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When growth stops for the winter, contrary to popular belief the tree's sap does not "go down into the roots/' It stays right where it is, stored in every part except the leaves of deciduous (shedding) trees, which wither and break off at the stem ends, where buds remain for new leaves next year. In fiercely cold winters, the moisture in a tree's extremities and outer tegument may freeze, with consequent damage to the containing cells. Roots suffer most in winters of deep frost under a scant snow blanket. Twigs and branches get hurt and frost cracks may open on limbs and trunks during cold snaps that follow "false spring" thaws. But as a rule trees have the hardihood to withstand the rigors of their accustomed climate: their sap stays fluid and viable, and is there to restart growth—in many species to put out blossoms—before the new leaves appear. 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 |
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