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CO2 Effects on Carbon and Nitrogen in a Scrub Oak Forest
Hungate, B.A., Johnson, D.W., Dijkstra, P., Hymus, G., Stiling, P., Megonigal, J.P., Pagel, A.L., Moan, J.L., Day, F., Li, J., Hinkle, C.R. and Drake, B.G. 2006. Nitrogen cycling during seven years of atmospheric CO2 enrichment in a scrub oak woodland. Ecology 87: 26-40.

What was done
In the third of Ecology's Special-Feature papers that address the progressive nitrogen limitation (PNL) hypothesis, Hungate et al. (2006) tested the concept against what they observed over a period of seven years in an open-top chamber study of a scrub oak woodland dominated by Quercus myrtifolia, Q. geminata and Q. chapmanii on an island enclosed within the borders of NASA's Kennedy Space Center on the coast of central Florida, USA, which study commenced just a few months after a complete burning of the ecosystem that is located on well-drained nutrient-poor soil.

What was learned
The researchers report that "litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO2, but the CO2 stimulation declined during years five through seven, concurrent with the accumulation of N in the O [soil] horizon and the apparent [our italics] restriction of plant N availability." These changes in N cycling, in their estimation, "are likely to reduce the response of plant production to elevated CO2." Yet, as they were forced to acknowledge, "at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent," as there was a persistent CO2-induced increase in aboveground plant carbon, which led them to conclude that "some mechanisms are partially alleviating progressive N limitation," just as was found by Finzi et al. (2006) in their study of loblolly pines, where by some unknown means the pines obtained the extra N they needed to thwart the negative consequences predicted by the PNL hypothesis.

It is interesting to note in this regard that an initial large CO2-induced increase in aboveground biomass production followed by a subsequent rapid, but then slower, decline in this parameter was also observed by Idso and Kimball in their long-term sour orange tree study (see our Editorial of 5 Mar 2003). Especially is this so when it is realized that the trees of their experiment were periodically fertilized so as to never lack for nitrogen. This being the case, the similar productivity vs. time pattern observed by Hungate et al. may well have had nothing to do with "restriction of plant N availability," which they correctly characterized as being merely "apparent." It is also important to note that the slow decline in the CO2-induced growth stimulation of the sour orange trees finally came to a halt at the ten-year point of the experiment, whereupon the growth stimulation leveled out and remained essentially constant at a sizeable value (80% biomass enhancement in response to a 75% increase in atmospheric CO2 concentration) for the last eight years of the study.

What it means
As in the studies of Norby and Iverson (2006) and Finzi et al. (2006), the pattern of CO2-induced growth stimulation in the scrub oak ecosystem studied by Hungate et al. provides no evidence for the real-world operation of the PNL hypothesis. In fact, it and other of their observations point to one or more unknown means of ecosystem N acquisition that allow the aerial fertilization effect of atmospheric CO2 enrichment to just keep chugging along (albeit at a somewhat lower level of impact than that of its peak manifestation), even in the face of "apparent" N limitations.

Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.

Norby, R.J. and Iversen, C.M. 2006. Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest. Ecology 87: 5-14.

Reviewed 19 April 2006