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Effects of Elevated Atmospheric CO2 and Temperature on Silver Birch Seedlings with and without Simulated Herbivory
Huttunen, L., Niemela, P., Peltola, H., Heiska, S., Rousi, M. and Kellomaki, S. 2007. Is a defoliated silver birch seedling able to overcompensate the growth under changing climate? Environmental and Experimental Botany 60: 227-238.

What was done
The authors grew silver birch (Betula pendula Roth) seedlings in pots filled with peat at three different levels of nitrogen (N) fertility - no N, moderate N (130 kg N ha-1), high N (270 kg N ha-1) - within climate-controlled closed-top chambers from mid-June to October of 2002 at the Mekrijarvi Research Station of the University of Joensuu, Finland, where the chambers were maintained at atmospheric CO2 concentrations of either 360 or 720 ppm and at either ambient air temperatures or elevated air temperatures that were 2C above ambient from June to August and 4C above ambient for the remainder of the growing season, while mimicing larval and adult leaf-feeding patterns exhibited during real-world defoliation by manually damaging the leaves of the seedlings by tearing off the apical halves of either 25% or 50% of all leaves greater than 1 cm in length on 1 July (mid-season) and again on 29 July (late-season), after which total plant shoot and root biomass was determined once the plants had gone dormant in October.

What was learned
As best we can determine from the authors' bar graphs of their results, and averaged over all three defoliation treatments, the elevated CO2 treatment increased the biomass of the seedlings in the moderate and high N fertility treatments much more than it increased the biomass of the seedlings in the no N fertility treatment (29 and 30%, respectively, vs. 13%). The same was also true of the combined elevated CO2 and elevated temperature treatment, where the corresponding treatment-induced biomass increases were 34 and 36% vs. 20%, which demonstrates that the heightened temperatures tended to augment the beneficial effects of the elevated CO2 treatment, with the greatest amplification being manifest in the no N fertility treatment (54% vs. 17 and 20% in the moderate and high N treatments, respectively). Finally, averaged over all three N fertility treatments, the effect of the elevated CO2 was to increase the plant biomass of the undefoliated seedlings by approximately 25%, that of the 25%-defoliated seedlings by 24%, and that of the 50%-defoliated seedlings by 22%, while the effect of the combined elevated CO2 and elevated temperature treatment was to increase the plant biomass of the same three categories of seedlings by approximately 31%, 30% and 29%, respectively.

What it means
In the words of the six Finnish scientists who conducted the study, "climatic change" - which they specifically define to mean elevated atmospheric temperature and CO2 - "will have a positive impact on the compensatory ability of defoliated silver birch seedlings." In fact, it may actually help them overcompensate for the effects of herbivory.

Reviewed 19 September 2007