Combustion of fossil fuels and deforestation. particularly in tropical regions, are rapidly increasing the concentration of CO₂ in the atmosphere (Bazzaz, 1990). Because of their imposing contribution to global productivity (Schlesinger, 1997), forest have the potential to reduce the anthropogenic increase in atmosphere CO₂.

Elevated CO₂ may influence the growth of forest trees. Greenhouse and growth chamber studies have shown that elevated CO₂ increase plant biomass compared with plant grown at lower CO₂ concentration (Curtis, 1996). Seedlings or saplings exposed to two times the current atmospheric concentration of CO₂ in growth chamber, greenhouse, or open-top chambers have about 31% greater biomass (Curtis and Wang, 1998). Most studies of trees, however, have been performed using seedlings or small saplings with non-limiting amounts of water and nutrients. Whether the initial stimulation in growth observed with elevated CO₂ will be sustained for trees growing in a forest ecosystem under natural conditions. It is difficult to extrapolate these studies to predict the response of mature trees and whole forest ecosystem. Lack of long-term forest level of CO₂ study limits my prediction of ecosystem response. Furthermore, nutrient limitation also complicates the problems. FACE-prototype ring is the longest site of elevated CO₂ study in the world which study both elevated CO₂ and nutrients in a forest ecosystems.

From chamber study and greenhouse study, growth enhancement of CO₂ is considerably reduced when plants receive sub-optimal amounts of other important resources such as nitrogen (Norby et al., 1992). The interact of CO₂ and nutrients has important implications for our ability to understand and model forest responses across the regional landscape, and predict how forest management should be conducted in the next 100 years to maintain yield with rising atmospheric CO₂. In this study (FACE-prototype), it tries to find whether resource limitation in natural ecosystems may constrain the potential for forest to respond to increasing concentration of CO₂.

From the FACE-prototype study, after initial enhancement of growth, the effect of CO₂ enhancement becomes diminish after several years. For the first three-years after CO₂ enrichment, there are about 20% growth enhancement. For the last three years, there is less than 10% growth enhancement (Figure 1.)

Results from the FACE-P should be interpreted with caution because we do not have replication during past. However, long term growth rate from FACE-prototype should indicate the growth trends. That results from the first year comparison of growth in the FACE FACTS-1 experiment show a similar enhancement of 20% of tree stem basal area growth (DeLucia et al., 1999) as in the FACE-P study is reassuring, and lends further support to the approach using the comparison of the FACE-P plot to its own reference plot. This comparasion suggests that our FACE-prototype is belongs to the elevated population and Reference site belong to control treatment of FACT-1 site.

A gas-delivery system was installed in a 10 years old loblolly pine (Pinus taeda L.) plantation in Duke forest, North Carolina in 1993. The free-air CO₂ enrichment (FACE) system increase the concentration of atmospheric CO₂ in a 30-m diameter plot to 550 ppm in active growing season.

The FACE ring 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 forest canopy and contain adjustable ports at 50-cm intervals. These ports are turned to control the atmosphere concentration of CO₂ through the entire volume of forest. Unlike closed growth chambers or open-top chambers, the FACE system control atmospheric CO₂ concentration without changing other variables.

The FACE-P plot consists of 89 dominant and co-dominant loblolly pine trees that have been exposed to elevated CO₂during the growing season (April-October) for the last seven consecutive years. Hence the entire foliage and likely much of the fine root structure was developed under 550 ppm elevated CO₂. FACE-prototype is the longest FACE of forest trees in the world.

Set up new sites and study.

FACE-P was divided into four pie-sections by opening narrow (30 cm) trench in the soil to 1 m depth (three-to-four times deeper than the depth of the fine roots in this stand) in May 1998, inserting an impermeable polyethylene sheets, and back-filling the trenches. In order to minimize disturbance, the soil dug out while trenching was stored along the trench on polyethylene sheets. The same procedure was followed in the reference plot, except that it was divided into two sections. Additionally, five new locations nearby were selected, in each of which a pair of 10 m X 10 m plots, with 20 m minimum buffer between plots was established (Fig. 2).

Pairing plots based on initial biomass, two pie-sections in the FACE and one in the Ref. plots were fertilized (including N, P, K), aiming to achieve the target concentration in the foliage (1.3% N, other nutrients in proportion to this). In addition, one plot of each newly established plot-pair was chosen similarly and fertilized. The plots were fertilized in July 1998 and March 1999 respectively.

Stainless steel dendrometer bands were installed on stems1.4 m above the soil surface on all trees in each plot. Diameter of all trees was measured at monthly interval in the beginning of growing season. In the late growing season, the measurement was done in weekly intervals.

The biomass of pine trees was calculated using site-specific allometric equations (Naidu et al, 1998).

The analysis relies on establishing a stand-wide baseline for fertilized and unfertilized behavior. This was done using six plot-pairs under ambient conditions. The null hypothesis is that there is no difference between the mean of the two unfertilized sections in FACE-P, and the population of unfertilized plots (which we expect not to be able to reject), and between the mean of the two fertilized sections in FACE-P and the population of fertilized plots (which we expect to reject in favor of a greater response in the FACE-P). A refinement on this approach is to test the hypothesis that the difference between fertilized and unfertilized sections in the FACE-P is not significantly greater than the population of differences for the paired plots (N=6, 5 new plot pairs and the original Ref. plot).

During the beginning of the growing season, the growth rate of different treatment is not much different (Figure 2). At the middle of growing season, treatments have effect on the growth rate. At the end of the growing season, treatment effect becomes very strong.

Fertilized plots of FACE-prototype show greater enhancement than ambient plots (Figure 3). The fertilized plots of prototype produced 290 g m^-2 y^-2 more than that of unfertilized portion. Ambient-Fertilized plots produced 175 g m^-2 y^-1 more than ambient-unfertilized plots. The enhancement of fertilization is much greater in elevated CO₂ plot than ambient plots (P=0.01). At the beginning of the growing season, fertilization effect of prototype is not significantly different from ambient plots. However, during late growing season, the net growth effect of fertilization in prototype is much greater than that of ambient plots. From this study, nutrients do limit the capacity of pine response to elevated CO₂ after long time of CO₂ exposure.

From the FACE-prototype study, the results show that elevated CO₂ enhance the growth more than 20% during the first 3 years. After the initial enhancement, the effect of CO₂ becomes very small. Other studies also have similar results. Individual trees exposed for long periods to elevated CO₂ (Idso, 1999) and forest near natural CO₂ sources (Hattenschwiler et al., 1997) show a rapid attenuation of CO₂ growth with age.

Most studies of tree rings (Graumlich, 1991; Luxmore and Wullschleger, 1993) show no increase in growth rate in response to the increase in atmospheric CO₂ that has occurred from the pre-industrial concentration of ~280 ppm to the current 360 ppm. It seems support that nutrients may play an important role in limiting the ecosystem response to CO₂.

After fertilization, CO₂ enhance the growth tremendously. However, it is unclear if the response of this pine stand will be sustained over many years or just transient phenomena with the fertilization. When fertilizer is applied, the stand grows close to optimum conditions. The response of fertilizer may be sustained in the future. If it really is the case, fertilization may be an option to reduce the anthropogenic increase in atmosphere CO₂.