Summary: Ecotypic Variation in Sugar Maples

[Susan J. Meades]

There were several responses to my query posted to ECOTEACH-L and
bionet.plants.education.  The question came from a student who noticed
that the sugar maple in her front yard, which had a damaged trunk, was
still green, while most (but not all) other sugar maples in town had
already turned color.  Apparently, I did not make it clear that nearly
all sugar maples in town and all sugar maple in the surrounding
countryside had already turned.  The original query and responses are
copied below. 

The replies offered five theories:

        - city microclimate vs. rural microclimate
        - local photoperiod differences
        - age of the tree
        - genetic variation (within one seed crop)
        - ecotype differences

1. There is a noticeable difference in flowering date, leaf flush, leaf
change, etc. between the city and the outlying rural areas, but in this
situation, the city vs. rural microclimate differences had been ruled
out because the few trees in question has retained their leaves much
longer than other trees in the city, in fact longer than adjacent sugar
maples on the same block, with the same exposure and light conditions. 
In mid November, the tree in my student's yard still has green leaves,
although they have since been killed by our cold temperatures.  
 
2. Effects of street lights were considered, but no street lights or
other obvious light sources occur near the green maples we checked. 
However, on checking sugar maples near street lights in parking lots,
the effect of photoperiod on leaf change/drop was obvious.  Looking at
individual trees in one parking lot with street lights that directed the
light downward, those leaves that were within the zone of illumination
were still on the trees.  Leaves on the same tree that did not receive
any direct light had already dropped.  The same situation could be seen
in other parking lots and along streets.
 
3. Age did not seem to be relevant in my student's tree either.  An
adjacent sugar maple of the same size, probably planted around the same
time, had dropped its leaves much earlier.  We did not check the age of
other maples.
 
4. Genetic differences: Fred Vogel's observations about the parentage of
his tree are interesting.  But without knowing the actual source of our
maple in question, it is impossible to rule out if our situation is due
to natural variation found in one seed crop or variation over a wide
geographic range (ecotypic) .  We live near the northern limit of sugar
maple; northern distribution limits are reportedly set by abiotic
factors, particularly temperature.  So, in northern Ontario, I would
expect a seedling, physiologically not ready for winter, to be selected
against at an early age.  Within STANDS in our (rural) woods, there is
very little variation in leaf change and drop; they all change color and
drop their leaves at about the same time, give or take a few days.  In
more southern climes, more variation in leaf change/drop is probably
retained in a population.  
 
5. So, recognizing that ecotypic differences are genetically controlled,
a difference in ecotype seems a plausible explanation for these few
green-leaved maples.  The best response came from Michael Dosmann, at
Iowa State University  (copied at the end of this post).  He states that
"Provenance and genetic source are a primary reason for the variability
in autumn leaf coloration", he then discusses the variation observed
between black and sugar maples on his campus and other possible factors.

Thanks very much to all who responded.   Your explanations certainly
started my students thinking.  And thanks to a suggestion by Johanna
Foster,  I plan to develop a short class exercise around this question.
I will try to determine the source of a number of city sugar maples
(nursery stock vs. transplanted from the nearby woods), then a future
class can compare times of leaf change/drop with microclimate, proximity
to artificial light, and source.

Susan J. Meades
Algoma University College, Sault Ste. Marie, ON

              >*<  >*<  >*<  >*<  >*<  >*<  >*<  >*<  >*<

Original Question: One of my students noticed that although most of the
sugar maples here in northern Ontario have turned gold and red (and have
since dropped their leaves), some of the sugar maples in town are still
green. Someone told her that it was due to the trunks being damaged, but
I think it probably is the source of these maples.  Trees planted in the
city are often garden stock from southern Ontario, where leaves don't
turn as early (southern ecotypes).  We've already had several frosts and
10 cm of snow last week, so these city maples now have dead green leaves
hanging from the twigs. Does anyone have an accurate explanation for
this phenomenon?

