Wow! Great to see some of your work already
Very creepy looking fellows, they look like nothing I've seen here. I especially like the skeletal additions on the branches.
For their rules, I would personally make them 'spirits', give them the special rule 'Terror' and a profile similar to this
Points cost: 30
F 4
S 4
D 5
A 1
W 2
C 3
Special rules -
Twisted Evil: The branches and roots around them become possessed, therefore all good models within 3" of a Possessed Worker suffer -1 on their roll to win a Fight. This is cumulative with other penalties.
Rooted: These evil spirits are able to draw the very roots of trees out from the ground. Calvalry models do not get their extra attack or knock to the ground special ability when charging Possessed Workers. Additionally these models received +1 to wound against Ents.
What do you think?