University
Park, PA -- September 27, 2016 -- In Pennsylvania, we have had an
extraordinarily hot and dry summer. Those who make their living from the land
are well aware that rain is changing. When it occurs, it is more intense and
has seemingly less value to crops. It seems that those less connected to the
land celebrate the warm days without rain – another sunny day is not always the
best day.
Imagine what
it is like to have your roots anchoring you in one place and depending on rain
from the sky to ensure there is adequate moisture in the soil to keep you
working. What kind of work does a tree do, you ask? Well, trees use carbon
dioxide from the air, water from the soil, and light from the sun to make sugar
through work called photosynthesis.
Photosynthesis
is a complex process that requires certain conditions. All of our trees have
leaves where the magic occurs. Tree roots collect and move water, which is
absolutely essential, along with minerals and nutrients through long soda straw
like tubes in the tree’s bole to the leaves. Photosynthesis involves combining
carbon dioxide, which enters the leaf through small openings called stomates,
water, and light in special cells called chloroplasts which contain chlorophyll
(the green color in leaves) to make sugars. Stomates are important part of the
process as they have the ability to open and close and thus control
photosynthesis.
Stomates
open and close by monitoring the amount of water available and air temperature.
If the temperature is too high, then water demand is too high, and the tree
stops making sugars necessary for its growth. When that happens, trees have to
respirate. That is, they use up sugars to carry out life functions. The
relationship between water in the soil and leaves is critical. And, on a hot
summer day without rain, a tree might spend more of its time using up its
sugars than using them to make wood, seeds, new twigs and buds, repairing
damage, and getting ready for winter.
There is a
lot going on with trees even when they are not growing. If things get really
hot and water is too scarce, trees and most other plants will wilt and loose
turgor pressure in their leaves. You have seen those wilting leaves. If water
comes soon enough or the air temperature drops as it does late in the day and
through the night, plants can recover; however, the stress of inadequate water
can take its toll.
Trees under
stress are susceptible to many threats. Insects and diseases are often lurking
in the environment to take advantage of tree defense mechanisms negatively
affected by heat and inadequate water. Healthy trees are constantly restoring
and repairing weakened or damaged defenses. For example, Armillaria mellia, a
common root rot, is always present the soil. When roots struggle to find water,
they may begin to decline as water is actually pulled from their fine roots by
the soil itself. Re-establishing water movement processes from those points to
the leaves takes resources, and the roots may lose their battle with the root
rot fungus and as a result begin a slow process of decline and, perhaps, death.
Across
Pennsylvania, trees are showing signs of stress. Already, as you look around
the neighborhood, you might see some trees are having leaf loss at the tops of
their crowns. Elsewhere in the crown, leaves are detaching and littering the
lawn with green rather than autumn colors.
You may have
also noticed trees on road cuts turning brown or showing premature yellow.
These cuts where the soil is shallow or facing south or west are often quick to
show moisture stress. When water is scarce, as it is now, it is common to see
maple and birch shedding leaves or going brown.
Elsewhere,
there are reports of patches of oak, red and sugar maple, and even tulip poplar
changing color sooner than expected or even appearing dead. It is difficult to
interpret what is happening in all cases, but in some, the site might be poor,
with shallow soils, or oriented to receive more direct light and heat; trees
are responding by casting leaves earlier than expected.
Water is
essential for plant growth. Heat and lack of rain make for difficult growing
conditions. Over the next few years, based on this summer alone, expect trees
to struggle even if conditions are better next year. As we approach the end of
the growing season, there is not much we can do for individual trees showing
stress responses, especially in the forests. Lawn trees might benefit from deep
watering. Make sure they get at least two inches of water under their crown
spread every 7 to 10 days until the soil freezes.
Written by
Jim Finley, Ibberson Professor of Forest Management and Director, Center for
Private Forests at Penn State
