I have a fast growing "tree" that has been growing in my yard all...
The tree seedling in your photo shows a compound leaf. The trees that have compound leaves include nut trees like hickories. Also, seedlings do not tend to look like the mature tree.
We would like to have more information such as what other trees are growing in your landscape. If you have multiple ones coming up they most likely are coming from a nearby tree. When you dig them up, what do you see? Is a nut attached?
Thank you for your response. I do not have any nut trees in my yard - mostly pines, but there is a tree of heaven and a Japanese maple. There is no nut attached when I pull this tree out - only roots.
It would be helpful to let one of the seedlings grow through the end of the season. At that time submit more photos of the whole tree and photos of a branch with the leaves. We may have more identifying features at that time. The foliage on immature seedlings can be different than a mature tree.
We have more questions to ask. In the first photo that you sent we notice there was a reddish growth on the main stem. Could this be a type of flower? Fruit or Seed head? Send more photos of this if possible. Also, when you pull out a seedling, what color is the root? Possibly yellow?
How tall is the seedling in the second photo? If you let the seedlings grow, how tall do they get????
It is possible the seedlings may be Koelreuteria paniculata, Goldenraintree. This is a cultivated tree and has a tendency to be very vigorous in growth. New growth has a bronze color. See the link from U of Connecticut http://www.hort.uconn.edu/Plants/detail.php?pid=240