Novice wrote:
Hi all, I was driving along the highway the other day and noticed that
some tall pine trees have lower branches and others don't. I would
suggest the reason for this is that in some groves of trees the sun
manages to reach the lower branches either because the grove has a
plain on one of its edges or because the upper branches are sparse
enough to allow a sufficient amount of sunlight through to allow the
lower branches to photosynthesize.

One other explanation would be that there are foresters who
intentionally go through and remove the lower growth from the trees so
that the trees don't waste undue energy on maintaining the lower
branches which aren't as capable of producing energy. Would another
possible explanation be simply a difference in the species of the
tree? For instance some trees are just better at maintaining lower
branches with minimal exposure to sunlight.

Thanks for any suggestions for why this phenomenon occurs,
Novice

Both of your ideas are probably right and are complementary as well.

From a molecular biology point of view, the cells in the tree branches
are pre-programmed to die, unless they get signals from the rest of the
tree telling them not to kill themselves. The farther the branch is from
the top of the tree and the less light (at a specific color aka
wavelength), the more likely the branch is to not get the signal to keep
itself from shutting down and cutting itself off from the rest of the plant.

If you're interested in getting really deep into this sort of thing at a
molecular level, try these for search words: P450, senescence,
apoptosis. (It's a very important topic for a lot of reasons.)