There are 3 kinds of rubber trees in Ultimate (at least). The rubber tree that comes from MineFactory Reloaded has some config problem.
When you collect and plant the saplings from this type of tree, they grow into Oak trees.
This is a problem for a tree that is supposed to be a renewable resource (the rubber -> plastic processing from this tree is used in pretty much every MFR recipe).
Can you please have a look to see what might be causing these saplings to grow up as the incorrect tree?