After one full year of exposure to these two CO2 regimes, Naidu and DeLucia (1999) reported that the extra CO2 increased the biomass of the dominant loblolly pines by about 14%, but that it had no significant impact on the growth rates of the four most abundant understorey species.  After two years of atmospheric CO2 enrichment, however, DeLucia et al. (1999) reported that the mean growth rate of the dominant canopy pines was about 26% greater than that of their counterparts in the ambient-treatment FACE arrays.  In addition, the elevated CO2 had by that time increased total ecosystem net primary productivity by about 25% relative to that of the plots maintained at ambient CO2.

Over the same period of time, Hymus et al. (1999) measured net photosynthetic rates of the dominant loblolly pine trees in both summer and winter.  During the warmer months of the year, the net photosynthetic rates of the CO2-enriched trees were as much as 65% greater than those of the ambient-treatment trees, but cool-season rates were only slightly greater.

Two years later, Hamilton et al. (2001) reported the results of respiration measurements they had conducted on loblolly pine and sweetgum foliage.  In both of the tested species, exposure to elevated CO2 did not appear to alter maintenance respiration, which is the amount of CO2 respired to maintain existing tissues.  There was also no significant short-term suppression of dark respiration rates in needles of loblolly pine, although there was an average reduction of 10% in sweetgum leaves.  Finally, in the case of growth respiration, which is the amount of CO2 respired when constructing new tissues, there was a 21% reduction in loblolly pine and a 39% reduction in sweetgum foliage at the top of the canopy.

LeDeau and Clark (2001) studied the effects of atmospheric CO2 enrichment on cone and seed production in the loblolly pine trees.  Since the trees were not mature at the start of the experiment, they did not produce any cones until a few rare ones appeared in 1998.  By the fall of 1999, however, the scientists found that, compared to the trees growing in ambient air, the CO2-enriched trees "were twice as likely to be reproductively mature and produced three times more cones per tree."  The trees growing in the CO2-enriched atmosphere also produced 2.4 times more cones than the ambient-treatment trees in the fall of 2000.  And from August 1999 through July 2000, LeDeau and Clark collected three times more seeds in the CO2-enriched FACE rings than they did in the control rings.

In a closely related study, Hussain et al. (2001) found that the seeds collected from the CO2-enriched loblolly pine trees were 91% heavier than those collected from the pine trees growing in ambient air.  In addition, the CO2-enriched seeds had a lipid content that was 265% greater than that observed in the seeds produced on the ambient-treatment trees.  Also, germination success for seeds developed under atmospheric CO2 enrichment was more than three times greater than that observed for control seeds developed at ambient CO2, irrespective of the atmospheric CO2 concentration at which the seeds were germinated.  Moreover, the seeds from the CO2-enriched trees germinated approximately five days earlier than their ambiently-produced counterparts, again irrespective of germination CO2 concentration.  And as if those responses were not benefit enough to be derived from the extra CO2, the seedlings that developed from seeds collected from CO2-enriched trees displayed significantly greater root lengths and needle numbers than the seedlings that developed from seeds collected from trees exposed to ambient air, also irrespective of the atmospheric CO2 concentration in which the seeds germinated and grew.

With respect to roots, Pritchard et al. (2001) found that after one year of treatment, total standing root length and root numbers per minirhizotron were 16 and 34% greater, respectively, in the CO2-enriched plots than in the ambient-air plots.  In addition, elevated CO2 increased the diameter of living and dead roots by 8 and 6%, respectively.

Last of all, Finzi and Schlesinger (2002) determined that elevated CO2 had little effect on green leaf and leaf litter nitrogen contents.  In addition, elevated CO2 did not significantly affect the efficiency of nitrogen retranslocation prior to leaf senescence.  Also, lignin and total nonstructural carbohydrate contents in leaf litter were unchanged by atmospheric CO2 enrichment.  In general, therefore, elevated CO2 did not alter leaf litter chemistry; and, consequently, the decomposition of leaf litter remained essentially unaffected by elevated CO2 in this pine forest ecosystem.

What is the bottom line with respect to the ultimate consequences of the extra 200 ppm of CO2 that is being supplied to the loblolly pines and other trees that comprise this North Carolina forest ecosystem?  No one yet knows.  As has been learned from the long-term sour orange tree experiment of Idso and Kimball (see our Editorial of 5 March 2003), significant changes in the growth response of long-lived woody plants to atmospheric CO2 enrichment can occur for at least a decade after the start of such a study; and close to two decades of data may well be required to be confident that a new growth equilibrium with the altered CO2 regime has actually been achieved.  In the interim, of course, one can always speculate, based on data derived to date; and doing so, suggests the following.

As the air's CO2 content continues to rise, loblolly pines will likely experience significant increases in growth rates and biomass production, even on nutrient-poor soils such as that of this experiment; and if the growth enhancement of the total forest ecosystem amounts to no more than the 25% documented by DeLucia et al. (1999) at the two-year point of the current study, that stimulation -- if extrapolated globally to all forests -- would, in the estimation of DeLucia et al., be sufficient to absorb and sequester fully half of the total anthropogenic CO2 emissions of the next century.

The observations of LaDeau and Clark (2001) and Hussain et al. (2001) also suggest that the reproductive prowess of loblolly pines will be greatly enhanced in the years and decades ahead.  Their findings bode particularly well for forests of the southeastern United States, where LaDeau and Clark report that under normal conditions, naturally-regenerating loblolly pine stands "are profoundly seed-limited for at least 25 years."

All in all, it's hard to say anything negative about future prospects for pines. Indeed, all indications are that things will only get better for them as the air's CO2 content continues to rise.

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.  2002.  Species control variation in litter decomposition in a pine forest exposed to elevated CO2.  Global Change Biology 8: 1217-1229.

Hamilton, J.G., Thomas, R.B. and DeLucia, E.H.  2001.  Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem.  Plant, Cell and Environment 24: 975-982.

Hussain, M., Kubiske, M.E. and Connor, K.F.  2001.  Germination of CO2-enriched Pinus taeda L. seeds and subsequent seedling growth responses to CO2 enrichment.  Functional Ecology 15: 344-350.

Hymus, G.J., Ellsworth, D.S., Baker, N.R. and Long, S.P.  1999.  Does free-air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different seasons?  A chlorophyll fluorescence study of mature loblolly pine.  Plant Physiology 120: 1183-1191.

LaDeau, S.L. and Clark, J.S.  2001.  Rising CO2 levels and the fecundity of forest trees.  Science 292: 95-98

Naidu, S.L. and Delucia, E.H.  1999.  First-year growth response of trees in an intact forest exposed to elevated CO2.  Global Change Biology 5: 609-613.

Pritchard, S.G., Rogers, H.H., Davis, M.A., Van Santen, E., Prior, S.A. and Schlesinger, W.H.  2001.  The influence of elevated atmospheric CO2 on fine root dynamics in an intact temperate forest.  Global Change Biology 7: 829-837.