I just want to make sure I'm not carefully tending volunteer seedlings of Tree of Heaven.

... mine should be NORTHERN pecans in case that makes a difference.
So according to the member who posted the above, the one I was most concerned about on the left with finely serrated leaves IS pecan.yeren wrote:Congratulation, that definitely looks like a Pecan tree to me. It has its characteristic young growth habit and finely-serrated leaf margins..
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Meanwhile, a Tree of Heaven (Ailanthus altissima) has slender leaves with small ears at their bases.
And they more closely resemble Sumac or Black Walnut trees than Pecan trees..