Gradually over the past two years, the tan colored gravel under my river birch trees has become a very dark almost black. The rhodos under the river birches also have a black sooty look on the leaves. The leaves on the river birches also have a black sooty looking something on the keaves. I don't have nearly the quantity of leaves On these three threes as in the past. Please advise. Thank you

Here, in Illinois, we have something called "sooty mold" which is a fungus that grows on the honeydew (nice word for poop) which is produced by aphids and scales. Take a sample to your local Master Gardeners, which can be found at: http://transylvania.ces.ncsu.edu/ and they can advise you on how to treat the problem. It should be addressed, because the mold will not allow the leaves to photosynthesize properly and will stress the plants. Hope that helped! B