There’s no such thing as a tree (phylogenetically)
I don't get the premise of the article. A tree is just a growth shape of a plant (plants are quite flexible regarding growth shapes, and often adapt to the environment), and a category that our minds place a plant species into. And its clearly an ancient ability common to all vascular plants to develop woody tissues. So why would we expect the tendency of a plant to become a tree to depend on its position in the phylogenetic tree? (the authors had me there for a moment. I thought they wanted to show that the phylogenetic tree is not really a tree!)