I've four poplar trees, in a line, about 6' apart, that form a nice barrier between my front yard and the street. I like them but one problem I seem to be having is that their roots are like thick vines that run about 6" under my front yard and make it difficult to maintain good top soil (and thus a lawn).
I'm wondering if there's anything else that can be planted in the front yard that will compliment them and, either replace what they're taking from the lawn or provide enough nourishment that they won't need to take as much from the lawn.
Ben, you could try a low growing clover like Dutch white clover, which will have fairly nice flowers, attract beneficial insects and feed the trees some nitrogen (they'll grow even faster, which with poplars is pretty fast). Another possibility if you want to keep more lawn than the clover might allow (and it could run into the lawn) is to plant some nitrogen-fixing shrubs, like siberian pea shrub, goumi (a relative of Russian olive but smaller, non-invasive, and with off-white flowers, silvery leaves, and edible berries) or bladder senna, which has cool-looking pinkish pods. All of those can be cut back if they get tall, which will dump a load of nitrogen into the soil as the roots die back. Shrubs like those will feed the poplars and cause the roots to be more concentrated where the shrubs are. Grass and fast-growing trees are not that compatible, since their roots, as you've seen, feed at the same depth. I imagine you need to keep up an acceptable appearance in the front yard to not freak out your neighbors, so you can't go too wild. But shifting over to some shrubs and beneficial-insect attracting flowers would be more compatible with the poplars than the grass. Let me know if you want some specifics for more shrubs. And (shameless plug) my book has many lists of plants that are compatible with trees and help build soil and habitat, while, in many cases, offering food as well.