Assuming you are correct in that the trees are pines and not spruce, fir,
larch, redwood, etc., there is a simple explanation for this sort of
observation. Pines are (almost all) shade intolerant: That means they do not
send new growth into areas where the intensity of sunlight is too low to
"justify the investment" in new tissue.
Additionally, some species (such as white pine) are much aggressive
self-pruners than others (e.g. the spruces). Self-pruning is the term we use
for when a tree sheds branches that have died, and in the majority these
branches died due to the low-intensity of sunlight on them as the
surrounding trees grow and shade each other. Trees are much more
compartmentalized as organisms than are animals, and if a particular branch
does respires more energy than it fixes via photosynthesis, the tree 'opts'
to kill and often shed it.
Hope this helps!
Mike LaMana, MS, CTE
Consulting Forester & Arborist
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