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What's bugging you? Dec. 5, 2018

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Last time I started a three-part series on 'Tree-ology,' discussing how trees grow and how that might influence our management of them. Roots were the first part of this puzzle. Now we move up to the trunk and branches.

Visually, the tree's trunk and branch arrangement will define what we see when we look at a tree. And each kind of tree will have a set of distinguishing characteristics. But the woody structure of all trees functions much the same.

When you look at a planted tree, you should see a thicker or wider part next to the ground. If the tree trunk arises from the ground as a straight, uniform diameter, you probably planted the tree too deep. Sometimes this poorly planted tree will survive. But more often than not it will decline in vigor and die a few years later.

Looking at a cross section of the tree's woody parts one will first see the woody or corky bark. Sometimes this bark is very thin such as with an aspen tree. Inside the corky bark section is the phloem. This tissue transports food manufactured by the leaves to the roots and other cells throughout the tree.

The next functional layer is the xylem or sapwood. Xylem cells conduct water from the roots to all other parts of the tree. This is the layer that sap is collected to make maple syrup. Heartwood is found in the center of the woody cross section and is composed of xylem that is no longer functional. It provides the core support of the tree and is darker in color.

You will also notice woody rays like spokes of a wheel. Cells in these rays transport water and nutrients from the xylem and phloem to interior areas of the trunk and branches. You will also see growth or annual rings. Favorable weather results in wider rings while narrow rings would indicate the tree was struggling. As the summer progresses, the xylem cells diminish in size, becoming densely packed, making up the dark part of the annual ring.

Last but not least is the cambium layer, between the phloem and xylem. It specializes in producing new cells for the phloem and xylem layers. The cambium is the only layer that is responsible for new growth.

So how does this transport system work? As sugars and associated water are moved from the leaves to the phloem sieve tubes, phloem cells swell to maximum capacity and eventually "squeeze" this solution from one sieve tube cell down to the next until the material reaches the roots.

Water and nutrients move upward to great heights thanks to cohesion. Water droplets tend to cling to each other forming ropes of droplets in the narrow xylem cells. As the tree's leaves give off water through transpiration, this pulls the water upward. This is a pretty awesome force!

As a tree grows, it adds a new layer or growth ring and the trunk increases in diameter. However, tree limbs never move up the tree from their original position. Higher limbs are new limbs.

So now you've endured my second class on tree growth and function. Hurray for you! But what good is this knowledge if it can't be put to practical use. Here are some tips. Remove any tree trunk wrap you used when you first planted your tree after one season. The cells in the bark of some trees manufacture sugars that provide energy to a recently transplanted tree. So uncover the trunk unless sunscald is a problem. If you must stake a newly planted tree, do it low on the trunk. Staking prevents the root ball from moving and shearing off the new roots. We want the higher parts of the trunk to move in the wind as this gives it strength.

It goes without saying that you do not want to damage the tree during construction, or with lawnmowers or weed whackers. This can permanently damage the cambium layer, disrupting food and water transport. This resultant damage can also provide an entry point for disease. And last but certainly not least, learn how to prune properly. I'll do a column on that subject in the near future.

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.

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Surface Creek
Jim Leser, What's Bugging You
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