Growing in the misty Sierra Nevada mountains, their massive trunks support the tallest known trees in the world.

生長在多霧的科奎內華達山脈，加州紅杉有著粗壯的樹幹，是世界所知最高的樹種。

But even these behemoths seem to have their limits.

但這種巨樹似乎也有極限。

No sequoia on record has been able to grow taller than 130 meters—and many researchers say these trees won't beat that cap, even if they live for thousands of years to come.

目前尚未有加州紅杉樹高超過 130 公尺，很多研究學者表示這種樹沒有辦法打破這個紀錄，即使它們再活幾千年也一樣。

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

所以到底是什麼因素導致這種樹永遠沒辦法長得更高？

It all comes down to sap.

那就是樹液。

In order for trees to grow, they need to bring sugars obtained from photosynthesis and nutrients brought in through the root system to wherever growth is happening.

樹木為了要生長，它們必須透過光合作用獲得糖分，並經由樹根獲取養分，再將它們運送至生長的部位。

And just like blood circulates in the human body, trees are designed to circulate two kinds of sap throughout their bodies—carrying all the substances a tree's cells need to live.

就像人體的血液系統，樹木的體內有兩種樹液在循環，以便運送樹木細胞存活所需的物質。

The first is phloem sap.

第一種是韌皮液。

Containing the sugars generated in leaves during photosynthesis, phloem sap is thick, like honey, and flows down the plant's phloem tissue to distribute sugar throughout the tree.

富含樹葉於光合作用獲得的養分；韌皮液質地濃稠，像蜂蜜一樣，流動至韌皮組織來將糖分運送至整棵樹木。

By the end of its journey, the phloem sap has thinned into a watery substance, pooling at the base of the tree.

在它旅程的最終，韌皮液會稀釋成水狀，集中至樹木底部。

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

在韌皮部的旁邊是樹木的另一種組織：木質部。

This tissue is packed with nutrients and ions like calcium, potassium, and iron, which the tree has absorbed through its roots.

木質部組織含有養分和鈣、鉀、鐵等離子，這些是樹木經由樹根獲取的。

Here at the tree's base, there are more of these particles in one tissue than the other so the water from the phloem sap is absorbed into the xylem to correct the balance.

這是樹木的底部。其中一個組織裏的例子比另一個組織多，所以韌皮液裡的水分會流至木質部以保持平衡。

This process, called osmotic movement, creates nutrient-rich xylem sap, which will then travel up the trunk to spread those nutrients through the tree.

這個過程叫做滲透作用，讓木質液富含養分，木質液會藉由樹幹將養分輸送至整個樹木。

But this journey faces a formidable obstacle: gravity.

但這個過程有個難以對付的障礙：重力。

To accomplish this Herculean task, the xylem relies on three forces: transpiration, capillary action, and root pressure.

為了完成這個艱鉅的任務，木質部靠的是三個作用：蒸散作用、毛細管作用還有根壓作用。

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

光合作用中，樹葉會把叫做氣孔的小孔打開再關閉。

These openings allow oxygen and carbon dioxide in and out of the leaf, but they also create an opening through which water evaporates.

氣孔打開時氧氣和二氧化碳得以進出樹葉，但這同時開了讓水分蒸發的通道。

This evaporation, called transpiration, creates negative pressure in the xylem, pulling watery xylem sap up the tree.

這種蒸發過程叫做蒸散作用，它讓木質部有了負壓足以將水狀的木質液往上拉。

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

這種現象受到水的基本作用幫助，叫做毛細孔作用。

In narrow tubes, the attraction between water molecules and the adhesive forces between the water and its environment can beat out gravity.

在窄小的管道中，水分子之間的吸引力以及水的附著力讓樹木得以戰勝重力。

This capillary motion is in full effect in xylem filaments thinner than human hair.

這種毛細管作用在木質部的絲狀組織進行，它比人類的頭髮還要細。

And where these two forces pull the sap, the osmotic movement at the tree's base creates root pressure, pushing fresh xylem sap up the trunk.

就在這兩種作用將木質液往上拉的地方，樹木底部的滲透作用製造根壓，將新鮮的木質液推至樹幹。

Together these forces launch sap to dizzying heights, distributing nutrients, and growing new leaves to photosynthesize—far above the tree's roots.

這些作用力將木質液發送至令人眼花撩亂的高度，運送養分並讓樹木長出新樹葉以進行光合作用，這些都在離樹根遠遠的上端。

But despite these sophisticated systems, every centimeter is a fight against gravity.

不過儘管樹木的這個系統奧妙精緻，運送中每公分都不斷與重力奮戰。

As trees grow taller and taller, the supply of these vital fluids begins to dwindle.

樹木越長越高，這些生命補給的作用會開始退化。

At a certain height, trees can no longer afford the lost water that evaporates during photosynthesis.

在一定的高度時，樹木將沒辦法承受光合作用中蒸發的水分。

And without the photosynthesis needed to support additional growth, the tree instead turns its resources towards existing branches.

沒有了光合作用來支持樹木生長，樹木會將現有的資源運送至既有的樹枝上。

This model, known as the “hydraulic limitation hypothesis”, is currently our best explanation for why trees have limited heights, even in perfect growing conditions.

這種現象被稱為「液壓限制假說」，這是目前針對樹木高度限制最好的解釋，即便在完美的成長條件下亦然。

And using this model alongside growth rates and known needs for nutrients and photosynthesis, researchers have been able to propose height limits for specific species.

根據這個現象搭配成長率、已知的養分和光合作用所需，研究學者們能夠提出某些品種的生長高度限制。

So far these limits have held up—even the world's tallest tree still falls about fifteen meters below the cap.

目前為止這些高度限制的紀錄尚未被打破，即便是世界上最高的樹也比學者提出的高度限制低大約15公尺。

Researchers are still investigating the possible explanations for this limit, and there may not be one universal reason why trees stop growing.

研究學者們仍然在研究這種高度限制的可能原因，而各種樹木停止生長的原因可能不只一個。

But until we learn more, the height of trees is yet another way that gravity, literally, shapes life on Earth.

但在我們發現更多以前，樹木的高度是我們目前可以看到重力影響地球上生命的最佳例子。