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Long-Term Effect of Elevated CO2 and Disturbance on a Pasture
Taylor, K. and Potvin, C.  1997.  Understanding the long-term effect of CO2 enrichment on a pasture: the importance of disturbance.  Canadian Journal of Botany 75: 1621-1627.

What was done
Diverse pasture species were grown for approximately three months in open-top chambers of 375 or 625 ppm atmospheric CO2 near Montreal, Quebec.  Each chamber was subdivided into six quadrants, three of which had all plant biomass removed to simulate disturbance, and three of which remained undisturbed, to investigate the effects of CO2 enrichment and disturbance on pasture species.  In addition, two seedlings of the C3 plant, Chenopodium album, which is a major weed of several economically important crops, were added to each quadrant to determine if elevated CO2 would increase its colonization in disturbed areas as commonly occurs in prairies following livestock grazing.

What was learned
In general, no significant interaction between CO2 and disturbance was found.  Elevated CO2 did not influence the number of species nor vegetation density per quadrant.  Disturbance, however, doubled the number of species relative to undisturbed quadrants and accounted for greater proportions of observed variability for nearly every biological measurement than did CO2 concentration.  The only exception to this general rule was that CO2 concentration had a slightly greater effect on the final dry mass of vegetation than did disturbance. 

With respect to Chenopodium album, elevated CO2 did not affect any measured growth traits, while disturbance nearly doubled and tripled plant height and branch number, respectively, while it increased final dry biomass more than 4-fold.  The one significant interaction between CO2 and disturbance was that plant height and branch number were significantly lower when grown at elevated CO2 with disturbance.

What it means
As the CO2 content of the air continues to rise over prairie ecosystems, the effect of CO2 fertilization on species richness will likely be less than that of disturbance, which clearly increases the number of species present.  Elevated CO2 will probably not favor the growth and colonization of CO2-responsive C3 plants, such as Chenopodium album, at the expense of slower growing prairie species following disturbance.  This observation suggests that the increasing CO2 content of the atmosphere will not decrease biodiversity within prairie ecosystems that are managed by livestock grazing.  On the other hand, increasing CO2 levels should stimulate photosynthesis and increase overall biomass production, which should help to maintain, if not eventually lead to, long-term increases in biodiversity.

Reviewed 1 December 1998