This basically depends on the type of forum a person creates. There are many popular forums that receive tons of organic traffic. Take for example the forum called Beermoney Forum. This forum seems to attract tons of organic traffic each day. The forum is dedicated to making money online.
Then we have forums that don't attract much traffic at all. They are forums dedicated to fishing or even car repair. They basically will receive traffic to the forum if a person is asking a specific question that has been asked or answered on the forum in the past.
Questions and answers site is the same. You have popular ones that many people use on a daily basis, and then you have ones that people don't really know about and seldom use. Just Answer is a very popular Q & A site that receive a lot of organic traffic each day. Another popular Q & A site is FixYa. They seem to attract tons of organic traffic and you can see many of the questions asked on this site appearing on the first page of many search engines.
I would have to say that both a forum and a Q & A site can receive tons of organic traffic if they are popular. If either of the two websites isn't popular then neither of them will receive a lot of organic traffic.
In order to make sure your forum or Q & A site receive a lot of organic traffic, it is up to you as a site owner to work on this, promote your site, and get people interested in your site so they will come back daily and tell others of it. This way your site can gain a lot of organic traffic daily.