How tall can a tree grow? - Valentin Hammoudi

譯者: Bruce Sung 審譯者: Marssi Draw

Reaching heights of over 100 meters,

加利福尼亞紅杉 可以高過 100 公尺，

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

在全球約 60,000 個 樹種之中出類拔萃。

Growing in the misty Sierra Nevada mountains,

生長在多水氣的內華達山脈裡，

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

它們巨大的樹幹支撐著這個 已知的世界最高樹種。

But even these behemoths seem to have their limits.

但即便是這種龐然大物 似乎也有它們的極限。

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

紀錄上紅衫最高 不超過 130 公尺——

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

而且許多研究者認為： 即使他們再多活幾千年

even if they live for thousands of years to come.

也突破不了那個上限。

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

究竟是什麼原因，讓它們 不能無止境地長高？

It all comes down to sap.

歸根究底就是樹液。

In order for trees to grow,

樹要能夠成長，

they need to bring sugars obtained from photosynthesis

就必須將光合作用產生的糖分，

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

以及根系吸收進來的養分 輸送到成長需要的地方。

And just like blood circulates in the human body,

就像血液在人體中循環，

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

樹木通體循環著兩種樹液，

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

輸送各個細胞生存所需的物質。

The first is phloem sap.

第一種是韌皮部樹液。

Containing the sugars generated in leaves during photosynthesis,

韌皮部樹液含有 樹葉經光合作用產生的糖，

phloem sap is thick, like honey,

像蜂蜜一樣濃稠。

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

它沿著植物的韌皮部組織向下 流動，將糖分配送到整棵樹上。

By the end of its journey,

到旅程結束時，

the phloem sap has thinned into a watery substance,

韌皮部樹液已經變得像水一樣稀，

pooling at the base of the tree.

匯集在樹的底部。

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

韌皮部旁邊，是樹的 另一種組織：木質部。

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

木質部充滿了營養成分 及離子，如鈣、鉀和鐵等，

which the tree has absorbed through its roots.

都是由根部吸收來的。

Here at the tree’s base,

在樹的底部，

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

在一個組織中，這些顆粒 比另一個組織中更多，

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

因此韌皮部樹液中的水 被吸收到木質部，

to correct the balance.

以調節平衡。

This process, called osmotic movement,

這個過程稱為「滲透運動」，

creates nutrient-rich xylem sap,

製造出營養豐富的木質部樹液，

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

然後沿著樹幹上行，將這些 營養物質傳播到整棵樹上。

But this journey faces a formidable obstacle: gravity.

但這個旅程面臨著 巨大的障礙：重力。

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

要完成這個費力的任務， 木質部依賴三種力量：

transpiration, capillary action, and root pressure.

蒸散作用、毛細管作用 和根部壓力。

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

在光合作用中，葉子 開關稱為「氣孔」的毛孔。

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

這些開口讓氧氣 和二氧化碳進出葉面，

but they also create an opening through which water evaporates.

但同時也讓水分蒸發。

This evaporation, called transpiration,

這種蒸發，稱為「蒸散作用」，

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

在木質部產生負壓， 將含水的木質部樹液向上拉。

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

這種上拉得力於水的一種特質， 稱為「毛細管作用」。

In narrow tubes,

在狹窄的管道中，

the attraction between water molecules

水分子間的吸引力

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

加上水和管壁間的粘著力 可以擊敗重力。

This capillary motion is in full effect in xylem filaments

在比人髮還細的木質纖維中，

thinner than human hair.

毛細管運動發揮了最大的功效。

And where these two forces pull the sap,

這兩股力量拉引樹液向上，

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

而樹底部的滲透運動 也產生根部壓力，

pushing fresh xylem sap up the trunk.

將新鮮的木質部樹液推上樹幹。

Together these forces launch sap to dizzying heights,

這些力量合起來，可送 樹液到令人目眩的高度，

distributing nutrients, and growing new leaves to photosynthesize –

傳輸養分到遠離樹根的高處，

far above the tree’s roots.

增長新葉來進行光合作用。

But despite these sophisticated systems,

儘管有這些複雜的系統，

every centimeter is a fight against gravity.

每一公分都是一場對抗重力的戰鬥。

As trees grow taller and taller,

隨著樹木越長越高，

the supply of these vital fluids begins to dwindle.

這些维生液體的供應也開始減少。

At a certain height,

到了某個高度，

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

樹木再也無法補充 在光合作用中蒸發的水分。

And without the photosynthesis needed to support additional growth,

沒了可支持額外增長的光合作用，

the tree instead turns its resources towards existing branches.

樹便將資源轉送到現有的枝葉上。

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

這模式稱作「液壓限制假說」，

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

是針對為何樹木即使在完美 生長條件下也有高度極限

even in perfect growing conditions.

我們現有最好的解釋。

And using this model alongside growth rates

利用這個模式，加上生長率

and known needs for nutrients and photosynthesis,

和已知的營養及光合作用需求，

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

研究人員已經能夠估算 特定物種的高度極限。

So far these limits have held up –

至今這些高度極限都還沒被打破——

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

即使是世界上最高的樹也仍 低於它的高度極限 15 公尺。

Researchers are still investigating the possible explanations for this limit,

研究人員仍在找對此 極限的可能解釋，

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

而且造成樹木停止生長 或許不只出於單一因素。

But until we learn more,

但在我們了解更多之前，

the height of trees is yet another way that gravity,

只能說樹的高度又是重力

literally, shapes life on Earth.

塑造了地球上生命型態的 另一個表現。