For elevated carbon dioxide and no climate changes, study findings indicate that the 

 overall response of net primary productivity of potential temperate forests in the 

 United States would be an increase of 5.1 percent over the 75-year period. However, 

 with projected elevated carbon dioxide and the resulting equilibrium climate, study 

 findings indicate the mean response would be an increase of 16.2 percent, reflecting 

 a few of the many possible synergistic interactions among the climate, the atmo- 

 sphere, and the vegetation. The range in the predicted increase in productivity among 

 temperate mixed, coniferous, and deciduous forests is between 14.6 and 21.8 

 percent. Higher responses are predicted for the more northerly forests and negative 

 responses were indicated for some southern forests, which is consistent with greater 

 warming at higher latitudes and projected precipitation changes. 



These changes in net primary productivity were assumed to be analogous to changes 

 in forest growth. Modifications to growth by timber type and RPA region were made in 

 the Aggregate Timberland Assessment System. 



Basic Assumptions 

 About Future 

 Determinants of 

 Demand and Supply 



If carbon dioxide content in the air doubles by 2065, if climate change occurs as 

 assumed in the four general circulation models, if the productivity effect of those 

 changes occurs as described in the Terrestrial Ecosystem model, and if 

 transient ecosystem changes and competition are not overwhelming, timber 

 growth would generally increase for mature forests in existence in 2065. The 

 increase would vary by species and region. 



In the future, as in the past, demands and supplies of renewable resource products 

 will be determined largely by growth in population, income, and economic activity; 

 technological and institutional changes; energy costs; capital availability; and levels of 

 private and public investments in human capital, forest, range, and water manage- 

 ment, utilization, and research. 



Population 



In the past five decades, the population of the United States has nearly doubled— 

 from 132 million people in 1940 to some 252 million in 1990 (app. table 2). Projec- 

 tions by the Wharton Econometrics Associates, using Bureau of the Census assump- 

 tions about future population demographics, indicate that population will continue to 

 grow to about 333 million in 2040 (fig. 15). The population assumptions are the 

 middle series projections developed by the Bureau of the Census, except that net 

 immigration is assumed to be 750,000 people per year rather than the 450,000- 

 person assumption used in the middle series. This adjustment in the immigration 

 assumption is to account for net illegal immigration. 



As this report was being written, the Bureau of the Census announced an updated 

 population projection that substantially revised previous estimates. For 2040, the new 

 projection shows a total population of 364 million, about 10 percent higher than the 

 projection used in the 1989 RPA Assessment and this update. Of a total population 



22 



