Reaching heights of over 100 meters, Californian sequoias tower over Earth’s other estimated 60,000 tree species. Growing in the misty Sierra Nevada mountains, their massive trunks support the tallest known trees in the world. But even these behemoths seem to have their limits. 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. So what exactly is stopping these trees from growing taller, forever?
加利福尼亞紅杉 可以高過 100 公尺, 在全球約 60,000 個 樹種之中出類拔萃。 生長在多水氣的內華達山脈裡, 它們巨大的樹幹支撐著這個 已知的世界最高樹種。 但即便是這種龐然大物 似乎也有它們的極限。 紀錄上紅衫最高 不超過 130 公尺—— 而且許多研究者認為: 即使他們再多活幾千年 也突破不了那個上限。 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, they need to bring sugars obtained from photosynthesis and nutrients brought in through the root system to wherever growth is happening. 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. The first is phloem sap. 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. By the end of its journey, the phloem sap has thinned into a watery substance, pooling at the base of the tree.
樹要能夠成長, 就必須將光合作用產生的糖分, 以及根系吸收進來的養分 輸送到成長需要的地方。 就像血液在人體中循環, 樹木通體循環著兩種樹液, 輸送各個細胞生存所需的物質。 第一種是韌皮部樹液。 韌皮部樹液含有 樹葉經光合作用產生的糖, 像蜂蜜一樣濃稠。 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。 到旅程結束時, 韌皮部樹液已經變得像水一樣稀, 匯集在樹的底部。
Right beside the phloem is the tree’s other tissue type: the xylem. This tissue is packed with nutrients and ions like calcium, potassium, and iron, which the tree has absorbed through its roots. 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. This process, called osmotic movement, creates nutrient-rich xylem sap, which will then travel up the trunk to spread those nutrients through the tree. But this journey faces a formidable obstacle: gravity. To accomplish this herculean task, the xylem relies on three forces: transpiration, capillary action, and root pressure.
韌皮部旁邊,是樹的 另一種組織:木質部。 木質部充滿了營養成分 及離子,如鈣、鉀和鐵等, 都是由根部吸收來的。 在樹的底部, 在一個組織中,這些顆粒 比另一個組織中更多, 因此韌皮部樹液中的水 被吸收到木質部, 以調節平衡。 這個過程稱為「滲透運動」, 製造出營養豐富的木質部樹液, 然後沿著樹幹上行,將這些 營養物質傳播到整棵樹上。 但這個旅程面臨著 巨大的障礙:重力。 要完成這個費力的任務, 木質部依賴三種力量: 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. These openings allow oxygen and carbon dioxide in and out of the leaf, but they also create an opening through which water evaporates. This evaporation, called transpiration, creates negative pressure in the xylem, pulling watery xylem sap up the tree. This pull is aided by a fundamental property of water called capillary action. In narrow tubes, the attraction between water molecules and the adhesive forces between the water and its environment can beat out gravity. This capillary motion is in full effect in xylem filaments thinner than human hair. 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. Together these forces launch sap to dizzying heights, distributing nutrients, and growing new leaves to photosynthesize – far above the tree’s roots.
在光合作用中,葉子 開關稱為「氣孔」的毛孔。 這些開口讓氧氣 和二氧化碳進出葉面, 但同時也讓水分蒸發。 這種蒸發,稱為「蒸散作用」, 在木質部產生負壓, 將含水的木質部樹液向上拉。 這種上拉得力於水的一種特質, 稱為「毛細管作用」。 在狹窄的管道中, 水分子間的吸引力 加上水和管壁間的粘著力 可以擊敗重力。 在比人髮還細的木質纖維中, 毛細管運動發揮了最大的功效。 這兩股力量拉引樹液向上, 而樹底部的滲透運動 也產生根部壓力, 將新鮮的木質部樹液推上樹幹。 這些力量合起來,可送 樹液到令人目眩的高度, 傳輸養分到遠離樹根的高處, 增長新葉來進行光合作用。
But despite these sophisticated systems, every centimeter is a fight against gravity. As trees grow taller and taller, the supply of these vital fluids begins to dwindle. At a certain height, trees can no longer afford the lost water that evaporates during photosynthesis. And without the photosynthesis needed to support additional growth, the tree instead turns its resources towards existing branches.
儘管有這些複雜的系統, 每一公分都是一場對抗重力的戰鬥。 隨著樹木越長越高, 這些维生液體的供應也開始減少。 到了某個高度, 樹木再也無法補充 在光合作用中蒸發的水分。 沒了可支持額外增長的光合作用, 樹便將資源轉送到現有的枝葉上。
This model, known as the “hydraulic limitation hypothesis,” is currently our best explanation for why trees have limited heights, even in perfect growing conditions. 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. So far these limits have held up – even the world’s tallest tree still falls about fifteen meters below the cap. Researchers are still investigating the possible explanations for this limit, and there may not be one universal reason why trees stop growing. But until we learn more, the height of trees is yet another way that gravity, literally, shapes life on Earth.
這模式稱作「液壓限制假說」, 是針對為何樹木即使在完美 生長條件下也有高度極限 我們現有最好的解釋。 利用這個模式,加上生長率 和已知的營養及光合作用需求, 研究人員已經能夠估算 特定物種的高度極限。 至今這些高度極限都還沒被打破—— 即使是世界上最高的樹也仍 低於它的高度極限 15 公尺。 研究人員仍在找對此 極限的可能解釋, 而且造成樹木停止生長 或許不只出於單一因素。 但在我們了解更多之前, 只能說樹的高度又是重力 塑造了地球上生命型態的 另一個表現。