Long-term CO2 enrichment alters growths and allometric relationships of loblolly pine

Background/Question/Methods

Rising atmospheric CO₂ concentration is expected to enhance terrestrial plant biomass and ecosystem carbon storage. Plant biomass in changing CO₂ concentration has been estimated using allometric equations with diameter measurements in many climate modeling and field studies, but assessments concerning assumptions of non-significant effects of high CO₂ concentration on diameter-height relationships, parameters of allometric equations for plant volume, and specific gravities are limited. Here, we tested these assumptions on loblolly pine (Pinus taeda L.) and regenerated broadleaved species, which have been exposed to elevated CO₂ (ECO₂; +200 µmol mol^-1) for 15 years at the Duke Free-Air CO₂ Enrichment (FACE) experiment site.

We measured the diameter at breast height (DBH) and tree height (H) of trees with DBH > 2 cm in both ambient and elevated CO₂ conditions from 1996 to 2010. At the end of the FACE experiment, ~40% of all trees were harvested to estimate aboveground biomass, and subsets of P. taeda and regenerated broadleaved species were selected for detailed analysis of allometric relationships. Using the biometric measurements and harvested samples, we investigated the effects of ECO₂ on diameter-height relationships, parameters of allometric equations and specific gravities of stem.

Results/Conclusions

The effect of ECO₂ on diameter-height relationship was significant for P. taeda, but not significant for broadleaved species. The average H at a given DBH of P. taeda was ~5% greater in ECO₂. Temporal changes in DBH and H of individual P. taeda showed that growth trajectories differed by trees’ social status and CO₂ treatment. The effect of ECO₂ on the parameters of allometric equations for stem volume with DBH as a predictor variable was significant only for P. taeda. The CO₂ effect on the equation disappeared after accounting for the significant difference in diameter-height relationship between the CO₂ treatments. While vertical patterns of specific gravities for stem wood and stem bark of P. taeda and a common broadleaved species at the site (Liquidambar styraciflua) were unaffected by ECO₂, changes in tree height in ECO₂ allowed small differences in tree-level specific gravities of P. taeda stems. These results underline that ignoring the effect of ECO₂ on the allometric relationships may cause discernible bias in long-term estimation of plant biomass and ecosystem carbon cycle.