Some tall pine trees have lower branches, others don't - why?

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
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Novice wrote:

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.)
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Pine trees are a crop just like cabbages. The lower branches are pruned regularly so that the wood which will be harvested will be straight grained and therefore fetch a much higher price - simple
Quilljar Website http://www.quilljar.btinternet.co.uk / Do not reply personally, false address
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this case the OP is talking about pine trees in general. There isn't a little tree gnome running around the woods trimming branches.
A pine growing out in the open will keep most branches because there's nothing competing with the branch for sunlight. In a woodland setting there are always other trees coming up, so the pine puts more effort into growing taller to stay in the sun. It's as simple as that.
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Ann, gardening in Zone 6a
South of Boston, Massachusetts
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Ann wrote:

Well, you may be right Ann, but he didn't actually specify wild woods. In my experience trees usually belong to somebody, and they get pretty annoyed if you mess about with them, camping and so on. Mind you, I live in the UK and our wilderness disappeared sometime around 1066 AD! All our forests are now tree farms.
Quilly
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Oh, well, I guess we're both posting from our differing perspectives. I've seen your forests, I had a wonderful early morning walk through what was called the New Forest somewhere down near Torquay (I think that's where it was). I loved the fact that it was an ancient forest with that name! What a wonderfully beautiful walk that was......:o)
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Ann, gardening in Zone 6a
South of Boston, Massachusetts
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I live very near The New Forest, a bit further East than you thought! So much of it has been cut down (originally to build warships to fight the French) that it is largely open heathland now. We like to walk there too...
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Quilly
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o doubt it is further east, I was on a Mass. Horticulture garden tour, started at the Chelsea flower show and went east then south then west, it was a wonderful trip. I could retire and die happily in Cornwall. You're so lucky, to live in such a gardening country. There's nothing like that over here.
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Ann, gardening in Zone 6a
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It's true that in the UK "all land belongs to somebody (quite often, wild areas are owned by us, via the National Trust) but I think you'll find the same is true of American wilderness land.
There are still remnants of ancient British wildwood left (see Oliver Rackhams books) and some of these are being very carefully preserved and regenerated (such as the Caledonian Forest in Scotland). The lottery-funded Millennium forestry project is regenerating huge areas of native mixed "forests for a thousand years" (non-commercial). You'll find plenty of info on the web.
Native Scots pine naturally sheds its lower branches so that old trees develop a high canopy above a massive bare trunk. Wherever you see them in the wild in Scotland, they achieved that form all by themselves.
Janet
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It is true. Most of the woods we explore in northern Maine is owned by one paper company or another (although there was just an enormous purchase up there by a conservation group); there are clearcuts, but the regenerated forests are good for wildlife. There isn't any virgin forest left up there, I think, but it's beautiful, nonetheless.
Here in Massachusetts there are actually a few stands of virgin forest, one in particular is threatened by the Mount Wauchusetts ski resort. Supposedly the expansion work there is going to be done to protect the area, but I don't think it'll work well. We'll see.
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Ann, gardening in Zone 6a
South of Boston, Massachusetts
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Dear Novice:
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! ML
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Mike LaMana, MS, CTE
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