Mushrooms that grow on the woody roots of trees are an indication that there is an extensive amount of interior decay within that tree. There is nothing you can do about that. Removing the mushrooms is sort of like dead heading a perennial plant: the flower is gone but the plant is very much alive and thriving. So is the fungal organism inside your tree...mushrooms are simply the reproductive body. It is likely that this tree has had this rot for many years.

I'm also worried about the sawdust, would could mean the presence of borers. Actually, each and every single symptom you've described indicates problems with your tree, I'm sorry to say.

You already seem to understand that it is growing in what can be accurately termed 'a hostile environment'. Given the location, which you can't do anything about, and the probable maturity of this tree, I don't think I could give you much hope.

However, a hand-on inspection might result in a completly different story! Why not get the yellow pages out and talk to an ISA CERTIFIED ARBORIST (ask to have proof of the certification). Please know that no responsible and educated arborist will recommend treating the tree rot (mushrooms), or filling the holes, or climbing the trees with spikes in order to prune the dead wood. (If the tree is in very bad condition, they won't be able to climb it at all!)

Good luck...maybe someone else has a more positive outlook than I do, lol! I know that you are attached to your tree.

Most mushrooms are the fruiting bodies of saprophytes, not parasites, so if they really are coming from the tree's root tissue, they would probably indicate rootwood decay. This would be consistent with the other evidence you cite of dead wood at and above ground level. Aphids are not a big deal as they don't kill the leaves. The restricted area of soil is probably something the tree has long adapted to. Roots can travel long distances and find water well away from their apparent location. Your tree sounds like it has nearly reached the end of its life span, and will likely start breaking up (or breaking down) in the coming years. Box-elders are not long-lived, and are very subject to decay. I wouldn't spend much money in efforts to prolong its life.

tenderkat,

rhizo 1 may have been on to something with the thought about borers...but I think a much more likely scenario (for a big ol' wild MAPLE such as what you have) would be an infestation of CARPENTER ANTS.

A 35' boxelder is a pretty sizeable specimen.
They normally wouldn't grow anywhere NEAR that large in the wild.
Not bad....for a species that normally exists in the wild typically, as a "transition tree", existing early on during the timeframe, somewhere between the gradual evolution that an abandoned farmer's hayfield might make before it would eventually become a mature beech / oak forest, a 100 or so years after the farmer died.

Still...wouldn't hurt to get a pro to maybe look at it...so long as they don't take your arm and/or leg with them for a consultation fee.

Can you post a few digital pics on here ?

I think the others are correct saying your tree is falling apart from rot- insects -you name it.I had a huge box elder that had to come down and another suffering the same fate.It will probably stand a few more years.
I planted a sugar maple to replace it so i'll have a few more years of shade while my maple gets a head start.
This is your chance to move up to a new model.

If by boxelder you mean Acer negundo (known as Manitoba maple by some), it is a fast growing, short lived tree with weak wood. Most consider it a nuisance, but I have seen nice specimens.

From the size you describe it is an old tree for boxelder. The hard surfacing that has been installed around it is not good for any tree, but boxelder would have tolerated it better than most. However this sounds like an old tree being affected by many different symptoms of age and decline. Without seeing it (e.g. how healthy the crown is) I can't tell how many more years the tree may have. A tree can live many years with rot, etc.

BUT I am far more concerned with the safety hazard a tree this size with rot in it poses to people and property.

I agree with earlier posts that it is not worth putting alot of money into, but do get an arborist to look at it to assess its safety. If you really like it the arbosrit can advise if some pruning now will improve its looks and safety for a little while.

However, I think you need to plan on remove and replace. If you want another boxelder, you can be assured it will grow fast to replace this one. Not a tree I would choose for a back yard, however.