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冠层
 雨林冠层. (Photo by R. Butler)
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写 Rhett Butler. 翻译 Che Fei
07/12/2007
冠层
在雨林里,据估计有70-90%的生命生存在位于阴凉地面之上的树中. 一般的热带雨林垂直方向上至少分为5层: 森林树冠层, 冠层, 林下叶层, 灌木层和地面表层. 每层都有各自与众不同的与周围生态系统相互影响的动植物. 森林树冠层指的是冠层外沿之上20-100高的树冠. 冠层是指那些紧密分隔开的树木和它们的树枝的稠密的顶层, 而林下叶层指形成冠层下面断折层的分隔更为广泛的、较小的树种和幼龄植株. 灌木层的特征是它只是由生长在森林地面5-20英尺的灌木和幼龄树木组成的. 地面表层是由许多树木、菌类植物真和低生长植物组成的森林地面表层. 这些分层并不总是有着明显的不同，它们也随着森林的不同而变化，但它们可以作为一种了解森林营养性、机械性结构的好的模型.
森林树冠层的特点是它由位于树冠层外沿之上的零散的超高树组成，一些树种有210英尺(65米)多高.在森林树冠层的下面,冠层横向扩展很大的距离，当从飞机上观察它们的时候，看上去从不间断的样子.然而，不管树枝有多重叠，冠层树木之间很少连锁甚至相互碰触.相反地是它们之间都分隔有几英尺的距离.这些树的树枝为什么相互不接触这仍是个谜,但人们认为主要是它们在保护自己不受食树毛虫和一些如同叶枯萎病一样的树木疾病的侵害.为了活下来，生存者冠层中的生物一定要有通过爬行、跳跃、滑行或飞行来顺利通过林间间隙的能力.
这数十亿计的冠层叶子，就像微型的太阳面板一样，通过光合作用将太阳能转变为叶片中化学能从而为森林提供能量来源.光合作用是这样一个过程，在这个过程中，植物将大气中的二氧化碳和水转变成氧气和单糖. 由于冠层树木光合作用速率很高，所以这些植物的果实、种子、花以及叶子的产量都较高，而这些正好用于支撑广泛的动物多样性.除了能够吸引大量的野生动植物外，树冠层在调节区域和全球气候方面起着重要的作用,因为它是最重要的热、水汽和大气气体的交换地.除了吸收太阳能和调节气候外，树冠层保护林下叶层免受强光、干风及大雨的影响，并且保持了下面森林的潮湿气候.因此，森林内部比起冠层顶部较高部分来说更稳定些.并且被外层保护着免受冠层极端条件(如温度的变动、太阳辐射的破坏、强风）的影响.在较低的冠层中，阳光弥漫着，好像受到了什么限制；湿气较重并且更始终如一；直接射进的阳光很少.
复习题:
- 雨林从哪获取能量?
- 大多数的雨林物种居住在什么地方?
[打印版]
续: 研究雨林冠层
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