Post 00325
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It is only a question of a few years until seeds are carried to every treeless locality. They may journey down-stream or across lakes on a log, fly with birds across mountain-ranges, ride by easy stages clinging to the fur of animals, or be blown in storms across deserts; but these adventurous seeds may find grass in possession of the locality and so thickly sodded that for a century or longer they may try in vain to establish a forest.
Commonly wind-blown seeds are first upon the ground and the most numerous. Though it is of advantage to be the first upon the ground, it is of immense importance that the seed which falls in an opening produce a seedling which thrives in the sun-glare,–which grows without shade. The seedlings of our great oaks and most strong and long-lived trees cannot thrive unless shielded from the sun, sheltered from the wind, and protected from the sudden temperature-changes which so often afflict openings. While these maintain the forest areas, they extend it but little. Only a small number of trees have the peculiar frontier characteristics. Young trees which cannot live in the sun are called tolerant,–they tolerate shade and need it. Species which conquer sunny territory are called intolerant,–they cannot stand shade and need sunlight. It will thus be seen that the acquirement of treeless territory by any species of tree demands not only that the tree get its seeds upon the earth in that territory, but also that the seeds, once there, have the ability to survive in the sunlight and endure the sudden changes of the shelterless opening. Most species of oaks, elms, firs, and spruces require shade during their first few years, and though they steadfastly defend possessions, they can do but little toward winning new territory. On the other hand, aspens, willows, gray birch, cottonwood, old-field pine, and lodge-pole pine produce seedlings that glory in the sunlight and seek to gain more territory,–to push forward the forest-frontier.
Again and again the forest has been swept away by fire; but again and again a few aggressive species have retaken speedily the lost territory. In this pioneer reclamation the aspen and the lodge-pole are leaders. The aspen follows the water-courses, running along the muddy places, while the lodge-pole occupies the dry and rocky slope of the burned area. Seen from a distance the aspen groves suggest bright ribbons and pockets on the sombre cloak with which lodge-pole drapes the mountain. And even beneath the trees the contrast between the methods of these two agents of reforestation is marked. The lodge-pole pine is all for business. Its forest floor is swept clean and remains uncarpeted. The aspen groves, on the other hand, seem like the haunts of little women. Here the floor has a carpet of grass gay with columbines, sweet peas, and wild roses. While the aspens and the lodge-poles are still young they begin to shelter the less hardy coniferous seedlings. But sooner or later both the aspens and the lodge-poles themselves are smothered by their nurslings. They then surrender their areas to forest trees that will live to be many times their age.