|
|
|
||
3. Komposisi Umur
Hutan seumur adalah Suatu hutan yang ditanam pada waktu bersamaan. Meskipun demikian ukurannya dapat berbeda karena perbedaan laju pertumbuhan.
Hutan segala umur terdiri dari pohon-pohon yang besar hingga tingkat semai. Jadi meliputi berbagai umur maupun ukuran.
Hutan tidak seumur adalah hutan yang hanya mempunyai dua atau tiga kelompok umur atau ukuran. Misalnya hutan yang terdiri dari pohon-pohon yang sudah masak tebang, miskin riap dan ukuran pancangnya saja.
Hutan segala umur biasanya penyebaran ukurannya lebih beragam dan umumnya jenis yang lebih toleran naungan. Sementara hutan seumur umumnya terdiri dari jenis intoleran. Gangguan alam seperti angin topan, kebakaran, bencana alam ataupun penebangan berlebihan menciptakan celah di dalam hutan sehingga menciptakan kelompok-kelompok yang tidak seumur.
Adanya variasi hutan menyebabkan sulitnya pelaksanaan praktek silvikultur, seperti pada hutan alam produksi di berbagai tempat di Indonesia.
4. Tipe Hutan
Tipe hutan ialah istilah yang digunakan bagi kelompok tegakan yang mempunyai ciri-ciri yang sama dalam susunan jenis dan perkembangannya. Ini disebabkan oleh faktor-faktor ekologi tertentu, merupakan kelompok alami atau asosiasi berbagai jenis pohon yang tumbuh bersama pada suatu daerah yang luas. Tipe hutan diberi nama menurut satu atau lebih jenis pohon yang dominan. Cara yang lazim digunakan di Indonesia menurut formasi hutan, yaitu suatu kelompok vegetasi yang mempunyai bentuk (life form) yang sama. Misalnya pembagian menurut Van Steenis (1950), seperti berikut ini.
Menurut Soerianegara dan Indrawan terdapat keragaman yang besar dalam vegetasi hutan di Indonesia, baik dari segi keadaan lingkungan dan tempat tumbuhnya, maupun dari susunan floristiknya, sehingga pada waktu ini masih belum mungkin untuk menyusun klasifikasi vegetasi Indonesia yang lengkap.
Photosynthesis is the process by which plants make food. The word photosynthesis means putting together with light. In green plants, sunlight captured by chlorophyll enables carbon dioxide from the air to unite with water and minerals from the soil and create food. This process also releases oxygen into the air. People and animals must have this oxygen to breathe. Most photosynthesis takes place in small bodies called chloroplasts within the cells of plant leaves. These chloroplasts contain chlorophyll, which absorbs sunlight. Energy from the sun splits water molecules into hydrogen and oxygen. The hydrogen joins with carbon from the carbon dioxide to produce sugar. The sugar helps a plant make the fat, protein, starch, vitamins, and other materials that it needs to survive. See PHOTOSYNTHESIS. Some plants, called parasites and saprophytes, have little or no chlorophyll and cannot produce their own food through photosynthesis. These plants must rely on outside sources for food. Parasites attach to living plants and take the nutrients they need from these plants. Saprophytes grow on dead and decaying organisms, or use organic substances produced by living organisms for food. Mistletoe and dodder are common parasites found in many parts of the world. Mistletoe grows on the trunks and branches of many trees. It is called a partial parasite because it also makes some of its own food. Indian pipe is a saprophyte that grows near fungi. It uses organic materials produced by fungi for food. A plant called giant rafflesia is a parasite that grows on the roots and stems of other plants. It bears the largest flower of any known plant. Rafflesia flowers may grow over 3 feet (91 centimeters) wide. Respiration breaks down food and releases energy for a plant. The plant uses the energy for growth, reproduction, and repair. Respiration involves the breakdown of sugar. Some of the products resulting from this breakdown combine with oxygen, releasing carbon dioxide, energy, and water. Unlike photosynthesis, which takes place only during daylight, respiration goes on day and night throughout the life of a plant. Respiration increases rapidly with the spring growth of buds and leaves, and it decreases as winter approaches. Factors affecting plant growth. A plant's growth is shaped by both its heredity and its environment. A plant's heredity, for example, determines such characteristics as a flower's color and general size. These hereditary factors are passed on from generation to generation. Environmental factors include sunlight, climate, and soil condition. Hereditary factors. Within the nucleus of all plant cells are tiny bodies called chromosomes that contain hereditary units called genes. These bodies contain "instructions" that direct the growth of the plant. As the cells divide and multiply, the "instructions" are passed on to each new cell. See CELL; HEREDITY. Substances made within a plant also play a part in regulating plant growth. These substances, called hormones, control such activities as the growing of roots and the production of flowers and fruit. Botanists do not know exactly how all plant hormones work. But they have learned that certain hormones, called auxins, affect the growth of buds, leaves, roots, and stems. Other growth hormones, called gibberellins, make plants grow larger, cause blossoming, and speed seed germination. Still other hormones called cytokinins make plant cells divide. Environmental factors. All plants need light, a suitable climate, and an ample supply of water and minerals from the soil. But some species grow best in the sun, and others thrive in the shade. Plants also differ in the amount of water they require and in the temperatures they can survive. Such environmental factors affect the rate of growth, the size, and the reproduction of all plants. The growth of plants also is affected by the length of the periods of light and dark they receive. Some plants, including lettuce and spinach, bloom only when the photoperiod (period of daylight) is long. Such plants are called long-day plants. On the other hand, asters, chrysanthemums, and poinsettias are short-day plants. They bloom only when the dark period is long. Still other plants, among them marigolds and tomatoes, are not affected by the length of the photoperiod. They are called day-neutral plants. Plants also are affected in other ways by their environment. For example, a plant may display a bending movement called a tropism. In a tropism, an outside stimulus (force) causes a plant to bend in one direction. A plant may have either a positive or a negative tropism, depending on whether the plant bends toward or away from the stimulus. Tropisms are named according to the stimuli that cause them. Phototropism is bending caused by light, geotropism is caused by gravity, and hydrotropism is caused by water. A plant placed in a window exhibits positive phototropism when its stems and leaves grow toward the source of light. Roots, on the other hand, display negative phototropism and grow away from light. However, roots demonstrate positive geotropism. Even if a seed or bulb is planted upside down, its roots grow downward-toward the source of gravity. The stem of the same bulb shows negative geotropism by growing upward-away from the source of gravity. Hydrotropism occurs chiefly in roots and is almost always positive. See TROPISM. Some plants are affected by being touched. When the sensitive plant, Mimosa pudica, is touched, its leaflets quickly fold and its branches fall against its stem. A change in pressure within certain cells of the plant causes this action. After the stimulus has been removed, the plant's branches and leaflets return to their original position.
