I forgot to mention that during my run on Monday, on the 1.5 mile segment (round trip) that goes north from the 9900 entrance to the Parkway, I saw about 20 or so new trees that had been planted along the path. I'm guessing they are Cottonwood trees since Cottonwoods are native to the area. Right now there are a lot of Russian Olive trees along the path, but those trees aren't native. In about 15 years the Cottonwoods will be big enough that the caretakers of the Parkway can start to thin out the olive trees.
I also saw during my run today quite a few trees that had been planted along the path. However, these trees are on the other side of the fence, and the trees left the path and followed the fence away from the path. That is an open area that gets a lot of wind, and the land-owner might have planted them as a wind-break.