I just had a look at the HTML version, which is - as I understand - the relevant version for search engines. On the forum home page, I did not found a meta description tag. On the topic pages, the meta description tag exists, but the value equals on every topic page to the main forum description.
I would suggest, to use the main forum description on the home page and the first 150 characters of the topic content as the description. This would allow the search engines to show a preview to the topic content.
