The auto pistol suggestions make me want to ask, "What exactly are the criteria for a trail/woods gun?" Maybe something like this...
NEEDS
--Rock solid reliability
--Powerful enough to defend against wildlife likely to be encountered***
--Ability to serve as a hunting/survival weapon if need be (small game and birds up to medium game like wild boar and deer)
--Lowest possible weight that still meets all of the above
***The caveat here is that no handgun is a great choice against large bears, for example, or against large game like elk or moose. Even in grizzly country some folks will be fine with less powerful handgun and a can of bear spray, while other folks will absolutely have to have that four-pound .454 Casull revolver.
DOES NOT NEED
--Large magazine capacity
--Fancy optics or lasers
One consideration on caliber that I don't thing has been mentioned--if you want to use pistol-caliber shotshells, then the smaller rounds have very little capacity. Looking at this table from the CCI web site --
http://www.cci-ammunition.com/products/pestcontrol_specialty.aspx -- it looks like a 9mm shotshell is pretty useless, .40 is only a little better, .45 is pretty good and any of the revolver calibers are almost as good or better.
Based on all of the above, I think it's very hard to beat a .357 revolver, though .38 +P is probably almost as good and .44 Special/Magnum is the best for shotshells. These give you the option of high-power loads optimized for deep penetration (FMJ or hard-cast wadcutter), high-power loads for terminal effect (hollow point), and low-power hunting loads and shotshells and it's very easy to switch between loads with no worries about cycling the action. The other nice thing about a revolver is that all those variations are readily available as factory loads, even very light "cowboy" loads for .44 Special.
I suppose that an auto pistol could do all of that, as long as you are willing to accept that the low-power hunting loads and shotshells may have to be single-loaded and may not cycle the gun properly. Also, the very low power loads will probably have to be handloads though shotshells are not hard to find.
For a revolver, any 3-4", 5-7 shot, .357 or .38 +P should be just fine, it all comes down to personal preferecnes and how much weight you want to carry. The Ruger LCRx 3" .38 +P stands out or maybe a Charter Arms Bulldog in .44 Special. A full-size double-stack auto pistol may not be the best choice if weight is a consideration but something like the Kahr CT45 might work very well--economical, reliable, light, decent shotshell capacity, though you'd need some very light handloads to leave any meat on the squirrel. ;-)
