Elevated CO2 Leads to Long-Term Forest Productivity and Increases in Carbon Flux

With rising levels of atmospheric CO₂ due to climate change, the earth is becoming more dependent on changes in ambient air composition and its effect on plant growth and productivity. Recent studies have shown significant variety in ecosystem responses to enriched CO₂ environments and only some studies demonstrated increased rates of NPP. In the following experiment, Drake et al. (2011) examine the physiological responses of loblolly pine (Pinus tadea) trees to increased CO₂ and determine the effects on nutrient availability and uptake. The experiment was performed on the grounds of Duke’s FACE program and the authors collected data on a variety of physiological plant factors in ambient and enriched CO₂ to determine if the information from this testing site can be applied to other ecosystems. It was predicted that increases in carbon flux, nitrogen-uptake and overall plant productivity would lead to long-term forest enrichment. —Taylor Jones

Drake, J. E., Gallet-Budynek, A., Hofmockel, K.S., Bernhardt, E. S., Billings, S. A., Jackson, R. B., Johnsen., K. S., Lichter, J., McCarthy, H. R., McCormack, M. L., Moore, D. J. P., Oren, R., Palmroth, S., Phillips, R. P., Pippen, J. S., Pritchard, S. G., Treseder, K. K., Schlesinger, W. H., DeLucia, E. H., Finzi, A. C., 2011. Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO₂. Ecology Letters 14, 348–357.

          John E. Drake and colleagues started the experiment with 3-year-old loblolly trees planted 2.4 by 2.4 meters apart. In 1994, they began collecting data on the paired reference plots and in 1996, began collecting data in all additional plots. Some data was taken from previous research at the FACE testing site and some data was new for this experiment. The standing pool of fine root biomass was measured every three months by collecting a 4.65 cm block of soil 15 cm deep and analyzing it for C content. The amount of C stored as CO₂ in the soil air space was calculated and the microbial biomass of N was measured.

          The rate of CO₂ diffusion out of the soil (soil CO₂ efflux) was measured with the closed IKGA system and the data were plotted as a temperature response curve. The fine root production was measured monthly and the production of various fungi was measured using microscopic methods. The on-site litterfall was collected monthly January–September and biweekly October–December in 12 baskets per site, measuring 0.218m² each. The authors also collected data on C-cycling, NPP and total belowground carbon flux (TBCF).

          The results showed that as CO₂ increased, the rate of C-cycling through the soil increased by 17%. Also, the TBCF increased 16% and the increased C entering the soil in an enriched CO₂  environment led to increases in the net biomass. N also increased, supporting an increase in NPP, supplied by a 25% increase in soil uptake. TBCF and N-uptake demonstrated an inverse relationship. NPP positively correlated with canopy N content and supported an increase in photosynthetic N-use efficiency.

          Drake et al. suggest that the increase in productivity is due to an exchange of tree C with soil N belowground, allowing the N levels to meet the growth requirements of the plant. Also, the long-term NPP increases are likely enabled by increases in TBCF that stimulate N-uptake and  canopy leaf area. Despite increased productivity, the experiment did not result in a net accumulation of  C in the mineral soil pool. The authors suggest this could be a result of the fixed C being added to the experiment replacing some of the C initially present in the soil and increases in microbial activity could account for changes in the soil composition. This effect on C pools is likely to model the response of soil to a long-term rise in CO₂. The authors recognize that this study provides an initial attempt to examine the physiological effects of increases CO₂, but is by no means comprehensive and further experiments representing a diversity of effects are necessary to better understand the long-term effects of increased CO₂. A simple experimental framework describing the most important processes that effect N availability and C uptake is necessary to understand the effects of CO₂ enrichment in the future.