Writing as background for their study, Ren et al. (2011) state that "in recent decades, there has been increased concern that elevated tropospheric ozone (O3) and climate change have [negatively] influenced the ability of China's ecosystems to provide people with essential goods and services." Consequently, in an effort designed to explore this concern, they investigated "the potential effects of elevated O3 along with climate change/variability on NPP [net primary production] and NCE [net carbon exchange] in China's forest ecosystems for the period 1961-2005 using a process-based dynamic land ecosystem model (DLEM, Tian et al., 2005, 2010a,b)," while additionally considering "other environmental factors such as land-cover/land-use change (LCLUC), increasing [atmospheric] CO2 and nitrogen deposition."

In describing their findings, Ren et al. report that O3 pollution had consistent negative effects on forest production, reducing total NPP by 0.2 to 1.6% from the 1960s to 2000-2005, such that without O3 pollution, carbon uptake rates would have increased by 3.5% in the 1960s and 12.6% in the 6 years 2000-2005. Climate change, on the other hand, had both negative and positive effects on NPP and NCE; and it was thus the major factor controlling the inter-annual variability of these two productivity parameters.

LCLUC also had negative impacts on NPP and NCE; but Ren et al. found that "nitrogen deposition alone could compensate for the combined negative effects of O3 and LCLUC in China." They also report that an increase in NPP occurred in the CO2-N combination simulation, which they say "was consistent with previous studies (e.g., Ollinger et al., 2002; Felzer et al., 2004; Hanson et al., 2005)." And they found that CO2 and nitrogen deposition working together "could offset the combined negative effects of O3 pollution, climate change and LCLUC on annual NCE." Therefore, it would appear that the combination of atmospheric CO2 enrichment and nitrogen deposition - both of which are by-products of the Industrial Revolution - provide powerful antidotes for the negative effects of ozone pollution, land-cover/land-use change and various deleterious climatic phenomena with regard to their impacts on NPP and NCE in China and, by inference, other parts of the world as well.

In another study, Su et al. (2004) used an ecosystem process model to explore the sensitivity of the net primary productivity (NPP) of an oak forest near Beijing (China) to the global climate changes projected to result from a doubling of the atmosphere's CO2 concentration from 355 to 710 ppm. The results of this work suggested that the aerial fertilization effect of the specified increase in the air's CO2 content would raise the forest's NPP by 14.0%, that a concomitant temperature increase of 2°C would boost the NPP increase to 15.7%, and that adding a 20% increase in precipitation would push the NPP increase all the way to 25.7%. Last of all, they calculated that a 20% increase in precipitation and a 4°C increase in temperature would also boost the forest's NPP by 25.7%.

In another model study, Su et al. (2007) used a process-based model (BIOME-BGC) "to investigate the response of Picea schrenkiana forest to future climate changes and atmospheric carbon dioxide concentration increases in the Tianshan Mountains of northwestern China," which they "validated by comparing simulated net primary productivity (NPP) under current climatic conditions with independent field-measured data." The specific climate change scenario employed in this endeavor was a double-CO2-induced temperature increase of 2.6°C and a precipitation increase of 25%. So what did they find?

When the predicted precipitation increase was considered by itself, the NPP of the P. schrenkiana forest increased by 14.5%; while the predicted temperature increase by itself increased forest NPP by 6.4%, and the CO2 increase by itself boosted NPP by only 2.7%. When the predicted increases in precipitation and temperature occurred together, forest NPP increased by a larger 18.6%, which is just slightly less than the sum of the two individual effects; but when the CO2 concentration increase was added to the mix and all three factors increased together, the Chinese researchers report that forest NPP "increased dramatically [italics added], with an average increase of about 30.4%."

Su et al. conclude that comparison of the results derived from the various scenarios of their study indicates that "the effects of precipitation and temperature change were simply additive, but that the synergy between the effects of climate change and doubled CO2 was important," as it made the whole response much larger than the sum of its separate responses, due to the fact that "feedback loops associated with the water and nitrogen cycles [which may be influenced significantly by atmospheric CO2 enrichment] ultimately influence the carbon assimilation response."

References
Felzer, B.S., Kicklighter, D.W., Melillo, J.M., Wang, C., Zhuang, Q. and Prinn, R. 2004. Effects of ozone on net primary production and carbon sequestration in the conterminous United States using a biogeochemistry model. Tellus 56B: 230-248.

Hanson, P.J., Wullschleger, S.D., Norby, R.J., Tschaplinski, T.J. and Gunderson, C.A. 2005. Importance of changing CO2, temperature, precipitation, and ozone on carbon and water cycles of an upland-oak forest: incorporating experimental results into model simulations. Global Change Biology 11: 1402-1423.

Ollinger, S.V., Aber, S.D., Reich, P.B. and Freuder, R.J. 2002. Interactive effects of nitrogen deposition, tropospheric ozone, elevated CO2 and land use history on the carbon dynamics of northern hardwood forests. Global Change Biology 8: 545-562.

Ren, W., Tian, H., Tao, B., Chappelka, A., Sun, G., Lu, C., Liu, M., Chen, G. and Xu, X. 2011. Impacts of topospheric ozone and climate change on net primary productivity and net carbon exchange of China's forest ecosystems. Global Ecology and Biogeography 20: 391-406.

Su, H.-X. and Sang, W.-G. 2004. Simulations and analysis of net primary productivity in Quercus liaotungensis forest of Donglingshan Mountain Range in response to different climate change scenarios. Acta Botanica Sinica 46: 1281-1291.

Su, H.-X., Sang, W.-G., Wang, Y. and Ma, K. 2007. Simulating Picea schrenkiana forest productivity under climatic changes and atmospheric CO2 increase in Tianshan Mountains, Xinjiang Autonomous Region, China. Forest Ecology and Management 246: 273-284.

Tian, H., Chen, G.S., Liu, M.L., Zhang, C., Sun, G., Lu, C.Q., Xu, X.F., Ren, W., Pan, S.F. and Chappelka, A. 2010a. Model estimates of net primary productivity, evapotranspiration, and water use efficiency in the terrestrial ecosystems of the southern United States during 1895-2007. Forest Ecology and Management 259: 1311-1327.

Tian, H.Q., Liu, M.L., Zhang, C., Ren, W., Chen, G.S., Xu, X.F. and Lu, C.Q. 2005. DLEM -- the Dynamic Land Ecosystem Model, user manual. Ecosystem Science and Regional Analysis Laboratory, Auburn University, Auburn, Alabama, USA.

Tian, H., Xu, S., Liu, M., Ren, W., Zhang, C., Chen, G. and Lu, C.Q. 2010b. Spatial and temporal patterns of CH4 and N2O fluxes in terrestrial ecosystems of North America during 1979-2008: application of a global biogeochemistry model. Biogeochemistry 7: 1-22.