Planting trees in cities is a great thing! Trees regulate temperatures, filter air, and
reduce runoff. And a tree-lined cityscape is beautiful!
But anyone who works as a tree health specialist knows that
if you want to find a sick tree, go to the city. Many urban trees are plagued
with a number of insults: reduced space for roots, pollution and other factors
can reduce the growth and vigor of trees. What about higher temperatures?
Cities are known to be “heat islands” and can be several
degrees warmer than surrounding rural environments, especially at night. This
is a result of heat produced by dense motor traffic and the concrete and
asphalt re-radiating the sun’s heat. If there is inadequate water heat can be
devastating to trees. The stomates, minute holes in leaves that allow for gas
exchange, stay open to evaporate water and cool the tree down, but this
requires water. If there is no water, the plant begins to wilt.
It appears that heat can affect tree health in more subtle
and indirect ways.
Recent work from the lab of Dr. Steven Frank at North
Carolina State University has shown that scale pests of some urban trees are
more abundant on trees in hotter parts of the city and that these insects have
a negative impact on the infested tree’s health.
Red maples and willow oaks are popular trees in the urban
landscape. Look closely at specimens of these trees in cities and you are
likely to find scale insects: gloomy scale (Melanaspis
tenebricosa) on maple, and oak lecanium (Parthenolecanium quercifex) on oaks. You would be hard-pressed to
find these scale insects on their hosts during a walk in the woods. The
leafless limbs on many maples and willow oaks planted in parking lots are often
a result of these pests.
 |
| Oak lecanium scales on willow oak. |
 |
| Gloomy scale on the bark of a maple. |
Why are they more common on urban specimens? Stressed trees
are often more susceptible to pests. They are too busy trying to survive and
allocate fewer resources to defending themselves. But this is not true for
sap-sucking insects like scales and aphids which are often more common on the
healthiest plants. Greenhouse experiments showed that increased temperature,
not lack of water or fertilizer, was the only measured variable accounting for
the increase in scale insects.
Like all insects, scales are cold-blooded and reproduce
faster at higher temperatures (just a note: this is true within an optimum
range of temperatures that varies according to the species; above a species’
maximum their survivorship rapidly declines).
But if it was simply a matter
of faster reproduction as a result of warmer temperatures we would expect other
insect pests to increase on trees in warmer areas, too, but that is not the
case. Although willow oaks and red maples have a number of insect pests
associated with them, including several other scale insect species, oak
lecanium and gloomy scale stand out as pests of oaks and maples in warmer
environments.
What does this tell us about
the impacts climate change will have on our forests? It tells us that it’s
complicated. There will be winners and there will be losers. There are many
trees that do very well in urban environments. In fact, oaks and maples are
great urban trees if planted in the right
place. It tells us that some insects are predisposed to take advantage of
warmer environments and these may become considerable pests in warmer
environments.
