> hog brought up a good point..never use a small tree..i look for trees at least
> a foot in dia..anything less might give way...
I prefer young, strong, undiseased and undamaged hardwood trees about 8-10" in diameter, which are strong enough that they don't bend when you lay in the hammock. Any tree can drop a limb without visible warming, but older, larger trees tend to drop bigger, more deadly limbs.
Of course, this is in the Ouachitas and Ozarks, oak/hickory forest, where there are plenty of this sort of tree -- more than any other type, except on desolate ridges where you tend to get obviously feeble oaks about 5-6' tall that you should take pity on and not hang from. Depending on where you are, you may not have the luxury of an abundance of such suitable young, vigorous trees.