I'm thinking like lisanti - aspens form thickets from root sprouts. Even separate trees will form root grafts, so you are right about no chemical control. Sounds like either remove the aspens or keep on mowin' and/or choppin'.

Are these saplings growing from seeds put down by those trees or are they suckers growing up from the tree roots?

If from seeds a plant poison will not harm the trees. These are most often best controlled simply by mowing any new growth down.

If they are suckers, new plants growing from the trees roots, spraying a plant poison could kill the trees that are there since those suckers will move the poison down to the trees roots.

New growth where older growth has been cut down may indicate a lot of viable seeds are germinating or it may indicate suckers growing, but suckers usually appear only fairly close to the parent tree.

kimmq is kimmsr

kimmsr's use of "fairly close" may be misleading; some plants sucker only in their immediate area, say a foot or less away, but some sucker much farther out than that. I have a sassafras tree that regularly sends up suckers 30 or 40 feet away (I mow them down, as they are in the middle of the lawn); a sumac from two yards away once suckered into my garden, and yes, it had the connecting root. It comes down to that there are no hard and fast rules about suckering distances.

I agree with the above comment, especially if you area dealing with aspens. They are notorious for producing root suckers - that is a primary form of reproduction for this species - and at an extended distance from the 'mother' plant.

Living in Utah, you should be familiar with the Pando aspen grove, which is considered to be the longest living single organism on the planet at some 80,000 years old. It covers about 160 acres and yes, is considered a single organism. All the trees are connected by their root system and interrelated, meaning that they are essentially all growths of the same exact plant.

Thanks everyone for all the replies.

They start about 5 feet from the trees but the bulk of them are 15-30 feet away. I am pretty sure they are suckers though, because any time in the past where I have let them grow large enough that they can be pulled up without breaking it has always pulled up a foot or two of a single root always in the direction of the trees. I guess it could be possible there's a mix, but I don't know how I could tell without digging them up.

It's sounding like my only real option is to keep mowing them down. It's too bad they seem to grow faster than the grass. If I'm lucky, removing that half-dead aspen will kill off some of them, too.