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Volume 3 Number 8:  15 April 2000

Editorial
Biodiversity and CO2: Scientific analyses suggest that the potential mass extinction of species constitutes "a problem with far more enduring impact than any other environmental problem."  We marshal evidence to further suggest that the rising CO2 content of earth's atmosphere may help to prevent this imminent disaster, which, if it were to occur, would take the earth literally millions of years to recover from.

Subject Index Summaries
Carbon Dioxide

Climate Oscillations

Journal Reviews
Ultra-Enhanced Spring Branch Growth in CO2-Enriched Trees: Experimental measurements reveal a dramatic ultra-enhancement in the early spring growth of new branches of sour orange trees exposed to an extra 300 ppm of CO2.

Soil Carbon Storage Below Woody Vegetation: Total soil carbon and particulate organic matter measured beneath woody plants at three western United States semiarid locations were found to be higher than beneath adjacent grasses, which suggests that more carbon will be sequestered in the soil as the air's CO2 content rises.

Yes, It Has Warmed Over the Past Five Centuries: Depth profiles of temperature obtained from over 600 boreholes scattered across the world reveal mean global air temperature to have risen approximately 1C over the past 500 years.

It's a Complex World: Part 1: A climate-model-driven attempt to explain the observed differences in the 1979 to 1998 trends in surface and lower tropospheric temperature trends ends in failure.

It's a Complex World: Part 2: Our most sophisticated climate models fail to reproduce real-world temperature characteristics of the tropical troposphere over the past two decades; and the researchers who made the attempt suggest that we probably never will be able to do so.

CO2 Exchange in Arctic Tundra Ecosystems: After eight years of observations, it was determined that carbon fluxes in arctic tundra ecosystems are much more sensitive to soil nutrient status than they are to rising air temperatures.  Nutrient fertilization, for example, increased ecosystem productivity nearly three-fold, while a 6C increase in temperature had little impact on plant photosynthesis and ecosystem respiration.  Therefore, as the CO2 content of the air rises, its aerial fertilization effect will likely exert a positive influence on arctic tundra ecosystems, increasing their carbon sequestering capabilities, even in the face of rising air temperatures.

Photosynthetic Responses of Four Hardwood Trees to Elevated CO2: After being exposed to an atmospheric CO2 concentration of 560 ppm for one year in a FACE experiment, four species of understory hardwood trees exhibited rates of net photosynthesis that were 50 to 160% greater than those of their respective controls exposed to ambient CO2 concentrations.  Thus, it is likely that saplings growing beneath dense forest canopies will exhibit significant increases in photosynthesis and growth as the CO2 content of the air continues to rise.

Fine Root Responses of Aspen to Elevated CO2 and Soil Nitrogen: Aspen cuttings grown for 2.5 years in open-top chambers receiving atmospheric CO2 concentrations of 700 ppm exhibited substantially thicker and longer roots than cuttings exposed to ambient air, regardless of soil nitrogen availability.  Fine root production, turnover, and biomass accumulation, however, were only significantly stimulated by atmospheric CO2 enrichment when soil nitrogen was not limiting to growth.  The increased carbon input to soils that resulted from enhanced root growth and turnover, particularly when nitrogen supply was adequate, can result in a substantial carbon sink increase in aspen stands that could be exploited to increase the carbon sequestering power of managed tree farms.

Biomass Responses of Aspen to Elevated CO2 and Soil Nitrogen: Aspen cuttings grown for 2.5 years in open-top chambers receiving atmospheric CO2 concentrations of 700 ppm exhibited substantially more biomass than those exposed to ambient CO2 concentrations, in a soil-nitrogen dependent manner.  When grown with high soil nitrogen availability, the CO2-induced increase in total biomass was more than twice as large as that observed in trees subjected to low soil nitrogen supply (38 vs. 16%, respectively).  In addition, when grown with high soil nitrogen availability, CO2-enriched trees accumulated more total nitrogen than their ambiently-grown controls.  There were no significant CO2 x genotype interactions reported for the six aspen clones studied, indicating that atmospheric CO2 enrichment affected all of them in a similar manner.  Thus, aspen diversity will likely be maintained in future atmospheres containing greater CO2 concentrations.

Effects of Elevated CO2 and Soil Nitrogen on Soil Microbial Communities: Although aspen cuttings grown for 2.5 years in open-top chambers receiving atmospheric CO2 concentrations of 700 ppm exhibited substantially greater fine root biomass and turnover rates than those exposed to ambient CO2 concentrations, the additional soil carbon inputs resulting from these processes had no significant effects on soil microbial composition, biomass, and functioning.  Thus, it is likely that the rising CO2 content of the air will maintain microbial diversity in soils beneath regenerating aspen stands.