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Plant Growth and Carbon Sequestration in Terrestrial Ecosystems: Model Calculations Based on Empirical Observations
Lloyd, J. 1999.  The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II.  Temperate and boreal forest productivity and the combined effects of increasing CO2 concentrations and increased nitrogen deposition at a global scale.  Functional Ecology 13: 439-459.

What was done
The author's main purpose in this "Essay Review" was to provide some new estimates of the magnitude and spatial distribution of the global terrestrial carbon sink in response to historical increases in the air's CO2 concentration and atmospheric nitrogen deposition, both of which phenomena are the results of mankind's burning of fossil fuels.  The model used in this exercise is a major step forward, as it is not subject to certain constraints employed in previous models that have subsequently been shown by empirical investigation to be uncharacteristic of the real world.

What was learned
In reviewing the literature, the author concludes that (1) "it is now generally accepted that the terrestrial biosphere is a major sink for anthropogenically released CO2" and that (2) "the two most important mechanisms leading to long-term net CO2 sequestration by the terrestrial biosphere are increases in plant productivity owing to increasing CO2 concentrations and, in some regions, increased rates of atmospheric nitrogen deposition."  The author notes, in this regard, that although it has been widely assumed that plants growing under conditions of sub-optimal nitrogen nutrition have a reduced ability to respond to atmospheric CO2 enrichment, the evidence for this belief is weak, and that when all the available data are considered, (3) "low-nitrogen plants are observed to have higher growth stimulations (compared to well-fertilized plants) almost as often as they have lower growth enhancements."

The reasons given by the author for this latter fact are (4) "low-nitrogen plants tend to have higher respiratory costs relative to their rate of carbon assimilation and this increases their sensitivity to increased atmospheric CO2" and (5) "they tend to have lower leaf areas and thus, reinvestment of any CO2 stimulated growth into extra leaf area with commensurate increases in light interception tends to amplify their proportional growth responses."

Initializing the model employed to assumed equilibrium conditions in 1730, the author calculates, for a mature temperate deciduous forest, that the increase in net primary production experienced by this ecosystem from that time to the early 1980s, due solely to the historical increase in atmospheric CO2 concentration over this period, was approximately 7%.  The author also calculates that the increase in ecosystem net productivity due to a modest proportional increase in nitrogen deposition over the same time period was about 25%.  However, when CO2 and nitrogen increased together in the model, the net productivity stimulation was 40%, which is more than the sum of their individual contributions.  And all this for a CO2 increase of only 25%!

In terms of the extra carbon sequestered by the ecosystem, the CO2 increase alone adds 1.3 mol C m-2 year-1, while the nitrogen increase alone adds nearly 4 mol C m-2 year-1 and the CO2 and nitrogen increases together add about 7.5 mol C m-2 year-1, again demonstrating this same principle: (6) "effects of increased nitrogen deposition and atmospheric CO2 increase are not simply additive."

In consequence of a number of other calculations performed by the model, the author concludes that (7) "in the absence of appreciable nitrogen deposition tropical forests are likely to be the major sink for CO2 on a ground area basis" and that (8) "this is attributable to higher temperatures during CO2 assimilation leading to a greater sensitivity of photosynthesis to atmospheric CO2 concentrations."  When atmospheric CO2 increases together with nitrogen deposition, however, (9) "temperate forests are modeled to have values [of carbon sequestration rates] that are comparable with tropical forests."

Calculations for other types of ecosystems reveal that (10) "15% of the earth's surface accounts for over 80% of the modeled sequestration."  These "hot spots," naturally, correspond to areas characterized by high nitrogen deposition, mostly forests and grasslands in Europe, China and North America and areas dominated by tropical rain forest in Africa, Asia and South America.  For the earth as a whole, (11) "the total global sink modeled ... is 0.194 Pmol C year-1."  However, the author's analysis (12) "does not provide much support for the highly localized terrestrial CO2 uptake in North America as proposed by Fan et al. (1998)," which truly does seem inordinately large.

What it means
The many interesting findings of this insightful paper, a dozen of which we have highlighted above, dramatically illustrate the tremendous positive response of the terrestrial biosphere to the significant increases in atmospheric CO2 concentration and nitrogen deposition that have been the legacy of the Industrial Revolution.  They also point to the great magnitude of biospheric benefits likely to be accrued in the future, as the air's CO2 content rises still higher.

Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P.  1998.  A large terrestrial sink in North America implied by atmospheric and oceanic carbon dioxide data and models.  Science 282: 754-759.

Reviewed 15 February 2000