It's a little hard to tell for sure from the picture, but I can't see the root flare at all. I would think it would be easy to see it in your picture -- especially, since it's such a big tree. If the root flare is covered, your tree is being slowly, but surely, suffocated.
The following links are from this site. To learn more about root flares -- and why they are important -- go to the home page on www.dirtdoctor.com
, hover over the "Learn" tab and then choose "Article Library". There are tons of articles on lots of helpful topics, including several about root flares.
To get you started, these are "improperly exposed":http://www.dirtdoctor.com/organic/garde ... n/id/3590/
and these are "properly exposed":http://www.dirtdoctor.com/organic/garde ... n/id/1609/
If you can afford to hire an arborist who knows how to handle this issue, that's one option for you. Moore Tree Care is apparently recommended by Howard Garrett, since the article (below) refers to them. The other is to work on your tree yourself. This article tells you more about doing that: http://www.dirtdoctor.com/organic/garde ... on/id/484/
We have a mature oak tree that was under severe stress when we bought our house. It had a bricked flower bed about 1-1/2' deep all around it. We removed the bed and the excess soil, amended the soil around the tree, removed a hot tub off the root zone, took out a deck (that was also on the root zone), and gave the tree lots of good mulch and nothing else in its' root zone for the last year and a half. Now, it is exhibiting many signs of wellness: large weeping gashes in the main trunk have closed up completely, "joints" in the tree that formerly were crawling with ants and oozing sap have healed up, there are no more woodpeckers visiting our tree (they are a sure sign of stress and the presence of insects), and some of the formerly dead looking branches have nice new leaves on them!
Howard has said that trees are very resilient. We are certainly finding that to be so. I hope you do, too!
**Take time to stop and smell the flowers
(or... as my ladybug refrigerator magnet says
"take time to stop and eat