(Continued from A Year in the Life of Your Tree - 2)
Liquid Transport Upward:
Just as inside the trunk, the large woody roots have their live old woody tissue, called xylem. A piece of xylem cut from the trunk to measured size is a piece of lumber, perhaps a 2x4. Water is collected from all the tiny non-woody roots into the large woody roots, through piping, vessels in the xylem and into the trunk. The woody roots have three main jobs: structural support, energy storage and conduit for the thirsty leaf mass far above.
It is these thousands of leaves that run this whole amazing system. To be photosynthetically efficient, all of the pores, mostly on the underside of leaves, need to be wide open and allowing as much air inside as the leaf needs. Without the carbon contained in the gas CO2 the tree could not produce sugar or build all of its structure made of carbon-based cellulose. Photosynthesis is a process where sunlight is changed to chemical energy, glucose. Carbon is "fixed " and oxygen is produced, released. As all this happens a lot of water vapor is released from the leaves in a process called transpiration. Without green plants there would be little oxygen, no plant basis of food chains, and no life on earth as we know it.
It turns out that the steady water use of the leaves creates a water loss that is sensed by the whole system. it is this steady draw that constantly brings water and nutrients up from the roots, through a complicated series of vessels. The active liquid transport vessels in evergreens are called tracheids. Like little pipes, a long series of separate vessels connect, all the way from the non-woody root hairs, through the woody roots and into the trunk, and on up into a branch, then a twig and into the leaf stem, the petiole. The whole trip from root hair to leaf is run by osmotic pressure, from the steady draw from the leaves.
We have discussed the process of photosynthesis in green leaves that contain chlorophyll, but not other energy-producing tissues. The entire tree is coated with a layer called the cortex, included in the bark. Scratch a young branch, but don't go deep enough to get to the wood, and find a bright green layer under the outer skin of the bark. As trees age and the outer bark or the cork gets thick the cortex is no longer present, but all the young bark has this layer, so even when a tree is without leaves and it is at least 4° Celsius, the tree is still producing energy.
So we have leaves and cortex that can produce energy; what about all the other tissues? How do the roots farthest from the leaves get the sugars they need to fuel all their life processes?
Some parts of the tree produce much more energy than they require. This is a good thing, they have extra and they share. It's a mutual help system; the roots and many other tissues perform necessary tasks, but produce no energy, but they are well paid for their work and every part gets what it needs.
Energy transfer starts in the leaves, and then the branch. When we consider the large amount of leaves on some branches, it is easy to see that the branch is generating much more energy than it needs. Apart from producing energy, all of the tree's parts have an inherent cost included in their continued maintenance. Indeed, over-prune any branch and watch it go into decline. Many broad leaf branches if cut hard will sucker or generate new shoots, very fast-growing shoots.
Conifers are a different story: cut the majority of the live needle tissue off any conifer branch and it will soon die. The main reason is that there was not enough leaf mass left to produce the energy needed to run the branch, let alone have anything extra to put back into the system. The branch soon dies. It is removed from the connected greater mass of all living parts.
(Continued in A Year in the Life of Your Tree - 4)