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| A Longleaf Pine showing the effects of drought. |
Although we all know that trees need water, scientists are
still discovering new information and implications about the process that
allows trees to “drink.” Thanks to recent scientific research, we now have a
greater insight into the mechanism of tree death from drought and a new
appreciation of how vulnerable trees are.
Trees draw water through their roots and into the
thread-like channels of their vascular system that distributes it to their most
remote needles and leaves. In order to photosynthesize and grow, trees need to
open the stomata (pores) on their leaves to take in carbon dioxide. As they do
this, water evaporates. The water loss creates a suction effect that goes down
all the way to the roots, where the water is replenished, similar to drinking
through a straw.
When water is unavailable, this suction pressure increases,
and air is drawn in. The air bubbles clog the channels and make it harder for a
tree to get fluid to its leaves – like drinking from a broken straw. This
“hydraulic failure” is the reason trees die from drought.
In addition to hydraulic failure, drought can impact a
tree’s ability to open its stomata. Even when water is again available, some
leaves are unable (or slow) to return to their work of photosynthesis, causing
further dieback.
Dr. Brendan Choat from the University of Western Sydney and
Dr. Steven Jansen from Ulm University in Germany lead a team of scientists that
have studied hydraulic failure in trees worldwide. The results of their
research show that about 70% of tree species have very little margin in the
amount of drought they can endure before they experience hydraulic failure.
Surprisingly, this was true for species that grow in marsh as well as desert.
The implications of this research are that a small change in the drought level
of an area could have serious consequences for large numbers of trees.
Watering trees in times of drought is extremely important.
Even the largest tree typically has its roots concentrated within the top 12”
of soil. They have no ability to suddenly find water when that layer is dry.
Trees will respond to drought by dropping leaves or needles, and then having whole
branches die. The damage can be irreversible.
Sophisticated systems are now available to monitor the
moisture content of soil and automatically adjust watering to the appropriate
level for plants, lawns or trees. These irrigation systems are now common in
arid regions and have been found to reduce total water consumption. Giving
trees the right amount of water at the right time can prevent hydraulic failure
and also prevent the unnecessary waste of water in landscapes by up to 60%. I
believe we’ll see this technology introduced in the northeast over the next
decade as water for our landscapes becomes a more precious commodity.
