There is never the right amount of water for perfect tree
growth. Most of the time,
supplies are less than ideal. Water shortages happen all the
time.

This condition — inadequate water in the soil for trees’
essential biological and
physical functions — is called drought stress.

As the soil dries, trees reach a point when they can’t
extract enough water to make a
living. Trees are sensitive to changes in water resources in
the soil and inside roots,
stems and leaves.

Leaves Sense Water
Woes

The main sensors of water problems are the leaves. They
house the green machinery that
makes food from carbon dioxide in the air. These leaf
factories
require water as a raw
material.

The leaves also require water to transport materials up
from
the roots. It’s hard for
leaves to produce food without critical soil materials.

Water also supports the living machinery in the cells. It’s
the solution and framework
within each cell that allows photosynthesis to function.

Trees, then, can’t gather, transport, dissolve, use or make
essential materials without
water. So they’ve developed ways to survive shortages. Low
soil-water concentrations start
a progression of mechanisms that constrain water use and
loss.

Trees ‘Know’ When Water Is
Short

A tree “knows” when water is in short supply. The
root tips act as front-line
sensors, chemically communicating with shoot tips.

Leaves lie at the top of a long line of water columns
stretching upward from the soil.
As water becomes harder to collect, the tension on the leaves
becomes greater.

One of a tree’s quickest reactions to drought conditions is
to close valves called
stomates in the leaves. This stops a large part of the active
water loss but also keeps
leaves from making food. As the leaves shut down, roots are
turned on.

One Step: Grow More
Roots

The shoots and leaves send the roots messages to use more
stored food to expand the
exploration and gathering of more water. So the tree starts
growing more absorbing roots.

Water uptake in a tree is roughly proportional to root
density and surface area. The
more little roots, the more potential for water uptake. As
soils dry even more, the leaves
and shoots send new messages to the roots.

If the new root growth is successful at gathering water,
trees keep growing roots. If
it’s not, the tree goes to the next step.

It can’t afford to let any of its water move from moist
roots into dry soil. So it
starts shutting down its roots and coating their surfaces with
a waterproofing material
called suberin.

Cutting
Liabilities

As water shortages persist, the tree next tries to shed any
water-losing tissue. It
closes off absorbing root fans and shuts down leaves in a
series of steps called
senescence. The tree pulls valuable materials, like nitrogen,
out of the leaves, which are
sealed off from the rest of the tree.

These leaves could not make food because they had no water.
They became a liability for
a tree trying to survive. Tree parts that are liabilities to
the survival of the whole
tree are quickly shed. The browned leaves beneath a tree in
summer are a visible sign of
water shortages and the tree’s reaction.

Once the leaves are off the tree, even if rain comes in
abundant supply, the damage has
already been done both above- and belowground. It takes trees
months to recover from one
moderate drought.

Trees don’t passively stand by as the environment becomes
harsher and more damaging.
They actively struggle to conserve water and survive. For all
trees, the struggle is
intense and can be permanently damaging.

Please water
your trees.