332 BOTKIN, JAIMAK, AND WALLIS 



THE EXPERIMENTS 



Although CO2 enrichment of photosynthesis has been known for a long 

 time, at this writing no quantitative descriptions exist concerning the effect of 

 an increase in CO2 concentration on the growth of any of the tree species found 

 in the Hubbard Brook forest. This forest includes 13 tree species and is typical 

 of forests found above 500 m in northern New England. This forest and its 

 general type have been described elsewhere, and only those features pertinent to 

 our subject are presented here (Bormann et al., 1970; Braun, 1950). Suppose, 

 lacking information about effects of C0 2 , we make the simple assumption that 

 an increase in CO2 concentration would increase the annual growth increment of 

 each tree in a forest by an equal percentage. One can then ask how great an 

 increase would be necessary to produce a significant change in the forest 

 Vegetation, and what kind of changes would occur. 



At first glance these questions seem simple and the answers obvious: an 

 increase of k percent in the growth of each tree each year would increase the 

 growth of the entire forest k percent. Second, at any time the standing crop of 

 the fertilized forest would be k percent greater than the normal forest. A further 

 deduction seems equally obvious and has been made public by leading scientists 

 quite recently: any increase in the atmospheric concentration of CO2 will result 

 in an increased growth of vegetation, an equal fractional increase in storage of 

 carbon in the biota, and therefore vegetation will serve to buffer the atmosphere 

 against such changes in C0 2 concentration. 



To some the answers may seem so simple and obvious as to demand no 

 further consideration. We have found to the contrary that applying this simple 

 initial treatment to our simulation leads to different but entirely reasonable 

 conclusions. These suggest that to accept the previously mentioned simplistic 

 conclusions at face value is to ignore the complex interactions that occur among 

 species in a natural ecosystem. 



Our simulated experiments are merely to compute the normal growth of 

 each tree each year, on the basis of the state of the forest in that year, and then 

 to increase its growth by a constant percentage. Thus AD, the annual diameter 

 increment of any tree, is 



AD = AD' X k (1) 



where AD' is the diameter increment computed normally, and k is the assumed 

 percent increase due to the fertilization effect of carbon dioxide. 



Suppose this treatment were carried out on a forest that had just undergone 

 clear-cutting and was beginning secondary succession. How would its develop- 

 ment compare to a similar just cutover forest under "normal" conditions? For 

 our experiments we chose an elevation of 762 m in northern New Hampshire. 

 This is a transition zone between conifer forests, typical of higher elevations and 

 more northerly latitudes, and hardwood forests, typical of lower elevations and 



