If I ran the world ... or if I ran this board ... or if I ran the search page on this board ...

... The default value under "Forum to search" would be "All Forums" (only I'd probably pluralize it "fora").

AND

... The default value under "Date Range" would be "All posts" (which I would pluralize "posts").

One suspects, rather strongly, that these are the values most often selected by most users of the search page and therefor ought, by default, be the defaults.