336 BOTKIN, JANAK, AND WALLIS 



and 1.64 : 1.0. The predicted effect of the fertilization is much less obvious than 

 one would have expected from a simple linear extrapolation. 



A naturalist hiking through the two forests would for much of the period 

 have a hard time distinguishing the treated from the normal on the basis of 

 appearance. Although this result may at first seem surprising, with a little 

 reflection it seems quite reasonable. There are three factors that account for it: 

 First, the model assumes that birth and death in a natural forest are noisy, and 

 this tends to obscure some seemingly drastic treatments. 



Second, the fertilization affects the importance of the different species in 

 the stand. Species characterized by the shade-tolerant strategy, such as spruce 

 [Fig. 3(a)] or sugar maple [Fig. 3(b)] , show greatly increased growth during the 

 first century; by year 100 the basal area of each is approximately twice that in 

 the control forest. 



The effect of the fertilization on the growth of white birch, which is 

 characterized by the shade-intolerant strategy, is considerably different 

 [Fig. 3(c)] . Although this species does slightly better in the first decade, its basal 

 area thereafter is not significantly different from its basal area in the control 

 forest, and after year 30 the mean value is lower in the treated than in the 

 untreated forest. 



The explanation for this is clear. Under the fertilization the shade-tolerant 

 species do so much better that they become stronger competitors against the 

 white birch. Spruce and sugar maple grow taller and develop more leaves faster 

 than normal, and their resultant ability to suppress white birch at least cancels 

 out the fertilization effect on the birch. 



The third effect is shown in the growth of balsam fir [Fig. 3(d)"] . This 

 species is shade tolerant, but has a lifetime that is short compared to spruce and 

 sugar maple. Although it does significantly better in the first 90 years, its 

 importance in the fertilized forest seems to be decreasing in the last 2 decades, 

 and its average basal area is not significantly different from the control in year 

 100. The fertilization appears to suppress the time scale for the balsam fir, so 

 that its peak basal area is reached sooner, but its final importance appears to be 

 unchanged. 



Similar experiments carried out with 10 and 20% treatments suggest that, in 

 general, the treated forests are not distinguishable from normal ones, either in 

 average total basal area or the average basal area of individual species. 



Long-term characteristics of the simulated forests are shown in Figs. 4 and 5 

 for a more extreme treatment, an increase of 100% in the annual diameter 

 growth of each tree. In this case the long-term average total basal area is 

 significantly different for the treated forest, but the difference is in a ratio of 

 approximately 1.5 to 1.0 for the means. The treated forest reaches a peak basal 

 area slightly earlier than the control, suggesting again that one effect of the 

 fertilization is to compress the time scale. 



The behavior of individual species is similar to that of the 50% increase 

 (Fig. 5). During the first century spruce and sugar maple do much better, white 



