The CO2dependence of photosynthesis, plant growth responses to elevated CO2concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and the combined effects of increasing CO2concentrations and increased nitrogen deposition at a global scale

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Abstract
1.Appropriate rates of carbon acquisition by temperate and boreal forests are re‐evaluated. Based on continental‐scale forestry data it is suggested that the productivity of temperate and boreal forests has been overestimated previously.2.Using these values, a model of the integrated response of ecosystems to carbon dioxide concentration and soil nitrogen availability is presented. The model does not assume constant C/N ratios in plant or soil and considers effects of increases in atmospheric CO2concentrations and nitrogen deposition separately or together.3.For temperate‐zone forests a co‐occurrence of a CO2increase and nitrogen deposition doubles the increase in net primary productivity and carbon sequestration that would be the case for nitrogen deposition occurring on its own. Considered separately, the effect of the atmospheric CO2increase is less than even moderate rates of anthropogenic N deposition for temperate or boreal forests. By contrast, for tropical forests, the atmospheric CO2increase is sufficient to induce large rates of carbon accumulation in plants and soil.4.Application of the model at the global scale suggests large localized sinks for CO2in either tropical rain forests or in forested or grassland areas of Europe and North America where appreciable N deposition occurs. Overall, the model suggests a terrestrial sink owing to CO2fertilization and N deposition of about 0·2 Pmol C per year. About half of this is in the mid‐latitudes of the northern hemisphere and about half in the tropics.