Our evaporating silver maple trees

[Don Peters]

If you've looked at as many silver maple trees over the past few years as I have, you may have noticed that they are changing in appearance. There appear to be two significant changes taking place. The first change is in the bark - it isn't as "silver" as it was in the past, and it seems to be falling off in spots, being replaced with a brown "under bark". The other change, responsible for the title of this posting, is that the canopy is getting thinner and thinner, and some leaves are turning yellow, curling up, and dropping off. Often the leaves will drop off a branch successively from the base to the tip, leaving just the branch tips with leaves. Note that this is NOT the typical "tar spot" that is causing this. Here is a picture of a stressed neighborhood silver maple tree:

Given a quick glance, you'd think this tree is healthy. But note the yellowed leaves on the lower-left branch.  Some of the leaves on this branch have curled up, turned brown, and fallen off. It's from this branch that I took a sample twig.

Since a twig cross-section usually tells a lot about the condition of the tree, I took a series of 45 digital microscope images of the bark edge at 400x and stitched them together with Microsoft's ICE program, yielding the following composite image:


To get an even better appreciation of what this picture shows, check out the full resolution picture at:
 http://www.whitecanker.net/pictures/2010-07-25 Composite Silver maple twig cross-section.jpg
(Click on a feature to zoom-in to it.)


First and foremost, note the almost complete lack of green phloem, the tree tissue that transports the tree's nutrients! Instead, this green phloem has been replaced by dead tissue and white fungus material. It's no wonder the leaves are curling up and dying as if they are starving for water - they are! But watering the ground around the tree probably wouldn't do much good at all, as there is almost nothing left transport this water to the leaves.

In addition, check out the xylem tissue below the phloem. It should be milk-white, but instead is a light brown, as if it, too, is dying.

The large quantity of white blobs mixed in with the normal bark tissue give a strong indication that this tree is dying of white canker.
 

[Don Peters]

If you've looked closely at many silver maple trees, you know why they call them "silver" - their bark is a silver color. And you've also probably noticed that the bark is shaggy and covers the entire trunk of the tree. White canker makes this description less true now. One characteristic of white canker is that it eats away at the inner bark, the phloem, and prevents the creation of new outer bark. Furthermore, chemicals that protect the outer bark from decomposition are also not produced. So, as a tree grows, the old bark decays away, and instead of new bark you get a poor substitute. In addition, the loss of chemical decay resistance means lichens are more likely to colonize the bark.

The previous posting mentioned this bark issue. This posting will show you some examples of it. Here's the first example:


To see a full-resolution version of this photo, click on www.whitecanker.net/pictures/2010-07-25 Silver maple-1 bark.jpg
(Left click to zoom in to a point, right click to save the picture to your PC or to print it.)


The first time you notice this, you will probably think somone has peeled the bark away. But then, as you more carefully examine these bare spots, you'll see that the underlying bark doesn't look healthy at all.

Here's another silver maple with the same problem:


To see a full-resolution version of this photo, click on www.whitecanker.net/pictures/2010-07-25 Silver maple-2 bark.jpg
(Left click to zoom in to a point, right click to save the picture to your PC or to print it.)

As you look at the remaining bark, you can see that it is losing its silver color, and instead turning black. The areas where the bark has fallen away often take on a light brown, or "peanut butter" color.


This silver maple hasn't lost too much bark yet, but it's getting there.

To see a full-resolution version of this photo, click on www.whitecanker.net/pictures/2010-07-31 Sick Silver Maple - 19 Briand.jpg
(Left click to zoom in to a point, right click to save the picture to your PC or to print it.)


This silver maple has lost more bark, and the lower area of bark loss even looks a bit rotten.
In fact, if you look closely, the upper-left part of the trunk shows some signs of bleeding.

To see a full-resolution version of this photo, click on www.whitecanker.net/pictures/2010-07-31 Sick Silver Maple - 25 Briand.jpg
(Left click to zoom in to a point, right click to save the picture to your PC or to print it.)


This large silver maple has lost quite a bit of bark from its lower trunk. The bark loss has gotten progressively worse the past few years.

To see a full-resolution version of this photo, click on www.whitecanker.net/pictures/2010-07-31 Silver maple - 6 Sanborn.jpg
(Left click to zoom in to a point, right click to save the picture to your PC or to print it.)


So, over time, people may begin to wonder why these trees are called "silver" maples!