Ask Mr. Smarty Plants

You are fortunate that your Fraxinus berlandieriana (Mexican ash), which is native to the area of Aransas County on the lower Texas Gulf Coast, lived as long as it did. All members of the Fraxinus (ash) genus are fast growing and short-lived. In fact, from our Conditions Comments on the page for the Mexican Ash in our Native Plant Database:

"This is deemed a weed tree by organizations (e.g., TX Forest and Agriculture Extension Service) because of its short life span, susceptibility to pests and disease, and habit of constantly dropping small, dead branches."

Let's begin with the problem of the roots of the ash. Yes, the roots should be removed; if you don't, they will  continue to try to send up sprouts for years, in an effort by the tree to survive. And, yes, roots left to decay in the soil will cause the earth to sink. Some experts recommend heaping dirt over the old roots to accelerate the decay, but this is going to take a while, and you will have to continue putting in fill dirt over the spot.

From the University of Minnesota Extension, we found this site on Removing Trees and Shrubs. It recommends, as do we, that you get a trained and licensed professional arborist to handle this job, including the stump grinding. And, from personal experience, we can tell you that the wood chips from the grinding make the most wonderful mulch or even can be left in the soil to compost in place.

So, now you want to know how to replace it. To begin with, replacing it with a "large" live oak probably means you are going to have to wait a few years for it to grow into "large." Oaks in general do not transplant well, they have a long taproot, possibly longer than the tree is tall. If that taproot is damaged in transplanting, the tree will eventually die.

Finally, on to the transplanting of a new tree. From Helium.com, read this article Tips for Transplanting Oak Trees. Please note: this author is talking about transplanting small oak trees; the rule of thumb we have seen on transplanting oaks is that they be no more than about 3 ft. tall.  If you really want to try a big live oak, that is, again, a job for professionals, and likely very expensive. This article from Popular Mechanics, Guide to Planting Mature Trees, should give you a good idea of the extent of this kind of project. Not a do-it-yourself project.

Our recommendation? First get rid of the tree roots; if you go with letting it decay in the ground, it will be several years. If you have the stump ground, the area should be ready for the new tree by late Fall, which is when you should transplant trees in South Texas. Make sure that any nursery stock is freshly dug, and that the roots are not wrapping around in the container, if it is in a container. And, then, be patient.