Sue Holden said it had been calculated that the NHS could save £2.1bn a year if everyone had access to green spaces.
She made the comments at an event to mark the culmination of the five-year Visit Woods project.
Ms Holden added that only an estimated 14% of the UK's population had "easy access to trees".
She told an audience of invited guests at the Houses of Parliament that the link between "healthy woods and healthy lives" was a "connection that really has to be made much more and much more often".
She added: "It is a connection that we know intrinsically, we believe it to be true but - increasingly - it is something that evidence is backing up as well. It has been calculated, for example, that £2.1bn of healthcare costs could be saved if everyone had access to green spaces."
Oh. And we'd be happier, too....;)