One way trees prevent themselves from having too many braches is simply by shedding (dropping off) branches once they have fulfilled their purpose. This happens as the tree gets bigger and grows new outer layers of foliage that shade the inner and lower branches. In most large trees, the center of the canopy contains only large branches, small branches and fine twigs are found only at the canopy’s edge. In the shaded center, the small branches that would once have occupied that space are long gone. Trees like the true cypresses regularly shed small twigs complete with leaves toward the end of summer. Most other trees shed only branches that prove unproductive. If a branch is not producing enough carbohydrate to cover its own running costs—i.e., it needs to be subsidized by other branches because, for example, it is being shaded and receives little light—it will usually be got rid of. This prevents unproductive branches from being a drain on the tree and removes the wind drag (the force of air resistance) from useless branches.Branches are shed for reasons other than lack of light. In dry parts of the world, it is common for trees and shrubs to lose smaller branches to save water. Small branches have the thinnest bark (the protective outer covering of a tree) and greatest surface area and thus are the source of most water loss once the leaves have been lost. The creosote bush of United States deserts self-prunes, or removes parts of itself, in the face of extreme heat or drought, starting from the highest and most exposed twigs and working downward to bigger and bigger branches; it’s a desperate act because if the creosote bush loses too much wood, it dies. Shedding branches can also be useful for self-propagation. Most poplar trees and willow trees characteristic of waterways will readily drop branches, which take root when washed up on muddy banks further downstream.How are branches shed? In the simplest cases, dead branches rot and fall off, or healthy branches are snapped off by wind, snow, and animals. Some willows have a brittle zone at the base of small branches that encourages breaking in the wind, seemingly for propagation. Other cases of “natural pruning” are more startling: elm trees, and to a certain extent others, such as oaks, have a reputation for dropping large branches (up to half a meter in diameter) with no warning on calm, hot afternoons. Such dramatic shedding appears to be due to a combination of internal water stress coupled with heat expansion affecting cracks and decayed wood. Many trees, however, shed branches deliberately. In this situation, branches are shed in the same way as foliage in autumn by the prior formation of a corky layer that leaves the wound sealed over with cork, which in turn is undergrown with wood the following year. In hardwoods, branches up to a meter in length and several centimeters in diameter can be shed normally after the leaves have fallen in the autumn (maples are unusual in casting branches mainly in spring and early summer). Oaks tend to shed small twigs up to the thickness of a pencil, beech may shed larger ones, and birches dump whole branches of dead twigs. Pine trees shed their clusters of needles (which really are short branches), and members of the redwood family shed their small branchlets with leaves. Typically in hardwood trees, something around 10 percent of terminal branches are lost each year through a mixture of deliberate shedding and being broken off.Another way of reducing potential congestion is to make some branches smaller than others. Branches in the shade grow smaller than those in the sun. But trees can also regulate branch length from within. In many trees there is a clear distinction between long and short branches or shoots. The long shoots build the framework of the tree, making it bigger. The job of the short shoots (called spur shoots by horticulturalists) is to produce leaves, and commonly flowers, at more or less the same position every year. To maintain flexibility, any one shoot can switch from long to short or vice versa depending on internal factors, light levels, and damage.

有一种方法可以使树木防止自己拥有太多分支的方法是，一旦它们达到了目的之后，它们就会脱落（掉下）枝条。这种情况发生在树变大并且生长新的树叶外层，遮蔽了内部和较低的分支。在大多数大树中，树冠的中心只包含大树枝，小树枝和细枝只能在树冠的边缘看见。在阴影中心，曾经占据那个空间的小分支早已不复存在。像真正的柏树这样的树木经常会在夏天结束的时候脱落小树枝和叶子。大多数其它树木落叶直证明是徒劳的。如果一个树枝没有足够的碳水化合物来弥补自己的身体运行，即它需要其它分支补贴，例如，因为它被遮蔽了，并且几乎没有光线，所以它通常会被清除掉。这样可以防止无效率的树枝在树上浪费营养，并除去无用树枝的风阻力（空气阻力）。 除了缺少光线以外，树枝也会被脱落。在世界的干旱地区，树木和灌木通常会失去较小的枝条以节约用水。小树枝有最薄的树皮（树木的保护壳）和最大的表面积，因此一旦树叶脱落，也就意味着丢失了大部分水分。在极度炎热或干旱的情况下，美国的杂木丛会自行修剪或者除去自身的部分，从最高和最暴露的细枝开始，向下扩大到更大的树枝;这是一个绝望的行为，因为如果石炭酸灌木失去了太多的木材，它就会死亡。脱落的树支也可以用于自我传播。大多数杨树和柳树的特点就是很容易掉落树枝，当在下游泥泞的河岸往上冲刷时，树枝会生根。 树枝如何脱落呢？在最简单的情况下，枯枝腐烂并脱落，或健康的树枝被风，雪和动物折断。一些柳树在小枝的底部有一个脆弱的区域，希望树枝在风中吹折，这看起来是为了繁殖。其它“自然修剪”的案例更令人吃惊：榆树，而在某种程度上，其它如橡树等，会在炎热的下午没有任何警告的情况下，有大的树枝掉落（直径达半米）。这种剧烈的脱落似乎是由于内部的水压力加上因热膨胀对裂缝和腐烂的木头造成的。 然而，许多树木却故意脱落树枝。在这种情况下，树枝脱落的方式就像秋天的树叶脱落一样，在软木层的前面形成了一个软木层，然后用软木塞盖住了软木，第二年又用软木来生长。在硬木中，长到一米长，直径几厘米的枝条在秋天树叶落下来后脱落（枫枝在浇灌枝头时主要在春季和初夏时是不寻常的）。橡树倾向于将小枝桠切成铅笔的厚度，山毛榉可能会掉下较大的树枝，而桦树会倾倒整枝的死枝。。松树脱落了一簇簇的针（实际上是短枝），红木家族的成员们用树叶脱落掉了它们的小分枝。通常在硬木树中，每年大约有10%的终末分支由于故意脱落和被破坏而丢失。 减少潜在拥塞的另一种方式是使一些树木分支比其他分支更小。在树荫下生长的树枝比太阳下的树枝小。但树木也可以从内部调节分支长度。在许多树木中，长枝和短枝或枝条之间有明显的区别。长芽形成了树的框架，使树更大。短芽（园艺学家称之为短枝芽）的作用就是每年产生大致相同位置的叶子，通常是花。为了保持其灵活性，任何一次发芽都可以根据内部因素，光线级别和损坏从长到短或反之亦然。