I parked bees in sometimes very small clearings in the middle of the
Jefferson and George Washington National Forests in VA, and WVA.

Given that the goal here was to gather Sourwood nectar from clear-cuts 
and burned areas, I would be placing trailers at "Point A" along 
a fire road or old logging road, and expecting the bees to find and
exploit blooming clearings at "Point B", sometimes as much as 1 mile
away.
Access to the exact clearings was often difficult or impossible due to
steep terrain and limited roads

They had little trouble making fine crops.

So:

1)  Trees are no barrier at all if the winds are calm.
    Bees can fly above the treetops with ease, 
    and will "terrain follow" above tree-top height
    without any problem, dropping down into clearings
    as they need to.  This is why a compass is such a
    useful tool when bee-lining, so you can at least
    get a direction vector for the bee you see flying
    up and away over a ridgeline.

2)  1/4 Mile is not going to assure you a complete lack of
    crosses, as any one hive will forage for several miles
    in all directions at a minimum.  1/4 mile certainly 
    is better than nothing, but if you have two kinds of
    blooms blooming at the same time 1/4 mile apart, any 
    hive within range of either plot would certainly be 
    within range of both plots.

3)  Will bees fly around wooded areas rather than over?
    If the day is windy, they may do this, assuming they 
    find a headwind if they go above treetop level. 
    A headwind is much more costly in terms of energy 
    than some extra distance to "go around".

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