The absolute simplest tree for Dendrologist is the
General tree; completing the whole tree only requires 20 ability points.
You can abuse
Places of Power for up to 25 bonus ability points in the base game (30 if you include Toussaint from BaW). Just using the guide for
Power Overwhelming will net you 5 bonus ability points early.
I definitely recommend either waiting until your build is complete and you don't need more ability points, or just savescumming back to before you spent your points. Either way, once you've collected enough ability points, dump it into the General tree to complete the achievement.