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? It all comes down to sap. 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. 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. 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. 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. 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.