How tall can a tree grow? - Valentin Hammoudi

翻译人员: Tony Yu 校对人员: Lipeng Chen

Reaching heights of over 100 meters,

高达100多米，

Californian sequoias tower over Earth’s other estimated 60,000 tree species.

加利福利亚红杉比地球上 其它6万多的树种都要高。

Growing in the misty Sierra Nevada mountains,

生长在雾蒙蒙的内华达山脉群山中，

their massive trunks support the tallest known trees in the world.

它们巨大的树干，支撑着 世界上最高的已知树种。

But even these behemoths seem to have their limits.

但是，即使这些庞然大物 高度似乎也有极限。

No sequoia on record has been able to grow taller than 130 meters –

在现有记录上，没有红杉高过130米——

and many researchers say these trees won’t beat that cap

很多研究者说，它们不可能超过这个高度

even if they live for thousands of years to come.

即使它们再生长上千年。

So what exactly is stopping these trees from growing taller, forever?

那么，到底是什么阻止它们的生长， 不能一直长上去呢？

It all comes down to sap.

这一切都跟树液有关。

In order for trees to grow,

树木要生长，

they need to bring sugars obtained from photosynthesis

它们要把光合作用产生的糖

and nutrients brought in through the root system to wherever growth is happening.

和根茎系统吸收的营养 送到生长的地方。

And just like blood circulates in the human body,

就像血液在人体循环一样，

trees are designed to circulate two kinds of sap throughout their bodies –

树木本身需要有两种树液通体循环——

carrying all the substances a tree’s cells need to live.

带着树细胞生长需要的所有物质。

The first is phloem sap.

第一种是韧皮树液。

Containing the sugars generated in leaves during photosynthesis,

包含树叶光合作用产生的糖分，

phloem sap is thick, like honey,

韧皮树液很稠，像蜂蜜，

and flows down the plant’s phloem tissue to distribute sugar throughout the tree.

沿着植物的韧皮组织向下流动 把糖份送到树的各个部分。

By the end of its journey,

这个旅程的最后，

the phloem sap has thinned into a watery substance,

韧皮树液变成很稀， 类似水一样的物质，

pooling at the base of the tree.

聚集在树的底部。

Right beside the phloem is the tree’s other tissue type: the xylem.

就在韧皮的旁边是树的 另外一种组织叫木质。

This tissue is packed with nutrients and ions like calcium, potassium, and iron,

这种组织充满了营养素和 像钙、钾、铁之类的离子，

which the tree has absorbed through its roots.

由树通过它的根部吸收。

Here at the tree’s base,

在树的根部，

there are more of these particles in one tissue than the other,

这些组织里的微粒分布不均匀，

so the water from the phloem sap is absorbed into the xylem

因此，韧皮树液中的水分 被吸收到木质中，

to correct the balance.

来调节平衡。

This process, called osmotic movement,

这个过程，叫做渗透运动，

creates nutrient-rich xylem sap,

会产生营养丰富的木质树液，

which will then travel up the trunk to spread those nutrients through the tree.

这些树液会沿着树干向上 把营养传送到整棵树。

But this journey faces a formidable obstacle: gravity.

但是，这项任务面临着 一个巨大的障碍：重力。

To accomplish this herculean task, the xylem relies on three forces:

要完成这样艰巨的任务， 木质要依赖于三种力量：

transpiration, capillary action, and root pressure.

蒸腾作用，毛细作用和根压。

As part of photosynthesis, leaves open and close pores called stomata.

作为光合作用的一部分，树叶会 打开和关闭它上面的细孔——叫气孔。

These openings allow oxygen and carbon dioxide in and out of the leaf,

这些气孔让氧气和二氧化碳 从叶子进出，

but they also create an opening through which water evaporates.

水分也通过这些气孔蒸发。

This evaporation, called transpiration,

这种蒸发，叫作蒸腾作用，

creates negative pressure in the xylem, pulling watery xylem sap up the tree.

在木质中产生一种负压， 把液体的木质树液往树上方拉。

This pull is aided by a fundamental property of water called capillary action.

这种拉力还得到水的一种基本特征—— 毛细作用的帮助。

In narrow tubes,

在微小的管道里，

the attraction between water molecules

水分子间的吸引力

and the adhesive forces between the water and its environment can beat out gravity.

和水与周边物质的粘结力 加起来超过重力的作用。

This capillary motion is in full effect in xylem filaments

在木质纤维中毛细作用无处不在，

thinner than human hair.

这些木质纤维比人的头发还细。

And where these two forces pull the sap,

这两种力量推动树液向上，

the osmotic movement at the tree’s base creates root pressure,

树根部的渗透作用也产生树压，

pushing fresh xylem sap up the trunk.

把新鲜的木质树液推上树干。

Together these forces launch sap to dizzying heights,

所有这些力量一起把树液推到惊人的高度，

distributing nutrients, and growing new leaves to photosynthesize –

进行营养分配，促进新叶子生长 进行光合作用——

far above the tree’s roots.

在远远高出树根的地方。

But despite these sophisticated systems,

尽管有这么精密的系统，

every centimeter is a fight against gravity.

树木的每一厘米增长 都要和重力做斗争。

As trees grow taller and taller,

随着树越来越高，

the supply of these vital fluids begins to dwindle.

这种生命之液也越来越少。

At a certain height,

到了一定的高度，

trees can no longer afford the lost water that evaporates during photosynthesis.

树再也提供不了 光合作用所蒸发的水分。

And without the photosynthesis needed to support additional growth,

没有了再生长所需的光合作用，

the tree instead turns its resources towards existing branches.

树就把它的营养送到现有的枝叶上了。

This model, known as the “hydraulic limitation hypothesis,”

这种理论，被叫做 “液压限制假定”，

is currently our best explanation for why trees have limited heights,

是目前最好的解释，为什么树的高度有限，

even in perfect growing conditions.

即使在最理想的生长环境中。

And using this model alongside growth rates

利用这个理论，加上生长率

and known needs for nutrients and photosynthesis,

以及已知营养及光合作用的需求量，

researchers have been able to propose height limits for specific species.

研究人员已经能推断出 一些树种的极限高度。

So far these limits have held up –

到目前为止，这些极限都很准确——

even the world’s tallest tree still falls about fifteen meters below the cap.

虽说世界上最高的树仍然 低于极限15米。

Researchers are still investigating the possible explanations for this limit,

研究人员还在试图解释 这些极限的原因，

and there may not be one universal reason why trees stop growing.

树木停止长高也许不会有统一的原因。

But until we learn more,

在我们了解更多之前，

the height of trees is yet another way that gravity,

树的高度应该是地球引力的 又一种表现，

literally, shapes life on Earth.

基本上，它在界定地球生命的形态。