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The Continuing Saga of the Duke Forest FACE Experiment
Volume 8, Number 28: 13 July 2005

The productivity of earth's temperate forests is often limited by the availability of soil nitrogen (Vitousek and Howarth, 1991).  Especially is this so in the southeastern United States, where pine-hardwood forests often remove so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization."  In light of these facts, it would seem only natural to presume, as many have, that the CO2-induced increase in the flux of carbon to the soil microbial community that typically occurs under elevated atmospheric CO2 concentrations, such as the extra 200 ppm employed in the Duke Forest FACE study, would increase the rate of N immobilization over the rate of N mineralization and ultimately lead to a decline in - and possibly the total negation of - the large initial CO2-induced stimulation of net primary production, such as that which was documented over the first two years of the Duke Forest FACE study by DeLucia et al. (1999) and Hamilton et al. (2002).  Is this "erosion" of the CO2 aerial fertilization effect occurring in the long-term Duke Forest FACE experiment, where loblolly pine trees have been growing for nearly a quarter of a century on a nutrient-poor Alfisol of the Enon series that is typical of many upland areas of the southeastern United States?

In a study designed to explore this question, Springer et al. (2005) examined the nature of the relationship between light-saturated net photosynthesis (ASAT) and area-based foliar nitrogen concentration (NA) on two tree species growing in the Duke Forest overstory - loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.) - and three tree species, in addition to sweetgum, growing in the forest understory - redbud (Cercis canadensis L.), hickory (Carya glabra Miller) and red maple (Acer rubrum L.) - in the early and late summers of the third, fifth and sixth years of this still-ongoing long-term experiment.  So what did they find?

Springer et al. report that the net photosynthetic rates of each and every species examined in the forest overstory and understory were stimulated by elevated CO2 at each and every measurement date, and that they found no effect of elevated CO2 on NA in any of the species.  More specifically, they report that averaged across all measurement periods and overstory canopy positions, the extra 200 ppm of CO2 strongly stimulated the ASAT of loblolly pine (44%, P < 0.0001) and sweetgum trees (40%, P = 0.005).  In addition, they state that the ASAT of fully-exposed sun leaves in sweetgum trees was stimulated by 60%, whereas the ASAT of shaded leaves was stimulated by 37%; in the loblolly pine trees the corresponding ASAT stimulations were 48% and 27%.  With respect to the smaller trees growing in the forest understory, they report that the ASAT of red maple was enhanced by 24%, that of sweetgum by 44%, that of hickory by 50%, and that of redbud by 61%.

It was also determined that the slopes of the ASAT vs. NA relationships of all of the species studied were not significantly altered by measurement date and that they were approximately 81% greater in the CO2-enriched air than in ambient air, indicative of a large and unchanging CO2-induced increase in photosynthetic nitrogen use efficiency.  In addition, the slopes of the ASAT vs. NA relationships were not significantly different when measured at the ambient and elevated atmospheric CO2 concentrations, nor were there any significant differences in their y-intercepts in these two circumstances, which is indicative of an absence of long-term photosynthetic down regulation or acclimation.

In commenting on these several observations, the authors say they "found a sustained positive response of photosynthesis to elevated CO2 after 6 years of treatment in the Duke Forest FACE experiment," noting that "there was little change in the CO2-stimulation of photosynthesis across season or year, even though the year 2002 was one of the driest summers on record for piedmont North Carolina."  In addition, they state that "the effect of elevated CO2 on photosynthesis of loblolly pine needles growing at the top of the canopy was very similar in magnitude to its effect during the first year of treatment (Meyers et al., 1999)," and that "the percent stimulation of photosynthesis by elevated CO2 for sun and shade sweetgum leaves was comparable to the stimulation of photosynthesis in overstory sweetgum trees averaged across the first three years of the Duke Forest FACE experiment (Herrick and Thomas, 1999)."

In summing up their findings, Springer et al. say they "found little evidence of CO2-induced changes in foliar N concentration or loss of stimulation of photosynthesis by elevated CO2 in the study trees," concluding that "the primary effect of long-term exposure to elevated CO2 in the Duke Forest FACE experiment has been the strong sustained enhancement of photosynthesis."  Consequently, and in spite of climate-alarmist claims to the contrary, a gradual long-term decline in the initial CO2-induced increase in forest photosynthetic prowess that is provided by a large step increase in atmospheric CO2 concentration need not occur, even in trees growing on soils that are notoriously low in nitrogen content.  And since the current yearly increase in the air's CO2 concentration is fully two orders of magnitude less than that provided by most CO2 enrichment experiments, it can be appreciated that real-world forests should continue to benefit from this phenomenon for as long as the "age of fossil fuels" continues.

Sherwood, Keith and Craig Idso

References
DeLucia, E.H., Hamilton, J.G., Naidu, S.L., Thomas, R.B., Andrews, J.A., Finzi, A., Lavine, M., Matamala, R., Mohan, J.E., Hendrey, G.R. and Schlesinger, W.H.  1999.  Net primary production of a forest ecosystem with experimental CO2 enrichment.  Science 284: 1177-1179.

Finzi, A.C. and Schlesinger, W.H.  2003.  Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide.  Ecosystems 6: 444-456.

Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H.  2002.  Forest carbon balance under elevated CO2Oecologia 131: 250-260.

Herrick, J.D. and Thomas, R.B.  1999.  Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua L.) in a forest ecosystem.  Tree Physiology 19: 779-786.

Myers, D.A., Thomas, R.B. and DeLucia, E.H.  1999.  Photosynthetic capacity of loblolly pine (Pinus taeda L.) trees during the first year of carbon dioxide enrichment in a forest ecosystem.  Plant, Cell and Environment 22: 473-481.

Springer, C.J., DeLucia, E.H. and Thomas, R.B.  2005.  Relationships between net photosynthesis and foliar nitrogen concentrations in a loblolly pine forest ecosystem grown in elevated atmospheric carbon dioxide.  Tree Physiology 25: 385-394.

Vitousek, P.M. and Howarth, R.W.  1991.  Nitrogen limitation on land and in the sea: how can it occur?  Biogeochemistry 13: 87-115.