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The greater importance and vulnerability of the downward flow of enriched sap, as contrasted to the upward water flow, are apparent. The phloem conduits are much thinner than the xylem conduits, and more exposed. Their burden is richer, containing all the tree's elaborated food, not just raw materials, as in the water column. External injury to the tree's cambium layer is thus much more serious than internal injury, to sapwood or heartwood. Trees even lightly "girdled"—cut or constricted all the way around— will die, not from the tops down, but from the bottoms up. Deprived of nourishment from above, the roots wither and cease sending up water to start the alimentary process. Exceptional in this respect are palm trees, whose trunks can suffer circumference damage up to their breaking point without the trees' health diminishing. This is because the palm family's phloem conduits are arranged in scattered bundles throughout the stem instead of in a circle around it. None of the moisture carried downward in the sapstream to the roots is returned into the soil. But in nature's economy, trees do reciprocate earth's gift of water by holding soil, and thus moisture, in place with their root meshes, and by lessening ground evaporation with their shade. This is why trees are planted around reservoirs, to check erosion and parching. Evergreens are most used for this purpose because they will grow fastest and densest with the least water requirement for themselves, and their roots run nearer the surface, where erosion begins. |
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