This dying issue all started 5 years ago with a cedar tree turning brown and then a small willow oak about 10 feet away dying and rotting within a few months. I didn't get pictures of those but then the pattern kept going. Last year a large 3-trunk willow oak had 1 trunk start dying and never leafed out but the other 2 did. I thought it may come back this year but it appears only one trunk leafed. The others are rotting away. Then a Rose Tree that was planted in a bed 30 feet away 2 years ago was growing great and then this year is just dead. I had the local county agriculture extension office come look at them but they were not arborists. They didn't see any signs of bugs or fungus on any of the dying trees. All of the trees so far in the back yard are fine except for some tall pines that have been slowly dying since we purchased the property 10 years ago. Also, a neighbor's large Post oak 40 feet downhill from the willow is dead all of a sudden last year. These trees were all healthy as of 2 years ago. What could have affected them so much in such a short time? Any advice is appreciated.

Three questions...

Where do you live?

Why haven't you consulted an arborist?

Do you have borers in your area?

We need information.
Please fill out your profile... we need to know what part of the world these trees are in.

Also... weather? Has there been a change?

And most likely in my thinking... Was there recent heavy equipment on the land?

Tree die off is predictable at new construction sites.
That heavy equipment compacts the soil and the roots can't function. No air in the soil, no ability to absorb nutrients...

It's sad, but those construction crews don't know or seem to care that they're destroying the few trees that they don't bulldoze.

I concur that heavy traffic might be the cause. A large, established tree's root system can spread out much further than the crown.

Less likely since I don't see any signs of it: Armillaria. One individual fungus can take down entire forests stands by invading through the roots with its large, black, shoe lace like rhizomorphs. These are visible right underneath the bark, but I don't see them in this picture.