Drought and Tree Stress
Most roots
are shallow, in the upper 12 to 18 inches of the soil,
especially the finer roots doing the critical work of
taking up water and nutrients. While there are also
deeper roots, these mostly provide support and stability
to the tree. The root system of a tree may extend
outward 2 to 3 times the width of the branches.
Stress symptoms range
from leaf burn, a thinning canopy, limbs dying, to death
of the tree. Some of the most common causes of stress in
trees are:
* Drought or
insufficient water:
This is the most obvious cause of stress in trees. It is
aggravated by summer heat. Large established trees can
tolerate short periods of drought, but young or newly
planted trees frequently show significant signs of
stress. Recurring summer droughts can severely impact
even old, long established trees.
Often folks will say, "My trees shouldn't look like
this. I water them." Consider this example. A 7-year-old
pecan tree in a well-drained soil needs (takes up) about
225 gallons of water a week in July and August to
maintain its vigor and health. Just think of how much
more a 70-year-old tree with a much greater root system
might need!
Water that is adequate for lawn grasses is not
sufficient for actively growing trees. During periods of
drought, occasionally supplement with deep watering at
the drip line (the ends of the branches) of trees, not
near the trunk.
* Excess water:
Just as not enough
water can injure tree roots, so can too much water. Tree
roots require oxygen to function properly. This oxygen
is forced out of the root zone when the soil is
saturated with water. Tree roots in waterlogged soils
stop growing, minerals are not absorbed, leaves then
turn yellow and remain small, and finally roots begin to
die. It can take a long time for a root system that has
been seriously injured to regenerate.
* Heat:
High temperatures cause trees to pump more water. As
temperatures approach 100 degrees, water loss
(transpiration) by some trees can equal or exceed the
ability of the roots to supply water, even when soil is
adequately moist.
* Stress-related pests.
Trees under stress are more prone to pests such as
borers and some root and canker diseases.
