Priorities in allocation of carbohydrates After many generations of natural selection, trees allocate carbohydrate in ways that will generally increase the trees ability to survive. Because some functions are more crucial than other, a priority schedule for allocation is apparent. Respiration has the highest priority because the living tissues would perish immediately without it. The greater the amount of crown surface per unit of living tissue, the greater will be the proportion of carbohydrate left for formation of new tissues. Very important is the indispensable renewal of foliage an fine roots, an this growth is the second highest priority. The extension of the crown an root system depends mainly on the room for expansion that thinning makes available or can be captured from less vigorous competitors. Part of this process is height growth, which is so important to the survival of competing trees that it commands the next highest priority. Being shaded by neighboring trees greatly lowers photosynthesis. Height growth varies surprisingly little with tree vigor unless vigor is so low that very little carbohydrate is left after respiration demands have been met. However, height growth differs greatly among species and is strongly affected by variation in site quality factors that control overall productivity. Another vital function that must be fulfilled is conduction of materials between crown and root. Phloem must be renewed annually, an so ordinarily must xylem (wood). How can be verified by observing how small a bridge of tissues is necessary to maintain an imperfectly girdled tree in good vigor. Most of the wood laid down in the stem an larger roots serves to support the crown. Although this is a crucial function, it does not have enough immediate survival value to command high priority. The production of secondary compounds that protect the tree from insect and disease attack is of similar low priority. Biologic success is basically the ability to reproduce. The priorities for carbohydrate are in direct relation to the importance in reaching reproductive size and then producing propagules. Seed production is so vital to species survival that whenever it takes place it does so at the expense of other functions. In comparing the relative growth rates of individual trees, it can be seen that the rich get richer. Trees that have sufficient carbohydrate after meeting basic respiratory needs are able to put more energy into larger crowns and root systems, stronger stems, and the production of secondary compounds that protect the foliage from herbivory and phleom from beetle attack. These factors all result in even greater photosynthesis, and the effect snowballs. The extent to which a tree increases mechanical support depends on the vigor of the tree and the amount of carbohydrate that remains after provision for more vital functions. The part of the growth that is of greatest economic concern for timber production is thus low enough on the scale of biological priority that it is subject to great variation; anything that reduces the amount of photosynthesis will result in a reduction in stem growth. It is for this reason that growth in diameter is so readily controllable by thinning or other means of regulating stand density and tree vigor.
Posted on: Mon, 10 Mar 2014 09:57:49 +0000
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