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

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 CO2 concentrations, preserving the allometric relations that prevailed under lower CO2 concentrations. 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 CO2 enrichment 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 taeda L.), the dominant tree species, and broad-leaved species. Using the same diameter-based allometric equations for biomass, it was estimated that plots with eCO2 contained 21% more aboveground biomass, consistent with previous studies. However, eCO2 significantly affected allometry, and these changes had an additional effect on biomass. In particular, P. taeda trees at a given diameter were observed to be taller under eCO2 than under ambient CO2 due to changes in both the allometric scaling exponent and intercept. Accounting for allometric change increased the treatment effect of eCO2 on 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. For P. taeda, it is concluded that eCO2 affects 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.