Yes, from the "Forum Tools" menu on the index page for that forum.

It works a bit differently from subscribing to a thread, though. Forum notifications and thread notifications get listed in different parts of your notifications page and I remember having to scroll down to find them. My subscriptions page (which I check a lot more often than my notifications page) only shows subscribed threads, not subscribed forums.

An alternative (from the same Forum Tools menu) is to mark the forum read, then scan the Forums page occasionally for signs of recent activity there.