Crown crowding...

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Don
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Crown crowding...

Post by Don » Mon Sep 10, 2018 5:11 pm

As a stand develops, say from a disturbance that cleared the area, there comes a stage where the crowns compete for light and attain crown closure. Almost. Looking up from underneath such a stand, one can often see 'corridors of sky' around the crowns.
Screen Shot 2018-08-27 at 1.38.11 PM.png
This is consistent with the training I had at Humboldt State University, perhaps now outdated? It was thought that it was simply the mechanics of physical abrasion. If my understanding is correct, the current thought is that a 'shading' of adjacent leaves diminished the solar input (think electromagnetic spectrum) to those adjacent leaves and left it with diminished photosynthetic energy to compete, and senesce.

I submit that these are two solutions, that in overlapping, provide a more complete answer.
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Grand Canyon National Park

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mdvaden
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Re: Crown crowding...

Post by mdvaden » Mon Sep 10, 2018 9:18 pm

Don wrote:As a stand develops, say from a disturbance that cleared the area, there comes a stage where the crowns compete for light and attain crown closure. Almost. Looking up from underneath such a stand, one can often see 'corridors of sky' around the crowns.
Screen Shot 2018-08-27 at 1.38.11 PM.png
This is consistent with the training I had at Humboldt State University, perhaps now outdated? It was thought that it was simply the mechanics of physical abrasion. If my understanding is correct, the current thought is that a 'shading' of adjacent leaves diminished the solar input (think electromagnetic spectrum) to those adjacent leaves and left it with diminished photosynthetic energy to compete, and senesce.

I submit that these are two solutions, that in overlapping, provide a more complete answer.
It would be interesting to measure with a meter the corridors of light you mentioned. It got me thinking of a candle or a flashlight. Such small amounts of light can be seen for miles, but how much light is it?

Working with photography, an umbrella or softbox with flash or strobe is clear as day to the eye from 4 ft. away, or even 32 ft. away. But the amount of light drastically depletes as the distance increases. If an umbrellas is moved from 4 ft. away to 8 ft. away, the light is not cut in half, but reduced to more like 25%. if the distance increases again to 16 ft. away, the light is only around 5% what it was at 4 ft. away. It's not very noticeable to the eye, but evident with a meter or in the final image exposure.

Trees on perimeters of groves typically grow fatter trunks with more foliage and photosynthesis, and receive more light total. But inside a tight stand, I'm not sure exactly what the actual difference in light is, in actual measure.
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gnmcmartin
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Re: Crown crowding...

Post by gnmcmartin » Tue Sep 11, 2018 9:42 pm

Don:

Having done thinning TSI in specific forests, both hardwoods and conifers, continuously for up to 45 years in some cases, maybe my observations can be useful here. BUT, I have not taken any precise measurements, and not taken careful notes of my more casual observations. Anyway, for what it's worth:

First, in general, I think that the primary factor is "the mechanics of physical abrasion." This is the result of the wind moving both the whole trees so that the trunks sway, causing the abrasion, and also the wind moving the branches so they abrade each other. If the trees are shorter, and/or the trunks more stout and/or stiff, the swaying of the trees is less, Ditto the branches. And, of course, some locations are more open to the winds than others, but the effect can be quite considerable when the winds and the swaying are only occasional.

One observation, supported by simple logic, is that in stands that are more dense, resulting in skinnier and more flexible trunks, the wind abrasion will be greater. But I can't document this with any measurements.

But one thing that I have most certainly observed, is that when a stand of pines, such as red or white is quite young, say 15 to 20 years or so, the branches from adjacent trees grow into the crowns of each other. As the trees grow older, this growing together of the crowns is reduced, and by the time the trees are 30 to 35 years old, the crowns of the trees are generally separate. But not always completely so. When thinning a stand of such trees, I would occasionally run into a rather difficult, and potentially dangerous problem. That is, when I cut one tree, and it began to fall, I would find that a branch from the tree I cut was hooked into a branch of an adjacent tree. This was one of the worst things I encountered. It could be very tricky, and dangerous trying to get the "hooked" tree free so it could fall. I was able to do it in each case, but I won't go into the details of the methods, which were a bit more dangerous than I usually like to get involved with.

The trees in this stand are now 90 feet tall. Of course, I have kept them thinned out, which reduces the opportunities for the crowns to grow into each other or for any branches to overlap. But quite on their own, the trees maintain a good distance from their neighbors. I still have to thin some out when they get too crowded, but they never come close to having branches from one crown grow into those of the neighboring tree at this stage of their lives. At this point, they are tall enough that the trunks are long relative to their diameter, they have become flexible, and in good winds they sway. I must say, I find this swaying in the wind beautiful, especially with tall Norway spruce trees, which in a stiff wind can make my heart "sing."

But there is also a shading effect, but I consider that relatively minimal. If the tree is more shade tolerant, such as spruce and hemlock trees, the crown will be longer. If one is looking up through the crowns of trees directly above, the crown space will be similar to, but perhaps a bit less than that in a stand of less shade tolerant species such as red and white pines. This may be because of the greater shade tolerance of the foliage, but it could also be because the branches are more numerous, more "dense." But even if the space between the trees is the same, if one is looking not so directly up, the longer crowns of the more shade tolerant trees will obstruct one's view of the sky somewhat more. And, of course, the light reaching the ground will be less, resulting in less undergrowth.

But I don't think this shading is an important factor, if any factor at all, in preventing the branches from one tree actually growing into those of another. BUT, if the branches were actually shade tolerant enough to continue growing after they are very much shaded, so that they persisted way down to the lower parts of a tree where the sway of the trunks was minimal, theoretically they could grow into each other. But I know of no tree with such terrific shade tolerance that these lower branches won't die long before they could grow enough to overlap the branches of an adjacent tree.

--Gaines

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Don
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Re: Crown crowding...

Post by Don » Thu Sep 13, 2018 9:54 pm

Gaines-
Between us we have many years in the woods, huh? I agree with your observations, particularly with those from the Western US. I was willing to consider that there might be something to the quality of light getting to the needles (portions of the electromagnetic spectrum...ultraviolets, infrareds, blues, greens...) having differing impacts on photosynthesis, thus vigor, turgor, sufficient to more effectively "abrade' adjacent tree branches with less vigor/turgor. With citations for support, I'd concede it might be both.
-Don
Don Bertolette - President/Moderator, WNTS BBS
Restoration Forester (Retired)
Science Center
Grand Canyon National Park

BJCP Apprentice Beer Judge

View my Alaska Big Tree List Webpage at:
http://www.akbigtreelist.org

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