After some playing around today Treecapitator now works properly with Slimy Trees from Tinkers Construct! As with other trees you can collect the leaves by having Shears on your hotbar when you cut down the tree.
Until the next client update is released this will only work in multiplayer.
Thanks to Drifter397 for highlighting the issue.
Thanks to Drifter397 for highlighting the issue.
Enjoy!