Druids are quite fond of using this to punish those that harm the forest. The really vengeful ones never turn the interloper back.

The type of tree the target turns into matches the bark used as a material component.

Witches have been known to use this on themselves, since their spell-casting abilities still work in such a form. They find it to be a good way to hide, without giving much up.

Affliction [57] 
    Advantage [+248%] 
        Alternate Form (Sentient Tree) [24.75] 
            Once On, Stays On [+50%] 
            Absorptive Change (Extra-Heavy) [+25%] 
            Magical [-10%] 
    Extended Duration (Permanent) [+150%] 
        Ends when the caster wills it 
    Malediction (As Regular Spell) [+100%] 
    Based on Activation Roll [+5%] 
    Magical [-10%] 
    Spell Components [-30%] 
        Must say "tree" in Latin 
        Must gesture with hands 
        Must hold tree bark 
Level 2 [62] 
    Add Multiple Targets 1 [+50%] 
    Alter the note on Extended Duration to this: 
        Ends (individually) whenever desired by those affected 
Levels 3+ [+5/level] 
    Increase Multiple Targets by 1 level [+50%] 

• Sentient Tree.gct