Should be interested to hear how you get on. I also have a forum and also haven't sent out a newsletter in the past.

Email is the usual method, and you would send actual news about how the forum or site is getting on rather than sending discussions (apologies if I've misread).

Once a month would be too much for me - I tend to block forums that send me info that frequently. Twice a year would work for me.

I'd see it more as community building rather than adsense generating, and would orientate my aim to achieve that. Hope for knock on benefits maybe.

I have an online magazine with a small forum area. Once a month (or once every two months) I send an email newsletter to all subscribers. Over the following 5-10 days I see an increase in traffic and Adsense earnings.

With forum sites, I think some members post for a short time and never return. Some members simply forget about the site, but may be pleasantly reminded once they receive an email update.

Either way, I wouldn't consider direct traffic (i.e. email link pointing to your forums) as low-quality.

If you receive traffic from a newsletter, make sure not to put ads on the landing page. Some people get in hot water for that as Google may interpret it as traffic generated from spam.

We have a mailing list of double opt-in subscribers and send them an email three times a week and the landing page has AdSense on it and has for four years. I suppose AdSense would shut us down if they didn't like the traffic, but the epcm on that site is the highest of any we own, including the ones that get organic search traffic.

I have email newsletters associated with some of my sites and get very good AdSense results whenever I send out an issue.

If your forum members have asked to subscribe and then actively confirmed the subscription (some call that double opt-in, others don't), that takes care of a lot of the poor quality traffic concern.

You also want to be regular. If you haven't sent something to subscribers in a number of months, a lot of people won't remember asking for a subscription and will consider it SPAM.

But you also need to make sure you have something useful to offer in the newsletter. If it smells like you're just sending something out to try and get a response, you'll lose some members/subscribers each time.

I've noticed that if my EPC is X for a few days and then I send out a newsletter, the EPC jumps up to 1.5X or more. Don't know why - but it happens.

FarmBoy

If you receive traffic from a newsletter, make sure not to put ads on the landing page. Some people get in hot water for that as Google may interpret it as traffic generated from spam.

I send nearly each week to 3700 German and 3000 English my newsletter.

There are aound 20 Links in a newsletter to new pages or pages mentioned special in the head of the newsletter.

Why should this pages have no ads?

I send about 16000 emails every single day to users that have asked for them. They all have at least 2 links to pages that have AS ads on them. I have never had any problems with AS.
I am not sure how this affects my AS earnings though, since emails are sent every single day and therefore their value is systematically "embedded" in my daily ecpm.

I am not sure how this affects my AS earnings though

My sites have in average 5000 visits a day.

When I sent out on Sunday 6700 newsletter, the result could be about 10% more on this Sunday.

I write "could" because the effect is to small to can tell an exact number.

I found a small but definite increase in hits when whenever I sent a newsletter. Not much difference in CTR but a far better take up on the affilite links. Of course I could put a puff in the newsletter whenever affiliate sales flagged which I couldn't do for Adsense.