A single-species sweetgum (Liquidambar styraciflua L.) plantation was established at the Oak Ridge National Environmental Research Park in Roane County, Tennessee, USA, in the spring of 1988. Within this stand, five circular FACE plots of 25 meters' diameter were constructed. Atmospheric CO2 enrichment to an average daytime concentration of 533 ppm (as opposed to the ambient concentration of 394 ppm) was begun in two of the plots in April of 1998, well after the 1996 date at which the young forest's canopy had achieved closure and the trees had entered into the stable linear growth phase in which yearly growth increments are approximately the same each year.
Monthly assessments of tree growth (based on bole circumference measurements at a height of 1.3 m above the surface of the ground) were begun in April of 1997, a full year before the start of differential CO2 treatments. These measurements were made on every tree within 10 meters of the centers of the plots. Several trees were then sacrificed to determine their aboveground biomass; and a relationship was developed between this parameter and tree basal area derived from the bole circumference measurements.
Based on the 1997 data, Norby et al. (2001) determined "there was no pretreatment bias to confound subsequent effects of CO2 on growth." Then, from the next two years of differential CO2 exposure, they determined that the 35% increase in atmospheric CO2 concentration employed in their study increased the biomass production of the trees by an average of 24%. "These results," in the words of the scientists, "indicate that large trees have the capacity to respond to elevated CO2 just as much as younger trees that are in exponential growth," something that until that time had been highly conjectural.
In a study conducted over the last five-month growing season of the first two years of atmospheric CO2 enrichment, Wullschleger and Norby (2001) used measurements of sap velocity to assess tree transpiration rates. Their data showed that elevated CO2 reduced tree sap flow by an average of 13% over the growing season, which led to an average reduction of 12% in tree transpiration rate. And after dividing each CO2 treatment's seasonal dry matter production by its seasonal transpiration total, they calculated a stand-level increase in water-use efficiency of 28% for the CO2-enriched trees.
In an evaluation of stand evapotranspiration, Wullschleger et al. (2002) found that the elevated CO2 of the FACE experiment reduced the stomatal conductances of individual leaves of the sweetgum trees by 23% across the growing season. When extrapolated to the entire canopy, however, the CO2-induced reduction in stomatal conductance declined to an effective per-tree value of 14%, similar to what was inferred by Wullschleger and Norby from sap velocity measurements, while the bottom line on a per-land-area basis was a 7% reduction in stand evapotranspiration.
After three years of differential CO2 exposure, Gunderson et al. (2002) determined that the elevated CO2 increased rates of net photosynthesis by 46% in both upper- and mid-canopy foliage. In addition, it reduced stomatal conductances by 24 and 14% in upper- and mid-canopy leaves, respectively, leading to increases in instantaneous water-use efficiencies of 68 and 78% in corresponding upper- and mid-canopy foliage. Also, Norby et al. (2002) determined that the elevated CO2 increased ecosystem net primary productivity by 21% in each of the three years, emphasizing that there was "no decline in enhancement over the 3 year period." Echoing the sentiments of Norby et al. (2001), they too emphasized that "this experiment has provided the first evidence that CO2 enrichment can increase productivity in a closed-canopy deciduous forest."
Sweetgum trees have also been studied in the Duke Forest FACE experiment [see FACE Experiments (Trees - Pine) in our Subject Index for more information about this mixed-tree study]. Specifically, Herrick and Thomas (1999) measured photosynthetic rates in both sun and shade leaves of sweetgum trees in June and August of the first year of this long-term FACE experiment, finding that the greatest CO2-induced photosynthetic stimulation occurred in August, when mean maximum air temperature was 4°C higher and monthly rainfall was 66% less than it was in June of that year. In less stressful June, for example, the extra CO2 increased photosynthetic rates of sun and shade leaves by 92 and 54%, respectively, while in more stressful August the corresponding increases were 166 and 68%. Thus, the positive photosynthetic response of sweetgum leaves to elevated CO2 increased significantly with the imposition of both heat and water stress.
Also in the same Duke Forest FACE experiment, DeLucia and Thomas (2000) reported a doubling of the mean photosynthetic rate of the foliage of the CO2-enriched sweetgum trees at the end of that experiment's first year; while Hamilton et al. (2001) found growth respiration, which is the amount of CO2 respired when constructing new tissues, to be reduced by 39% in leaves at the tops of the sweetgum trees. Last of all, a full seven years after the start of the study, Herrick and Thomas (2003) determined there were still large increases in the net photosynthetic rates of the leaves of the CO2-enriched sweetgum trees: 51 to 96% in sun leaves and 23 to 51% in shade leaves.
In reviewing the results of these several FACE experiments, there is little room for doubting that the ongoing rise in the air's CO2 content will significantly stimulate the growth and water-use efficiency of the planet's sweetgum trees, as it works its wonders with earth's biosphere.
References
DeLucia, E.H. and Thomas, R.B. 2000. Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory. Oecologia 122: 11-19.
Gunderson, C.A., Sholtis, J.D., Wullschleger, S.D., Tissue, D.T., Hanson, P.J. and Norby, R.J. 2002. Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment. Plant, Cell and Environment 25: 379-393.
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.
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) in a forest ecosystem. Tree Physiology 19: 779-786.
Herrick, J.D. and Thomas, R.B. 2003. Leaf senescence and late-season net photosynthesis of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown in elevated and ambient carbon dioxide concentrations. Tree Physiology 23: 109-118.
Norby, R.J., Hanson, P.J., O'Neill, E.G., Tschaplinski, T.J., Weltzin, J.F., Hansen, R.A., Cheng, W., Wullschleger, S.D., Gunderson, C.A., Edwards, N.T. and Johnson, D.W. 2002. Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage. Ecological Applications 12: 1261-1266.
Norby, R.J., Todd, D.E., Fults, J. and Johnson, D.W. 2001. Allometric determination of tree growth in a CO2-enriched sweetgum stand. New Phytologist 150: 477-487.
Wullschleger, S.D., Gunderson, C.A., Hanson, P.J., Wilson. K.B. and Norby, R.J. 2002. Sensitivity of stomatal and canopy conductance to elevated CO2 concentration - interacting variables and perspectives of scale. New Phytologist 153: 485-496.
Wullschleger, S.D. and Norby, R.J. 2001. Sap velocity and canopy transpiration in a sweetgum stand exposed to free-air CO2 enrichment (FACE). New Phytologist 150: 489-498.