Elevated Co2

One might expect that on nutrient deficient soils, the stimulation of tree growth should abate as forests outpace the capacity of soils to provide N and other nutrients. Oren et al. (2001) and Finzi et al. (2002) have demonstrated that at least early in the loblolly pine FACE experiment, the growth of pine trees is co-limited by CO2 and N. The N and C cycles are tightly coupled, and it has been hypothesized that elevated CO2, by increasing carbon and decreasing the N concentration of foliage, will reduce N mineralization rates from decomposing litter, thereby retarding the growth stimulation by elevated CO2 (Zak et al., 1993; Finzi et al., 2002). Evidence to date tacitly supports this hypothesis, but it is far from conclusive.

The annual N requirement for pine grown under elevated CO2 increased by 16% (Finzi et al., 2002), but there is no evidence yet that elevated CO2 has altered microbial N cycling in this forest or in the sweetgum forest (Zak et al., 2003). Greater litter production in these forests when exposed to elevated CO2 has not yet altered the supply of microbial N for tree growth. Thus, it appears that the N demand under elevated CO2 is outpacing supply, but this potential imbalance has not yet reduced the growth stimulation. Johnson et al. (2004) concluded that in the sweetgum experiment, increased demand for N is small relative to its availability, and an N limitation is not likely to constrain the growth response to elevated CO2 in the foreseeable future. It is reasonable to anticipate that the stimulation in growth without a corresponding increase in the supply of N will lead to a reduction in the response to elevated CO2, but given the potentially large storage of N in tree stems and soils, the relatively high spatial variation of N in various components of these ecosystems and low experimental replication, it may take several years for an N limitation to become evident.

At just over 6 years of exposure to elevated CO2, the Duke experiments provide a unique opportunity to examine the strength of the growth stimulation with time as well as its interaction with changing environmental conditions. The reduction in stomatal conductance often observed for plants grown under elevated CO2 (Curtis, 1996; Medlyn et al., 2001) leads to the hypothesis that the growth enhancement should be disproportionately greater in drought years (Strain and Bazzaz, 1983). And, because photorespiration becomes a larger drain on carbon assimilation as temperature increases, it also has been suggested that the stimulation of photosynthesis, and perhaps growth, will be greatest at high temperatures (Long, 1991; Drake et al., 1997). Although manipulative experiments to directly test these hypotheses at the scale of an intact forest ecosystem are not yet possible, an examination of the interannual variation in the response of NPP to CO2 may provide an indirect test.

From the first year of the treatment in 1997 for the pine forest and in 1998 for the sweetgum forest, elevated CO2 caused a substantial and sustained increase in NPP (~12% to ~38%) (Figure 8.3). In addition to being responsive to CO2 and soil N availability, regression analyses revealed that NPP in this pine forest was highly responsive to precipitation during the growing season. Precipitation during the growing season varied from approximately 500 to 800 mm over the 6 years of this experiment, and this variation caused a ~27% increase in NPP. In contrast to one of the hypotheses posed above, the pine forest plots exposed to ambient and elevated CO2 responded similarly to increasing precipitation (e.g., there was no trend in the percent stimulation of NPP with rainfall, Figure 8.4). The absence of an interaction between NPP and rainfall stems from the observation that unlike many angiosperms, the stomata of loblolly pine needles are relatively insensitive to growth under elevated CO2 (Ellsworth, 1999). Because more litter accumulated on the forest floor, thereby retarding soil evaporation, soil moisture was somewhat greater in plots exposed to elevated CO2 (Schäfer et al., 2002), but this had a negligible effect on NPP. The proportional response to elevated CO2 was, however, greatest in warm years, as predicted by the kinetic properties of the primary carboyxlating enzyme in C3 photosynthesis (Drake et al., 1997).

500 600 700 800 2500 3000 3500 4000 4500 Rainfall (mm) Growing Degree Days (°C)

Figure 8.4 Net primary production (NPP; g DM m-2 year-1) for loblolly pine forest plots exposed to ambient (open symbols, ~370 pl l-1) and elevated atmospheric CO2 (~570 pl l-1, closed symbols), and its percent stimulation, plotted as a function of total rainfall during the growing season and growing degree days. The shaded symbol represents data collected in 1997; foliage in this year developed before the treatment. Data are for the Duke free-air CO2 enrichment experiment, and each point represents a mean value for a given year (1997-2002). The coefficients of determination and p values for all regressions were >0.5 and <0.05, respectively, and where a single line is shown, the regressions for the control and treatment plots did not differ. (Data are from D. Moore and E. DeLucia, unpublished, 2004.)

500 600 700 800 2500 3000 3500 4000 4500 Rainfall (mm) Growing Degree Days (°C)

Figure 8.4 Net primary production (NPP; g DM m-2 year-1) for loblolly pine forest plots exposed to ambient (open symbols, ~370 pl l-1) and elevated atmospheric CO2 (~570 pl l-1, closed symbols), and its percent stimulation, plotted as a function of total rainfall during the growing season and growing degree days. The shaded symbol represents data collected in 1997; foliage in this year developed before the treatment. Data are for the Duke free-air CO2 enrichment experiment, and each point represents a mean value for a given year (1997-2002). The coefficients of determination and p values for all regressions were >0.5 and <0.05, respectively, and where a single line is shown, the regressions for the control and treatment plots did not differ. (Data are from D. Moore and E. DeLucia, unpublished, 2004.)

As inferred from interannual variation in the growth response, elevated temperature stimulated NPP in cool years but caused NPP to decrease in warm years (Figure 8.4). Respiration consumes a major portion of the carbon fixed by GPP (57% to 71%) (Table 8.1), and is profoundly temperature dependent (Atkin and Tjoelker, 2003), thus explaining the decline in NPP in warm years. Unlike the response to precipitation, pine forest plots exposed to elevated CO2 responded to temperature differently from those under ambient CO2; the percent stimulation caused by elevated CO2 increased with increasing temperature. Although this relationship must be interpreted with caution, as it was derived from a correlation, it is consistent with the theoretical prediction for photosynthesis and GPP.

Photosynthesis in C3 plants is responsive to CO2 because rubisco, the primary carboxylating enzyme, is not saturated at current concentrations, and because the reaction catalyzed by this enzyme is competitively inhibited by O2 (Zelitch, 1973). Moreover, the specificity of rubisco, its relative affinity for CO2 vs. O2, is temperature dependent, decreasing strongly with rising temperature (Long and Drake, 1992). A consequence of this decline in specificity is that the stimulation of photosynthesis by elevated CO2 progressively increases with increasing temperature (Long, 1991; Long and Drake, 1992). A greater percentage stimulation of photosynthesis by elevated CO2 has been confirmed for loblolly pine (Myers et al., 1999), and it is therefore likely that this disproportionate increase in the stimulation of photosynthesis and GPP explain the observed increase in the percent stimulation of NPP in warm years. Further affirmation of this mechanism stems from the use of process-based models. Application of the PnET-II model (Aber et al., 1995, 1996) to this forest produced the same pattern of increasing percent stimulation of NPP by CO2 in warm years, as observed in Figure 8.4 (C.J. Springer and R.B. Thomas, unpublished data, 2004).

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