Biomass increases attributed to both faster tree growth and altered allometric relationships under long‐term carbon dioxide enrichment at a temperate forest

Abstract Increases in atmospheric carbon dioxide (CO2) concentrations are expected to lead to increases in the rate of tree biomass accumulation, at least temporarily. On the one hand, trees may simply grow faster under higher CO2concentrations, preserving the allometric relations that prevailed under lower CO2concentrations. Alternatively, the allometric relations themselves may change. In this study, the effects of elevated CO2(eCO2) on tree biomass and allometric relations were jointly assessed. Over 100 trees, grown at Duke Forest, NC, USA, were harvested from eight plots. Half of the plots had been subjected to CO2enrichment from 1996 to 2010. Several subplots had also been subjected to nitrogen fertilization from 2005 to 2010. Allometric equations were developed to predict tree height, stem volume, and aboveground biomass components for loblolly pine (Pinus taedaL.), the dominant tree species, and broad‐leaved species. Using the same diameter‐based allometric equations for biomass, it was estimated that plots with eCO2contained 21% more aboveground biomass, consistent with previous studies. However, eCO2significantly affected allometry, and these changes had an additional effect on biomass. In particular,P. taedatrees at a given diameter were observed to be taller under eCO2than under ambient CO2due to changes in both the allometric scaling exponent and intercept. Accounting for allometric change increased the treatment effect of eCO2on aboveground biomass from a 21% to a 27% increase. No allometric changes for the nondominant broad‐leaved species were identified, nor were allometric changes associated with nitrogen fertilization. ForP. taeda, it is concluded that eCO2affects allometries, and that knowledge of allometry changes is necessary to accurately compute biomass under eCO2. Further observations are needed to determine whether this assessment holds for other taxa.