Forests contain perennial trees that remove CO2 from the atmosphere during photosynthesis and store it's carbon within their woody tissues for decades to periods sometimes in excess of a thousand years. Thus, it is important to understand how increases in the air's CO2 content affect forest productivity and carbon sequestration, which has a great impact on the rate of rise of the air's CO2 concentration. In this summary, we review several recent scientific publications pertaining to these subjects.
By examining various properties of tree rings, researchers can deduce how historical increases in the air's CO2 concentration have already affected tree productivity and water use efficiency. Duquesnay et al. (1998), for example, analyzed the relative amounts of 12C and 13C present in yearly growth rings of beech trees raised in silviculture regimes in northeastern France, determining that their intrinsic water use efficiencies rose by approximately 33% during the past century, as the air's CO2 concentration rose from approximately 280 to 360 ppm. In another case, Rathgeber et al. (2000) used tree-ring density data to create an historical productivity baseline for forest stands of Pinus halepensis in southeastern France, from which they determined that the net productivity of such forests would increase by 8 to 55% with a doubling of the air's CO2 content. Finally, when running a forest growth model based on empirical observations reported in the literature, Lloyd (1999) determined that the rise in the atmospheric CO2 concentration since the onset of the Industrial Revolution likely increased the net primary productivity of mature temperate deciduous forests by about 7%. In addition, he determined that a proportional increase in anthropogenic nitrogen deposition likely increased forest net primary productivity by 25%. And when he combined the two effects, the net primary productivity stimulation rose to 40%, which is more than the sum of the individual growth enhancements resulting from the increases in CO2 and nitrogen.
The results of these studies demonstrate that historic increases in the air's CO2 content have already conferred great benefits upon earth's forests. But will future increases in the air's CO2 concentration continue to do so? Several research teams have embarked on long-term studies of various forest communities in an attempt to address this important question. What follows are some important observations that have been made from their mostly-ongoing CO2-enrichment studies.
Back in 1996, circular FACE plots (30-m diameter) receiving atmospheric CO2 concentrations of 360 and 560 ppm were established in a 15-year-old loblolly pine (Pinus taeda) plantation in North Carolina, USA, to study the effects of elevated CO2 on the growth and productivity of this particular forest community, which also had several hardwood species present in the understory beneath the primary coniferous canopy. Using this experimental set-up as a platform for several experiments, Hymus et al. (1999) reported that net photosynthetic rates of CO2-enriched loblolly pines trees were 65% greater than rates observed in control tress exposed to ambient air. These greater rates of carbon fixation contributed to the 24% greater growth rates observed in the CO2-enriched pine trees in the first year of this long-term study (Naidu and DeLucia 1999). In addition, DeLucia and Thomas (2000) reported that the elevated CO2 increased rates of net photosynthesis by 50 to 160% in four subdominant hardwood species present in the forest understory. Moreover, for one species - sweetgum (Liquidambar styraciflua) - the extra CO2 enhanced rates of net photosynthesis in sun and shade leaves by 166 and 68%, respectively, even when the trees were naturally subjected to summer seasonal stresses imposed by high temperature and low soil water availability. Consequently, after two years of atmospheric CO2 enrichment, total ecosystem net primary productivity in the CO2-enriched plots was 25% greater than that measured in control plots fumigated with ambient air.
In a similar large-scale study, circular (25-m diameter) FACE plots receiving atmospheric CO2 concentrations of 400 and 530 ppm were constructed within a ten-year-old sweetgum plantation in Tennessee, USA, to study the effects of elevated CO2 on the growth and productivity of this forest community. After two years of treatment, Norby et al. (2001) reported that the modest 35% increase in the air's CO2 content boosted tree biomass production by an average of 24%. In addition, Wullschleger and Norby (2001) noted that CO2-enriched trees displayed rates of transpirational water loss that were approximately 10% lower than those exhibited by control trees grown in ambient air. Consequently, elevated CO2 enhanced seasonal water use efficiencies of these mature sweetgum trees by 28 to 35%.
On a smaller scale, Pritchard et al. (2001) constructed idealized ecosystems (containing five different species) representative of regenerating longleaf pine (Pinus palustris Mill.) communities of the southeastern USA, fumigating them for 18 months with air containing 365 and 720 ppm CO2 to study the effects of elevated CO2 on this forest community. They reported that elevated CO2 increased the above- and belowground biomass of the dominant longleaf pine individuals by 20 and 62%, respectively. At the ecosystem level, elevated CO2 stimulated total aboveground biomass production by an average of 35%. Similar results for regenerating temperate forest communities have been reported by Berntson and Bazzaz (1998), who documented a 31% increase in Transition Hardwood-White Pine-Hemlock forest mesocosm biomass in response to two years of fumigation with twice-ambient concentrations of atmospheric CO2.
It is thus clear that as the air's CO2 concentration continues to rise, forests will likely respond by exhibiting significant increases in total primary productivity and biomass production. Consequently, forests will likely grow much more robustly and significantly expand their ranges, as has already been documented in many parts of the world (see our Trees-Range Expansion Summary), including gallery forest in Kansas, USA (Knight et al., 1994), and the Budal and Sjodal valleys in Norway (Olsson et al., 2000). Such CO2-induced increases in growth and range expansion should ultimately result in large increases in global carbon sequestration within forests (see our Carbon Sequestration - Forests Summary), and may actually reduce the rate of rise of the air's CO2 concentration.
References
Berntson, G.M. and Bazzaz, F.A. 1998. Regenerating temperate forest mesocosms in elevated CO2: belowground growth and nitrogen cycling. Oecologia 113: 115-125.
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.
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.
Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring ð13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572.
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.
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.
Knight, C.L., Briggs, J.M. and Nellis, M.D. 1994. Expansion of gallery forest on Konza Prairie Research Natural Area, Kansas, USA. Landscape Ecology 9: 117-125.
Lloyd, J. 1999. The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interaction with soil nutrient status, II. Temperate and boreal forest productivity and the combined effects of increasing CO2 concentrations and increased nitrogen deposition at a global scale. Functional Ecology 13: 439-459.
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.
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.
Olsson, E.G.A., Austrheim, G. and Grenne, S.N. 2000. Landscape change patterns in mountains, land use and environmental diversity, Mid-Norway 1960-1993. Landscape Ecology 15: 155-170.
Pritchard, S.G., Davis, M.A., Mitchell, R.J., Prior, A.S., Boykin, D.L., Rogers, H.H. and Runion, G.B. 2001. Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO2 enrichment. Environmental and Experimental Botany 46: 35-69.
Rathgeber, C., Nicault, A., Guiot, J., Keller, T., Guibal, F. and Roche, P. 2000. Simulated responses of Pinus halepensis forest productivity to climatic change and CO2 increase using a statistical model. Global and Planetary Change 26: 405-421.
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.