An addition that I don't feel would be overpowered or in any way effect how the balance of the Treelord is given, while at the same time giving them a reason for nature mages to summon them would be:

Give them a base Nature Gem Income of +~5.

Suddenly they are a Nature Mage with their own source of gems to cast their own spells.

Another Idea would be to have the Province that the Treelord is in, protected by a Forest Dome.