Sitemap for a forum

[Tourist]

Hy i have a small problem: I am making a sitemap for my site that have a forum but one day i have let say 90 pages next day i have 65 .
How can i solve the problem ? Will the pages that are not in the sitemap anymore show as errors in google webmaster?
Please help . If i exclude all pages from the/forum from the sitemap and show only the static html pages of the site how will this look too google?

I forgot to mention or not been to clear that I have a site and to that site i have attached a forum so i do not have a stand alone forum. So i will need a sitemap at least for the site and exclude the forum from the sitemap(my opinion). But i do not know if or what errors i will recive in webmaster tools.