What causes different colored growth rings?

[Klunker]

What causes the extraordinarily dark growth ring as seen in the center of the picture?
This is from an approx 150 yr old 30" White Oak. The picture does not do it justice.
I sanded down the end grain to check the age of the tree.
Its hard to see in the picture but this darker ring can be seen even in the area that was not sanded.
It goes all the way around the tree.
This ring is about 2/3 out from the center/pith of the tree.

Thanks if anyone knows.

[Jeff]

I sure dont know but eagerly await an answer from someone that does!

[WDH]

I could make up something that sounds fancy, but I really do not know either . 

[clearcut]

Trees can regulate the amount of protective chemicals deposited in the wood when they face environmental distress such as drought, insect attack or disease. 

The mechanisms are not well understood, but trees appear to receive signals from mycorrhizae, and changes in infrared light from surrounding distressed trees. They can then channel energy into protective chemicals like tannins. 

If you have ever seen a false color infrared aerial image, stressed trees will reflect a different range of IR light. Surrounding trees will respond to this light by changing the amount and composition of tannins and such to defend against the stressing agent. 

If you can estimate the date of the dark ring, it may be possible to link an historical event that could have impacted the tree. Gypsy moth or other invasive pest boom year?  Large scale fire blocking the sun. Volcanic eruption? Do other nearby oaks exhibit the same dark ring?

That is my guess. Does that sound fancy enough?  Also looking forward to interesting alternative explanations. 

[Woodpecker52]

What year was it in?

[WDH]

Clearcut,

Stress may just be the cause.  All living things respond to the environment.

[barbender]

I got have some red oak off of a sale we cut. It has dark and off color growth rings, the forester suspected they coincided with a period where we had draught conditions and an infestation of two lined chestnut borer (back around 2000). We lost a lot of red oak in that period. These rings almost look like lightning strikes, but it affected most of the trees in the stand.

[BaldBob]

Clearcut's theory seems the most logical. Although I don't have a better theory, the fact that that growth ring and the one the following year don't show a measurable reduction in growth relative to other growth rings immediately before or for some time after, would seem to argue against that theory.

[Jeff]

Perhaps it has something to do with food quality and amount that year. (Sunlight)

[Don P]

I wonder whether that ring was apparent in the sapwood or just the heartwood, in other words when did the coloration happen, is it from extractives or "meat"?