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Effects of Elevated CO2 and Simulated Herbivory on a Tropical Tree
Lovelock, C.E., Posada, J. and Winter, K.  1999.  Effects of elevated CO2 and defoliation on compensatory growth and photosynthesis of seedlings in a tropical tree, Copaifera aromaticaBiotropica 31: 279-287.

What was done
Seedlings of the tropical tree Copaifera aromatica were grown for 50 days in pots placed within open-top chambers receiving atmospheric CO2 concentrations of 390 and 860 ppm.  Additionally, after 14 days of differential CO2 exposure, half of the seedlings in each treatment were subjected to mechanical defoliation, which removed about 40% of their leaf area.  Thus, the authors were able to study the influence of simulated herbivory on the CO2 growth response of this species.

What was learned
During the entire experiment, seedlings grown in elevated CO2 displayed rates of net photosynthesis that were between 50 and 100% greater than those exhibited by plants grown in ambient CO2, regardless of defoliation, which had little to no impact on photosynthesis in either CO2 treatment.  Mechanical defoliation did, however, temporally reduce seedling leaf area and leaf relative growth rates in both CO2 environments.  By the end of the experiment, leaf relative growth rates had recovered, and there were no differences between defoliated and undefoliated seedlings in either CO2 treatment.  In contrast, the leaf area of defoliated seedlings never recovered to match that of undefoliated controls in either CO2 treatment.  However, defoliated seedlings grown at ambient CO2 were able to ultimately attain leaf areas that were 77% of their undefoliated controls, while those exposed to elevated CO2 only attained leaf areas that were 67% of their respective controls.

What it doesn't mean
The authors state that "in plants growing under elevated CO2, the compensatory response failed to lead to the replacement of lost tissue within 5 1/2 weeks of the experiment;" and they therefore conclude that under elevated CO2 concentrations "plants that undergo herbivory may be at an even greater competitive disadvantage compared to their undefoliated neighbors than they are under current ambient CO2 concentrations."  This conclusion is misleading, however, for the authors neglect to acknowledge that the compensatory response also failed to replace the lost tissue in seedlings grown in ambient CO2.  And even more importantly, their data show that defoliated seedlings grown in elevated CO2 still possessed about 20% more leaf area than defoliated plants grown in ambient CO2.  Furthermore, final plant dry weights, which better represent the total impact of any stress upon a plant--and which indicate how well a plant is able to deal with a stress--were 15% greater in defoliated seedlings exposed to elevated CO2 than they were in defoliated seedlings growing in ambient CO2.  Thus, the authors' data do not support their conclusion on this important point.

What it does mean
As the CO2 content of the air continues to rise, it is likely that seedlings of the tropical plant Copaifera aromatica will exhibit substantial increases in their photosynthetic rates.  This phenomenon can lead to enhancements in many physical and physiological characteristics of plants that impact their abilities to grow and respond to various stimuli in their environments.  In this particular species, the data suggest that seedlings will be better prepared to cope with stresses induced by herbivory, and that they will likely maintain greater leaf areas and total biomass at higher CO2 levels than they would at lower CO2 concentrations under such conditions.  Thus, even with leaf destruction resulting from herbivory, Copaifera aromatica seedlings will likely demonstrate increased photosynthetic rates and greater biomass accumulation in the future, as the air's CO2 content continues to rise.

Reviewed 1 April 2000