CARBON FERTILIZATION EFFECTS IN A SIMULATED ECOSYSTEM 341 



birch no better than normal, and balsam fir reaches a high peak in the treated 

 forest from which it declines. It is of interest that, as the forest approaches a 

 steady state, the roles of the individual species in the treated and untreated 

 forests become more difficult to distinguish, and eventually the importance of 

 balsam fir is indistinguishable. 



DISCUSSION 



In the simulated forest the stochastic properties of birth and death tend to 

 mask the effects of fertilization on the structure and productivity of the forest, 

 while interactions among species buffer the ecosystem. If a real forest were less 

 noisy, the effects of the treatment would be more readily distinguishable, but 

 our normal forest is at least as noisy as the observed Hubbard Brook plots. 



As can be seen in Fig. 4, by year 200 both treated and untreated forests 

 reach a total basal area statistically equivalent to their respective year 500 values. 

 Once this stable value is reached, there is no net change in the stored carbon, and 

 the treated forest would cease to cause any further net change in the 

 atmospheric C0 2 concentration. It is important to recognize that in our 

 experiments the only change made was to increase annual diameter growth of 

 each tree by a constant percentage; in particular, maximum allowable tree 

 diameters, heights, and ages, as well as chance of death and sapling entry, have 

 not been changed. It is possible that fertilization might change what we have 

 considered to be the fundamental species constants given in Table 1, and it is 

 even conceivable that, if the appropriate constants were properly scaled, the sole 

 effect of C0 2 fertilization might be to compress the time scale and in no way 

 change the final state of the forest. 



A carefully managed monoculture, where unhealthy individuals were 

 eliminated early in life and healthy individuals protected, could have quite 

 different properties than a natural ecosystem of many species such as that 

 modeled here. A monoculture might be managed so that the population growth 

 resembled that of the simulated spruce during the first century following 

 clear-cutting. Our simulation points out the importance of species interactions 

 and suggests that the behavior of a natural mixed-species ecosystem is different 

 in kind from the behavior of single species in monoculture. 



In our discussion we have used basal area exclusively as an index of the 

 forest's status. However, we made similar calculations for stem density, leaf 

 weight, and total biomass. In general, stem density tends to decrease with 

 increases in basal area; leaf weight and total biomass tend to increase in 

 approximately the same proportion as basal area. 



CONCLUSIONS 



The simulated experiments suggest the following answers to the questions 

 posed at the beginning of this paper. Unless the effect of C0 2 increase in the 

 atmosphere is sufficient to fertilize the annual growth of each tree approxi- 



