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.