Responses:

Perhaps environmental conditions are a little different in the city as
well?  -  Grant Cramer, Univ. of Nevada 
----------
Could well be a few degrees warmer in the city.  - Adrian Bell, UK
----------
I've noticed in both Eastern Nebraska and the Front Range area of
Colorado, that trees of similar species and age (cottonwoods most
noticeably here along the Front Range) loose there leaves later in the
Fall and bud out earlier in the Spring when they are in town, compared
to those "in the country".  Some of the cottonwoods here in Greeley were
planted from seedlings collected from adjacent stream banks in the late
1800's and early 1900's, yet still show the shedding discrepancy.  I
thought it might be a response to the "micro-climate" created by the
city.  -  Gerry Saunders, University of Northern Colorado
----------
Your example of city trees being southern ecotypes is interesting. 
Could the lack of leaf drop also come from a warm city compared to a
rural environment?  I couldn't tell from your e-mail if the maples that
have dropped their leaves live in the country or in a city.  If you have
a "country tree" versus a "city tree" then you may be noticing the
warming effect that cities have on an environment (often called the
"urban heat island"). ... Cities, with so much more concrete and
asphalt, tend to make the air mass above them warmer than the rural
areas, thus it can snow on the edge of town and rain in town.  - Johanna
Foster, Johnson County Community College, Kansas
----------
Your explanation is probably correct.  However, it is also usually the
case that cities are a bit warmer than the non-urban areas and thus
could prolong leaf senescence for that reason.  There is some debate as
to whether senescence is triggered by day length or temperature.  Dirk
Vanderklein, Montclair State University, New Jersey             
--------------------
Could it be due to artificial lights in the city?  Street lights that
are often on all night might be fooling the trees with an extra long
photoperiod.  - Guy Farish, Adams State College, Colorado
----------
One possibility is that the trees growing in the city are exposed to
street lamps that are artificially extending the daylength.  Fall leaf
abscission and senescence is a short-day, photoperiodically induced
phenomenon, and is only hastened by low temperatures. - Gerry Deitzer,
University of Maryland
--------------------
....could it be that the planted trees are younger? I notice down here
in S. Illinois that on occasion what appear to be young oak trees will
retain their dead leaves throughout the winter. However, I think that
your explanation sounds to be the best.  - David Gibson, Southern
Illinois University 
 
In response to the above comment on the relative age of oaks in S. Ill.,
Dave McNeely, University of Texas at Browsnville wrote:  Oaks retain
their leaves when the water content of the leaves is higher than usual. 
Basket-makers select oaks for wythes based on whether they keep their
leaves through the winter or not.  Those that keep their leaves make
better (more supple) basket wythes. 
--------------------
Maples being grown from seed are subject to wide variations in
characture.  An example is our neighbor grew hundreds of maples from one
tree and we have one in our back yard.  It doesn't change color at all
while the tree it came from turns red and gold.  -  Fred Vogel, no
institution given
--------------------
I think you are correct, that these trees are a different race.  When I
took woody plants, I learned that non-native trees tend to hold their
leaves later than native trees.  I think both daylength and temperature
have something to do with it.  -  Douglas Jensen, Hollins College,
Virginia
----------
I think you hit the nail right on the head.  Provenance and genetic
source are a primary reason for the variability in autumn leaf
coloration.  Here on the ISU campus, there is extreme variability both
in time of senescence and color.  A majority of the 'hard maples' on
campus are Acer nigrum, the black maple (A. saccharum ssp. nigrum for
you lumpers out there!), while there are some selections of true sugar
maple (A. saccharum) also on campus.  Since the black maples are
primarily all native or from native source, time of color development is
pretty synchronized.  Sugar maples, though, have been unpredictable,
some still green, others already defoliated.  Regarding trunk damage and
coloration, it has been my observation that some of the best color I
have seen is often associated with stressed trees.  Don't forget
microclimate, either.  While light intensity and photoperiod are
relatively the same in a localized area, microclimate (and evening
temperatures) can vary dramatically, also contributing to variable
coloration.  -  Michael Dosmann, Iowa State University