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Cleaning and weeding are two similar terms referring to the practice of selecting particularly desirable trees in a young stand and removing or killing trees that threaten their survival or development.〔Smith et al. (1997), p.147〕 Used correctly, the term cleaning refers to the removal or killing of overtopping competitors that are significantly taller than the desired trees, while the term weeding refers to the removal of trees that are of the same height, but still competing for the resources that could be used by the selected trees. Colloquially, these treatments are often referred to as crop tree release when they are practiced in sapling sized stands. Both treatments are conducted when the stand is young, in the seedling or sapling stage. Cleaning is common in softwood plantations, clearcuts, and overstory removals, where the desired conifers are overtopped by rapidly growing early-successional hardwood species. Herbicides are often used in these cleaning operations because the correct chemical at the correct dose and time will kill only broadleaved species, leaving the conifers unharmed and free to grow. Where chemicals are impractical or unpalatable to the landowner, a brush saw is used to cut competing trees close to the ground. Foresters will often conduct these treatments as early as possible, but not so early that new growth of hardwoods can overtake the planted seedlings again. Their goal is to maintain the health and vigor of tree species that are preferred for some use, often structural material. Liberation cutting is similar to cleaning, with the exception that the competing trees are much older than the desired trees. Weeding is often done at a later stage, to allocate growing space to selected individuals that have demonstrated superior quality. This usually means a straight trunk that will make a good sawlog, or perhaps a healthy crown on a mast-producing tree. In any case, desirable qualities have been identified in each particular tree, and competing trees are removed to promote the desired trees. To help distinguish between cleaning and weeding, consider these two images from the northeastern United States. Both pictures focus on an eastern white pine of good future sawlog quality. In the first, the short-lived and undesired balsam fir has overtopped the pine: the good quality pine will likely die. This is a cleaning situation, albeit an abnormally late one. In the second, the balsam fir compete from the sides: the good quality pine will almost certainly not die, but it is ready to grow more quickly. This is a normal weeding situation. File:Cleaning Required.jpg|Cleaning File:Weeding Needed.jpg|Weeding In many situations where the trees are in the large sapling stage, the distinction between cleaning and weeding is blurred from tree to tree: this example provides a distinction that is not always made in practice. Most cleanings are conducted at the seedling stage with herbicides in plantations and clearcuts in order to control the species composition and guide the stand to a desired future condition as early as possible. This treatment is a standard and well developed feature of management on large landbases that produce pulp and softwood lumber for building. Later intervention is usually a weeding, and requires a certain amount of brute force and precision. Small woodlot owners often perform this operation in large sapling sized stands that have been acquired from defunct timber companies. They do this to improve the quality of their woodlots and to produce their own firewood. Competing trees at this stage are often large enough to burn, but small enough for fit and properly trained landowners to comfortably handle falling and transporting. File:Weeding Before.jpg|Eastern white pine in need of weeding File:Weeding During.jpg|Weeding in progress File:Weeding After.jpg|Weeding completed File:Weeding After 3.jpg|Brush after a weeding operation, before firewood collection File:Weeding After 4.jpg|Aftermath of a weeding operation with all brush removed To conceptualize the allocation of growing space, imagine a party where there is one pizza: if there are ten guests, each gets one slice and wants more, but if there are only one or two guests they are well fed. Trees get their energy from the sun, and there is only so much sunlight falling on a given area. If that area is occupied by a very large number of trees, each receives a small portion. There may be a lot of energy being captured, but it is being distributed between so many stems that none grow very quickly. If the growing space is occupied by fewer trees (but still enough to eventually grow into a closed canopy), each will receive a greater portion of energy and the individuals will grow in diameter faster, yield a heartier seed crop, and produce more defensive compounds to respond to wounds, drought, and insect attack 〔Smith et al., p.48〕 ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cleaning (forestry)」の詳細全文を読む スポンサード リンク
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