Khaki button-down long-sleeved or short-sleeved shirts are worn over cotton t-shirts, usually open at the collar (unless the ranger is working in an office, in which case a khaki tie with a brass tie-tack would be appropriate).
This has led to many styles of microfiber shirt lines - from the simple t-shirt offered by Silver Eagle Outfitters to the Hemisphere short-sleeve button-down shirt from TravelSmith, one of the biggest microfiber retailers.
The short sleeved button-down style is also very popular with travelers, especially since some (like Travelsmith's "Trip'r" line) incorporate UV protection and specially-designed pockets useful for tickets and other gear.
Unlike other fabrics, fleece is not usually blended with spandex, so there is no extra "give." If you are unsure of your size, choose a size larger than what you would wear in a button-down shirt style.
Button-down cardigan sweaters for men were a staple of academics - it's the rare portrayal of a male professor in popular culture that doesn't show him wearing a thick cardigan, often while smoking his pipe.