Just posting this here in case anyone else is looking for more info.
I was able to locate the files Minecraft uses for it's trees (I decided to borrow data from the Dark Oak Tree). Files to look for:
DarkOakTree.java
DarkOakTreeFeature.java
DarkOakTreePlacement.java
And the various .json files related to the dark oak tree, sapling, and leaves.
From the looks of it, I'm able to copy the .json data over and have it work with the block I have created.
However, it seems I won't be able to just copy and paste the code from the .java files and will need to try to find more.
I must say, the documentation for Forge leaves much to be desired. You would think a project with such a huge user base would be documented better.
But I digress.
If anyone else has any tips or info on making custom trees, feel free to reply here! Thanks!