Net Primary Production of a Forest Ecosystem with Experimental CO2 Enrichment

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Science  14 May 1999:
Vol. 284, Issue 5417, pp. 1177-1179
DOI: 10.1126/science.284.5417.1177


The concentration of atmospheric carbon dioxide was increased by 200 microliters per liter in a forest plantation, where competition between organisms, resource limitations, and environmental stresses may modulate biotic responses. After 2 years the growth rate of the dominant pine trees increased by about 26 percent relative to trees under ambient conditions. Carbon dioxide enrichment also increased litterfall and fine-root increment. These changes increased the total net primary production by 25 percent. Such an increase in forest net primary production globally would fix about 50 percent of the anthropogenic carbon dioxide projected to be released into the atmosphere in the year 2050. The response of this young, rapidly growing forest to carbon dioxide may represent the upper limit for forest carbon sequestration.

Combustion of fossil fuels and deforestation, particularly in tropical regions, are rapidly increasing the concentration of CO2 in the atmosphere (1,2). Trees that use the C3 mechanism of photosynthesis are carbon-limited at the current atmospheric CO2concentration (3); therefore, the stimulation of photosynthesis by elevated CO2 may increase the capacity of forests to store carbon in wood and soil organic matter. Because of their imposing contribution to global productivity (2), forests have the potential to reduce the anthropogenic increase in atmospheric CO2.

Seedlings or saplings exposed to two times the current atmospheric concentration of CO2 in growth chambers, greenhouses, or open-top chambers have ∼54% greater photosynthesis and ∼31% greater biomass (4). These enhancements are considerably reduced when plants receive suboptimal amounts of other important resources such as nitrogen (5). Most studies of tree rings (6) show no increase in growth rate in response to the increase in atmospheric CO2 that has occurred from the pre-industrial concentration of ∼280 μl liter−1 to the current 360 μl liter−1 . Resource limitations in natural ecosystems and other ecological interactions including competition (7) may constrain the potential for forests to respond to increasing concentrations of CO2.

To examine the response of an intact forest ecosystem to projected elevated concentrations of CO2, we installed a gas-delivery system in a 13-year-old loblolly pine (Pinus taeda L.) plantation in the Piedmont region of North Carolina (35°97′N 79°09′W) (8). The free-air CO2 enrichment (FACE) system (9) increases the concentration of atmospheric CO2 in 30-m-diameter experimental plots nested within this continuous pine forest (Fig. 1). Each FACE ring (plot) consists of a large circular plenum that delivers air to an array of 32 vertical pipes. The pipes extend from the forest floor through the 14-m-tall forest canopy and contain adjustable ports at 50-cm intervals. These ports are tuned to control the atmospheric concentration of CO2 ([CO2]) through the entire volume of forest. In the three elevated CO2 plots, CO2was injected to maintain the atmosphere at ambient [CO2] plus 200 μl liter−1 (∼560 μl liter−1); three ambient CO2 plots were treated identically but without the addition of CO2 (10). Unlike closed growth chambers or open-top chambers, the FACE system controls atmospheric [CO2] without changing other variables. Moreover, its size permits the experimental manipulation of an entire forest ecosystem, including vegetation and soil components. The injection of CO2 was initiated on 27 August 1996.

Figure 1

Free-air CO2 enrichment (FACE) rings in a pine plantation in North Carolina, USA. Each ring is 30 m in diameter and circumscribes about 100 trees. The distance from the single ring in the southwest (top right) to the two rings in the north (bottom) is ∼500 m. The single ring in the background is a prototype. There are six experimental rings; three rings receive ambient air and three receive ambient plus 200 μl liter−1 CO2(photo: Will Owens).

At monthly intervals beginning in March 1996 we measured the diameter of 203 canopy pine trees distributed across the ambient and elevated plots (11). In 1997 and 1998 we made additional measurements of 112 subcanopy hardwood trees. Before the fumigation was initiated, the seasonal increase in basal area was similar for canopy trees in the ambient and elevated plots (Fig. 2). The basal area increment began to diverge soon after the fumigation started in August 1996, and by 1997 and 1998 the average basal areas for trees in the elevated plots were ∼2.6 and 4.5% larger, respectively, than those in the ambient plots.

Figure 2

Average basal area (±1 SE) for loblolly pine trees growing in ambient (N = 102) and elevated (N= 101) CO2. Values are expressed as the percentage of the initial basal area. The insert shows the absolute difference between the basal area of elevated and ambient trees, and the arrows indicate when the CO2 fumigation was initiated.

