BUR OAKS: What's Wrong with the Oaks?

  

September 16, 2004

What’s wrong with the oaks?
by Haley Catton, M.Sc. student, Department of Plant Science

Southern Manitoba is home to many fine tree species such as elm, ash,
and maple but what makes this region unique is the bur oak, the only
oak found on the Canadian prairies. Because Winnipeg is near the
northwestern boundary of the bur oak’s native range, it is the only
major city in western Canada to have a large population of naturally
occurring oak trees in its urban forest.

The bur oak is admired for its large, majestic build and very long
lifespan. In fact, in the right conditions, bur oaks have been known
to live for over 400 years! In recent years however, many of
Winnipeg’s oaks have shown signs of suffering.

In the 1980s, city forest workers noticed that a significant number of
oaks had dead tops (crown dieback), branches sprouting from their
trunks and increasing signs of insect damage. Although a wood-boring
beetle was found in the distressed trees, the pest was not known to
attack healthy trees and was ruled out as the primary cause of the
problem.

After further investigation, Winnipeg’s oaks were diagnosed as
suffering from a syndrome known as “oak decline”. Oaks in decline show
gradual signs of sickness not related to a single, obvious cause.

A healthy oak next to a declined oak in Winnipeg.

As a graduate student in the Department of Plant Science at University
of Manitoba, I set out to investigate. My two main questions were: 1)
how long have oaks been declining and 2) why.

I began my study by selecting 180 oak trees across the city and
classifying them as either healthy, medium, or declined, based on
their dieback symptoms. Then, I collected information about each tree,
most importantly their annual growth ring data. To do this, trees were
“cored”, a harmless procedure that involves extracting a small
cylinder of wood from the outside to the centre of the trunk. Doing
this allowed me to see the tree rings so I could determine the trees’
ages and growth patterns throughout their lifetimes.

From this information, I found out that the declined trees had an
average age of 112 years and were older than the healthy trees at 95
years of age. And when I compared growth patterns over the years, I
saw that declined trees have been growing slower than healthy trees
since the 1940s. In other words, when oak decline was noticed in the
1980s, it had been happening for 40 years already. The answer to my
first question was that trees that look sick now have been troubled
for decades.

Answering the second question is more complex. What caused the
declined oaks to slow down? Was it simply a genetic difference?
Possibly, but we think it had more to do with habitat.

Bur oak is adapted to living on upper riverbanks where soil is moist,
but not wet, and root disturbance isn’t an issue. This is the opposite
of city life, where excavation, construction, and concrete injure root
systems and change drainage patterns. Bur oaks living happily in dry
soils can be suddenly overwhelmed with wetness; they cannot handle
having “wet feet”, and this takes a toll on their growth. The earliest
beginnings of oak decline in Winnipeg were in years with especially
heavy spring rains.

City life encroached on many healthy oaks in Winnipeg in the 1940s,
when oak decline began, as construction was spurred by the post-war
housing shortage. That burst of urban development disturbed the
habitats of many of the existing oaks in the city and those that
weren’t killed are probably among the ones that appear to be in
decline today.

Trees are completely dependent on their root systems for water and
nutrient supply and any kind of root damage can lead to loss of vigour
or even death. Older, larger trees are even less able to adapt to
changes than younger trees, although the damaging effects can take
years to show in the crown. So the answer to the why question is that
urbanization compromises oak root systems and leads to decline.

The take-home message from all of this is that oak health can be
fragile when the trees are not in their natural environment,
especially if changes occur when they are older and established. Our
advice: do not change the drainage patterns around oaks unless you
suspect they are suffering from “wet feet”; protect their root systems
by not digging or otherwise disturbing the soil close to the trees,
especially under their crowns. With good TLC, your oaks will stay
healthy for many years.

 Faculty of Agricultural & Food Sciences
University of Manitoba - Winnipeg, MB, Canada - R3T 2N2
Tel: (204) 474-9295  Fax: (204) 474-7525
Questions or comments?  email agfoodsci@umanitoba.ca

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