Global environmental changes accompanying increases in atmospheric greenhouse gas concentrations includes many factors affecting plant growth. Some factors enhance plant growth and their ability to create new biomass and other factors are detrimental. Shaw et al. (2002) studied the effects of increased CO2, higher temperatures, increased precipitation, and elevated nitrogen deposition taken together and separately on net primary production (NPP) on a California annual grassland. They found that enhanced CO2 increased NPP but only if the interacting effects of other changes were not present. When several factors were allowed to change the effect of higher CO2 was to suppress root growth and to decrease some of the positive effects on NPP due to higher temperature, precipitation, and nitrogen deposition. These results suggest that assessing the full effects of all global changes will be more complicated than simply adding together of individual influences taken separately.
Reference
Shaw, M. R., E. S. Zavaleta, N. R. Chiariello, E. E. Cleland, H. A. Mooney, and C. B Field, 2002: Grassland responses to global environmental changes suppressed by elevated CO2. Science, 298, 1987-1990.