Diameters at the beginning and the end of each growing season were used to calculate the relative basal area increment of each tree [RBAI = (BADecember – BAJanuary)/BAJanuary, where BA is basal area] (12). A mild drought in 1997 and a severe drought during the summer of 1998 [modified Palmer drought index: −3 (13)] caused lower RBAI in the ambient plots relative to 1996 (Table 1). The addition of CO2 to the experimental plots in the late summer and fall of 1996 produced no statistical effect on RBAI between ambient and elevated plots (0.094 compared with 0.098 cm2 cm−2year−1) in that year. During the two full years of exposure, however, elevated CO2 caused significant increases in RBAI. The 26% stimulation in RBAI was similar to the growth stimulation observed for potted loblolly pine seedlings maintained at two times [CO2] but with suboptimal soil N and P, and considerably less than the maximum response observed for this species under optimal growth conditions (14, 15). For the subcanopy hardwood species, a stimulation in RBAI by elevated CO2 was statistically detectable only in Ulmus alata (for 1997: ambient = 0.055, elevated = 0.072 cm2 cm−2year−1, P = 0.07, N = 24; for 1998: ambient = 0.09, elevated = 0.118 cm2cm−2 year−1, P = 0.027).

Table 1

The mean (±1 SD) relative basal area increment (RBAI; cm2 cm−2 year−1) for loblolly pine trees growing in ambient and elevated atmospheric CO2plots. The average RBAI was calculated for 30 to 40 trees in each plot. The RBAI for ambient and elevated plots for each year was compared with a paired-sample t test (one-tailed, N = 3).

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Net primary production represents the flux of carbon into ecosystems. Some of this carbon is returned to the atmosphere by respiration from soil microbes and herbivores—the remaining carbon is stored as net ecosystem production. Net primary production (NPP; gross photosynthesis minus plant respiration) for the ambient and elevated CO2 plots was calculated as the summation of the annual increment in standing biomass of trees (including roots), saplings, shrubs, and vines, plus the turnover of foliage (litterfall) and fine roots (16). We calculated the biomass of the dominant pine trees from diameter using site-specific allometric equations (17), and equations from the literature were used to convert diameter to total biomass for the subcanopy hardwoods (18). Herbivory was not included in our estimates, but losses by herbivory from forest ecosystems typically are <10% (19).

Net primary production for the entire ecosystem in 1998 was 1662 g of dry matter per square meter per year in control plots and 2082 g m−2 year−1 in experimental plots (Table 2). Our estimates of biomass increment for the canopy pines (685 to 1087 g m−2year−1) are within the range reported for other loblolly pine forests (20). The annual biomass increment in canopy pines plus litterfall accounted for 78% of NPP (1998), followed by contributions from fine roots, subcanopy hardwoods, and saplings, shrubs, and vines. Elevated CO2 caused a consistent increase in NPP during the two full years of treatment (1997 and 1998). There was a trend of higher fine-root turnover and a significant increase in fine-root increment (86%) in the elevated-CO2plots in 1998. Higher fine-root turnover under CO2enrichment is consistent with higher rates of CO2 efflux from the soil in fumigated compared with ambient plots [1066 ± 46 g of C per square meter per year in 1997 and 928 ± 19 gC m−2 year−1 in 1998 in ambient plots; 1183 ± 8 gC m−2 year−1 in 1997 and 1175 ± 132 gC m−2 year−1 in 1998 in elevated plots; paired t test within each year:P = 0.04 for both years, N = 3 (21)]. Model simulations of terrestrial ecosystems predict an 8% increase in NPP for the contiguous United States (22) and a ∼9% increase for temperate coniferous forests with a doubling of CO2 (23). It was therefore striking to find 25% stimulation in NPP with only a 1.5-fold increase in CO2.

Table 2

Net primary production (production of dry matter; g m−2 year−1) for a pine ecosystem under ambient or elevated atmospheric CO2 during fumigation in 1997 and 1998. Subcanopy hardwoods are trees with a diameter ≥2.5 cm. The “sapling” category includes trees (<2 m tall), shrubs, and vines. Litterfall is the amount of dead biomass in foliage, branches, and reproductive structures falling to the ground annually. Net primary production (“Production”) is the sum of all components. For years where data were not available for one or more components, they were not included in the calculation of NPP (for example, fine roots in 1996 and 1997 and subcanopy hardwoods and sapling production in 1996). The “Percent CO2 effect” is the percentage difference between the elevated and ambient plots. Values for ambient and elevated plots were compared with a paired-sample t test (one-tailed, N = 3).

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It is unclear if the response of this young, fast-growing southeastern forest will be sustained over many years or if other vegetation types will respond similarly. In simulations with process-based models (24), the initial increases in forest NPP after a step doubling of CO2 declined dramatically with time as tree growth exceeded the rate of soil nitrogen mineralization. Similarly, individual trees exposed for long periods to elevated CO2(25) and forests near natural CO2 sources (26) show a rapid attenuation of the CO2growth response with age. Thus, the growth stimulation observed for this pine ecosystem under CO2 enrichment may represent the maximum response. If it applies to forests globally, the 25% increase in NPP that we observed suggests that enhanced uptake of CO2 by forests will not exceed 50% of the CO2emitted from fossil fuel combustion in the year 2050, when the atmospheric [CO2] is expected to reach 560 μl liter−1 (1, 27).

  • * To whom correspondence should be addressed. E-mail: delucia{at}


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