Grassland Root Biomass Response to Atmospheric CO₂ Enrichment

Anderson et al. studied various root responses of a C₃-C₄ grassland community at Temple, Texas (USA) over a CO₂ concentration gradient stretching from 230 to 550 ppm, which they created in two CO₂-gradient above-ground "tunnels" of clear polyethylene film. One of the 60-m-long and 1.5-m-wide chambers had ambient air pumped into one end of it; and by the time that air exited the chamber through its other end, its CO₂ concentration was reduced by the photosynthetic activity of the plants within the chamber to a value of approximately 230 ppm. At the same time, the other chamber had air enriched to a CO₂ concentration of 550 ppm pumped into one end of it; and as this air exited out the other end of that chamber, its CO₂ concentration was reduced to a value approximately equivalent to that of the ambient air (~380 ppm). Concurrently, community ingrowth root biomass was assessed along the lengths of the tunnels every two to four months from May 1997 through November 1999, via the help of two ingrowth cores in each 5-meter chamber section; and root biomass response was calculated as the ratio of each measurement date's result to that prevailing at the start of the experiment in May 1997.

Upon the conclusion of their study, Anderson et al. report that based on the linear relationship they derived from all twenty of the ingrowth biomass assessments they conducted, there was "a 40% increase in the ingrowth root biomass ratio from 380 to 480 ppm as compared with a 36% increase from 280 to 380 ppm," but they say that excluding one extremely variable data point, and using a power function they fit to the data, "the contrast is even greater: a 50% increase from 380 to 480 ppm vs. a 41% increase from 280 to 380 ppm." And we additionally note, in this regard, that in going from the linear relationship to the power function, the r² value of the relationship jumped from 0.10 to 0.50, and that P dropped from 0.095 to less than 0.001.

Thus, the six scientists say "these data suggest that root biomass in grasslands may have changed markedly as atmospheric CO₂ increased since the last glacial period, but that more substantial changes are ahead if the air's CO₂ content doubles by the end of this century as predicted." And, these anticipated "changes" should all be positive, implying ever greater grassland root biomass -- and all that that phenomenon implies -- as the air's CO₂ content continues to climb ever higher.