Ok, I've done some research of my own, and the following seems to be the case:

##Tree tubing does work, but not with dirt

Using dirt tubes of height 2 and 3, normal trees still grow:

However, using cobblestone tubes, large trees always grow:

Ok, I've done some research of my own, and the following seems to be the case:

Tree tubing does work, but not with dirt

Using dirt tubes of height 2 and 3, normal trees still grow:

However, using cobblestone tubes, large trees always grow: