intolerant trees, like aspen and tulip, Fig. 66, clean themselves well and hence grow with long, straight boles, while tolerant trees, like spruce and fir, retain their branches longer.

[Ill.u.s.tration: Fig. 66. Intolerant Tulip. Notice the long, straight boles. _U. S. Forest Service._]

The distribution of a species may also be determined by geographical barriers, like mountain ranges and oceans. This is why the western forests differ radically from the eastern forests and why the forest of Australasia is sharply distinct from any other forest in the world.

Any one or several of these factors, soil, moisture, heat, and light, may be the determining factor in the make-up of a forest, or it may be that a particular tree may survive, because of a faster rate of growth, thus enabling it to overtop its fellows and cut off their light. The struggle for survival is constant, and that tree survives which can take the best advantage of the existent conditions.

Besides these topographical and climatic factors which help determine the distribution of trees, a very important factor is the historical one. For example, the only reason by which the location of the few isolated groves of big trees in California can be accounted for is the rise and fall of glacial sheets, which left them, as it were, islands stranded in a sea of ice. As the glaciers retreated, the region gradually became re-forested, those trees coming up first which were best able to take advantage of the conditions, whether due to the character of their seeds, their tolerance, their endurance of moisture or whatever. This process is still going on and hardwoods are probably gaining ground.

Besides these external factors which determine the composition and organic life of the forest, the trees themselves furnish an important factor in their methods of reproduction. These, in general, are two, (1) by sprouts, and (2) by seeds.

(1) Most conifers have no power of sprouting. The chief exceptions are pitch pine and, to a remarkable degree, the redwood, Fig. 67. This power, however, is common in broad-leaved trees, as may be seen after a fire has swept thru second growth, hardwood timber. Altho all the young trees are killed down to the ground, the young sprouts spring up from the still living roots. This may happen repeatedly. Coppice woods, as of chestnut and oak, which sprout with great freedom, are the result of this ability. The wood is poor so that it is chiefly used for fuel.

[Ill.u.s.tration: Fig. 67. Sprouting Redwood Stumps. Glen Blair, Calif.

_U. S. Forest Service._]

(2) Most trees, however, are reproduced by seeds. Trees yield these in great abundance, to provide for waste,--nature's method. Many seeds never ripen, many perish, many are eaten by animals, many fall on barren ground or rocks, and many sprout, only to die. The weight of seeds has much to do with their distribution. Heavy seeds like acorns, chestnuts, hickory and other nuts, grow where they fall, unless carried down hill by gravity or by water, or scattered by birds and squirrels.

Trees with winged seeds, however, Fig. 68, as ba.s.s, maple and pine, or with light seeds, as poplar, often have their seeds carried by the wind to great distances.

[Ill.u.s.tration: Fig. 68. Winged Seeds. 1, Ba.s.swood; 2, Box-elder; 3, Elm; 4, Fir; 5, 6, 7, 8, Pines. _U. S. Forest Service._]

Again some trees, as spruce, are very fertile, while others, like beech, have only occasional seed-bearing seasons, once in three or four years. Willow seeds lose their power of germination in a few days, and hence, unless they soon reach ground where there is plenty of moisture, they die. This is why they grow mostly along water courses. On the other hand, black locust pods and the cones of some pines keep their seeds perfect for many years, often until a fire bursts them open, and so they live at the expense of their compet.i.tors.

It is such facts as these that help to account for some of the acts of forest composition,--why in one place at one time there is a growth of aspens, at another time pines, at still another oaks; and why beeches spring up one year and not another. That red cedars grow in avenues along fences, is explained by the fact that the seeds are dropped there by birds, Fig. 69.

[Ill.u.s.tration: Fig. 69. Red Cedar Avenue. Seeds dropped by birds which perched on the fences. Indiana. _U. S. Forest Service._]

The fact that conifers, as the longleaf pine, Fig. 46, p. 200, and spruce, Fig. 55, p. 212, are more apt to grow in pure stands than broad-leaved trees, is largely accounted for by their winged seeds; whereas the broad-leaved trees grow mostly in mixed stands because their heavy seeds are not plentifully and widely scattered. This is a rule not without exceptions, for beech sometimes covers a whole mountain side, as Slide Mountain in the Catskills, and aspens come in over a wide area after a fire; but later other trees creep in until at length it becomes a mixed forest.

The essential facts of the relation of trees to each other in the forest has been clearly stated by Gifford Pinchot thus:[1]

The history of the life of a forest is a story of the help and harm which trees receive from one another. On one side every tree is engaged in a relentless struggle against its neighbors for light, water and food, the three things trees need most.

On the other side each tree is constantly working with all its neighbors, even those which stand at some distance, to bring about the best condition of the soil and air for the growth and fighting power of every other tree.

The trees in a forest help each other by enriching the soil in which they stand with their fallen leaves and twigs, which are not quickly blown or washed away as are those under a tree in the open. This collection of ”duff” or ”the forest floor” retains the moisture about their roots, and this moist ma.s.s tends to keep the temperature of the forest warmer in winter and cooler in summer. The forest cover, Fig.

55, p. 212, consisting largely of foliage, has the same effect, and in addition protects the bark, the roots, and the seedlings of the trees from the direct and continuous hot rays of the sun. Without the shade of the leaves, many trees, as white pine, would quickly die, as may readily be seen by transplanting them to the open. The ma.s.s of standing trees tempers the force of the wind, which might overthrow some of them, and hinders the drying up of the duff.

[Ill.u.s.tration: Fig. 70. Shallow Roots of Hemlock. Bronx Park, New York, N. Y.]

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