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It does not take many years for a cavity the size of the one in Fig. 7, which started from a small branch stub, to enlarge to one the size shown in Fig. 11. Here the heartwood has been invaded so far as to cause structural weakness. When such a cavity has formed in a branch or minor leader, the simplest procedure is to amputate the whole member below the cavity. In a main leader or trunk, salvage is still possible by full excavation of the decay, through a frontal channel down its whole length (which matters much less than its width), followed by the installation of internal iron bracing rods. (See below.) These are criss-crossed, at different levels, at spots where the cavity's sapwood-and-bark shell affords good purchase. Tree "surgery" as it was first sold years ago consisted largely, and more truly, in tree "dentistry": plugging up cavities with a variety of fillers, chiefly concrete, which were supposed to arrest decay and strengthen the tree. Seldom did cavity-filling do any such thing for trees. Unless a perfect seal is achieved, decay persists more surely in a "filled" cavity than in one left open, well drained and periodically painted. Structural strength is as often lessened as it is increased by the filler's inert, non-integral bulk, against which the tree's living tissues weave and chafe. Nowadays, cavities are seldom filled except for appearance's sake; and this work, which is at once the most expensive and least important aspect of tree surgery, should be left to experts only. With this exception: Where a cavity extends down a trunk into the root-crown, it often pays a tree owner—after he has cleaned out, painted, and rodded the hollow above-ground—to excavate the decay downward as far as he can, then lay a base of small stones and pour filler (an asphalt-sawdust mix) into the hollow until its surface rises a few inches above ground level. When it hardens, this fill by its weight alone may improve the tree's balance and anchorage. The top of the fill can be kept sloped and moisture-sealed against the tree's butt shell with thick tree paint or a plastic. |
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