gebo said:
I was also wondering how long I could expect them to keep growing. Don't they have some limit? By the way, in my area there is a pretty solid clay base under a shallow soil layer, so that may be why those roots are so close to the top. They have nowhere else to go to get the nurishment.
Tree roots just keep growing, they can grow far further than the drip line of the tree, up to 7 times and even further in some species. Of course they'll grow in dia size as well.
Now, regarding clay soils, we have a lot of that here. It really comes down to species selection to suit existing conditions. Some trees don't mind heavy soils others hate them.
But in general, tree roots need air, yep, just like us. So in heavy soils such as clay there is a lot less air in the soil, consequently roots tend to be closer to the surface ... just think the opposite such as sand, plenty of air and roots stay a little deeper. So really, your in bit of a whammy, increasing the soil level will encourage more surface root growth.
Tree roots sitting in wet boggy conditions can also die and rot, so clay is a real bugger. You could try improving the soil profile but that's not going to solve the immediate problem either.
Looks like your going to be having more garden, mulch is good for trees, kill off the lawn, mulch, some little plants here and there ....