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Effects of Elevated CO2 on Mature Sweetgum Trees: Much More Than Meets the Eye
Volume 7, Number 45: 10 November 2004

The Oak Ridge National Environmental Research Park in Roane County, Tennessee, USA is home to one of the world's premier forest FACE experiments.  The origins of this experiment stretch back to 1988, when one-year-old bare-rooted sweetgum (Liquidambar styraciflua L.) seedlings were planted in the ground to create the forest.  Eight years later, five 25-m-diameter FACE rings were constructed so as to enclose about 90 trees each.  A year later, when pretreatment measurements were made, the trees were about l2 m tall with an average diameter of 11 cm.  At this point, the trees were in a linear growth phase and the canopy was no longer expanding, while a year later in April of 1998, exposure to elevated CO2 was begun in two of the plots and has continued each year thereafter throughout the growing season (April to November).

Over the last four years of the study (years 3-6), the atmospheric CO2 concentrations of the ambient and CO2-enriched plots have averaged 391 ppm and 544 ppm (39% more than ambient), respectively.  During this period, the net primary production of the CO2-enriched plots has averaged 22% more than that of the ambient plots; but there has been no discernible "bulking up" of the trees.  So where has the extra biomass attributable to the extra CO2 been going?  Norby et al. (2004) believe they have found the answer.

Back in July of 1997, the Oak Ridge scientists had installed five minirhizotron tubes in each FACE plot.  These transparent tubes extend to a depth of 60 cm below the soil surface and are inclined at a 60-degree angle from the vertical.  Each is equipped with a video recorder that collects images biweekly throughout the growing season; and these data have been digitized and used to calculate a number of different root parameters on the same biweekly basis.  So what's been learned?

Norby et al. report that "the CO2 effect on annual [root] production was highly significant, with production 2.2-fold higher in CO2-enriched plots from 2000-2003."  They also found that "CO2 enrichment significantly increased peak-standing root crop by altering allocation such that the potential for root occupancy of the soil volume was increased," noting that "this response was manifested especially in the deeper distribution of roots in the soil profile."  In particular, they write that the peak-standing root crop exhibited "3-fold more length at 30-45 cm and 4-fold more at 45-60 cm," which is truly amazing considering the enhancement of the air's CO2 concentration employed in this study was only 39%.

The Oak Ridge investigators also determined that the mass of fine roots produced in a given year accounted for 11-34% of forest net primary production; and they say that this "preferential allocation to fine roots should significantly reduce the potential for additional C [carbon] sequestration in trees in elevated CO2," which has indeed proven to be the case in their study.  "However," as they continue, "sequestration of some of that C in the forest remains a possibility," for "as fine roots die, their C enters the soil system where there is the potential for movement into long-lived organic matter pools."  Indeed, they go on to say that "soil analysis indicates that there is increased accumulation of new C in CO2-enriched plots, particularly in microaggregate fractions that facilitate movement of C into pools with long residence times."  And they say that "it may become especially important that the greatest increases in root production in elevated CO2 occur in deeper soil, where sequestration into longer-lived pools may be more likely."

Norby et al. further note that "the CO2-induced increase in fine-root standing crop in summer could also be an important mechanism for conferring increased resistance to late-season droughts," and that "the stimulation of root growth in deeper soil could be particularly important in buffering trees against seasonal droughts."  This being the case, the huge allocation of net primary production that the CO2-enriched trees send belowground and distribute to greater depths may yet enable them to sequester more biomass in their aboveground woody tissues, if it helps them to continue to be able to produce biomass during droughty periods that could possibly bring the net productivity of trees growing in ambient air to a screeching halt.

It will be interesting to see if this scenario develops in some of the drier years to come, which is another reason for continuing such studies as this one for as long as it is humanly (and financially) possible.  There is simply no other way to acquire the important knowledge that these long-term real-world studies generate.

Sherwood, Keith and Craig Idso

Reference
Norby, R.J., Ledford, J., Reilly, C.D., Miller, N.E. and O'Neill, E.G.  2004.  Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment.  Proceedings of the National Academy of Sciences USA 101: 9689-9693.