8. Forestry Yield Cycle (FORYIELD)

The FORYIELD model in Figure IV-8a has the essence of a forest plantation and its interface with the economy, which buys the wood products. Operating on available sunlight (J0) and using soil nutrients for fertility (SW), organic matter (W) is produced including wood of trunks, limbs, and roots. Some of the organic matter is used continuously by respiration of the plant tissues and other consumers such as microorganisms and animals.

When the wood reaches a threshold stock (TW), the wood is cut and sold for money at the prevailing price (P1). This process of cutting, transporting, and selling requires inputs of labor, machinery, and other goods and services. These inputs cost money that is paid out from the available money (M) at price (P2). When the cutting process reduces the stock of wood below a threshold (TC), cutting and selling are stopped. In the program, X is used to turn the cutting off and on: if X is 0, it is not cut; if X is 1, cutting is switched on.

After forest cutting, the money that has been earned from sales has to last until the next cutting cycle. During the regrowth time, money is paid out at price (P3) to fertilize soil and carry out related management. The simulation plots the stock of money in the upper graph (M), and the stock of wood biomass (W) and soil nutrients (SW) in the lower graph. Nutrients in the soil increase after cutting due to fertilizing and decrease again as the trees get larger and bind nutrients into biomass.

Examples of Forest Yield Systems

Plantation forests are found all over the world, using different species in different places depending on soil and climate. Plantation species are selected for rapid net growth. They are usually the species originally found as successional species that were dominant after the initial cover of wind-seeded, small plants were established, and before the diversity of long-lived, high quality, slow growing mature species became dominant.

The plantation species rarely develop as much gross production as a fully developed mature forest, but the higher net production of wood meets the needs of the human economy for wood for housing, paper, cooking, and many other uses. To keep plantations from being displaced by the wild species, requires management including replanting after cutting, weeding, and often fertilizing the soil. Plantation forestry is a form of agriculture, but one which has a longer cycle of 20 to 50 years to get a crop.

In general, as forest resources become scarce, sale prices rise, which causes more plantations to be economic. On the other hand, if prices of input fuels, goods, and services rise, more money is required to pay costs. Prices of inputs may increase faster than the price of the wood products being sold. The amount of purchased inputs, from the right in the diagram, which can be bought, becomes less, and the yield is less. If so, the work of the environment, on the left, becomes proportionately more, favoring the natural forests as source of wood. Although this model was calibrated for a forest plantation, similar models apply to fisheries.

"What If" Experimental Problems

  1. What happens to yield and money earned if the environmental inputs supporting the plantation increase (double J0)?

  2. What happens to the plantation economy if the price of the wood on the markets falls to a low value? Change P1 to a value 1/10th the initial value. What happens if the price of the wood increases to 1.5 times the initial value? Change PI to 0.024 and observe the effect on the income.

  3. If the initial condition of the soil is fertile (SW = 2), and here is not much fertilization, what happens to the yield? Reduce K8 to 0.001.

  4. For some purposes such as paper manufacture, trees do not have to be large, whereas larger trees are needed for telephone poles and construction. What happens in this model when the threshold for cutting (TW) is increased? Increase TW to 3000.


Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
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