Published on 04/25/00

Trees Actively Struggle to Survive Drought

How Much Water Is
Enough for a Tree?

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

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Photo: Sharon Omahen

When water is short, trees start shutting down their leaves. The damage can't be reversed, but it's one of the things trees do to survive droughts.

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.

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.