Defensive traits of Fagus crenata leaves after long-term CO2 exposure in free-air CO2 enrichment (FACE) system

We aim to elucidate the long-term effects of elevated CO2 concentrations on the defensive traits of forest tree leaves using Fagus crenata saplings grown for 7-9 years in the free-air CO2 enrichment (FACE) system from 2003 to 2011. During 2009-2011, we checked the leaf phenology from budburst to leaf expansion and seasonal changes in defensive traits of leaves such as leaf mass per area (LMA) and concentration of condensed tannin because phenological synchrony between the leaf maturing process and herbivorous insects' emergence and feeding timing is essential. As a result, there was little difference in leaf phenology and the emergence of herbivorous insects between FACE and control sites. LMA increased significantly in 2010 and 2011, and carbon/nitrogen ratio increased significantly in 2009 and 2011 in FACE sites than in control sites. There were significantly higher total phenolics and condensed tannin concentrations at FACE sites than at control sites in only 2009. On the other hand, microscopic observation revealed no differences in the distribution of phenolic substances between FACE and control sites. Our results after 9 years of CO2 exposure showed that the effect of CO2 on each defensive trait varied from year to year. This could be because the experimental CO2 concentration was not high enough to have an effect or because it was a field experiment and other environmental factors were more critical.