In reviewing studies that have been conducted on individual trees, it is clear that elevated levels of atmospheric CO2 increase photosynthesis and growth in both broad-leaved and coniferous species.  When broad-leaved trembling aspen (Populus tremuloides) were exposed to twice-ambient levels of atmospheric CO2 for 2.5 years, for example, Pregitzer et al. (2000) reported 17 and 65% increases in fine root biomass at low and high levels of soil nitrogen, respectively; while Zak et al. (2000) observed 16 and 38% CO2-induced increases in total tree biomass when subjected to the same respective levels of soil nitrogen.

Similar results have been reported for coniferous trees.  When branches of Sitka spruce (Picea sitchensis) were fumigated with air of 700 ppm CO2 for four years, rates of net photosynthesis in current and second-year needles were 100 and 43% higher, respectively, than photosynthetic rates of needles exposed to ambient air (Barton and Jarvis, 1999).  In addition, ponderosa pine (Pinus ponderosa) grown at 700 ppm CO2 for close to 2.5 years exhibited rates of net photosynthesis in current-year needles that were 49% greater than those of needles exposed to air containing 350 ppm CO2 (Houpis et al., 1999).  Many more such responses of trees to atmospheric CO2 enrichment can also be found in our Subject Index under the general heading of Trees.

From the preceding material, it is clear that elevated CO2 enhances photosynthetic rates and biomass production in forest trees, both of which phenomena lead to greater amounts of carbon sequestration.  In addition, elevated CO2 enhances carbon sequestration by reducing carbon losses arising from plant respiration.  Karnosky et al. (1999), for example, reported that aspen seedlings grown for one year at 560 ppm CO2 displayed dark respiration rates that were 24% lower than rates exhibited by trembling aspen grown at 360 ppm CO2.  Also, elevated CO2 has been shown to decrease maintenance respiration, which it did by 60% in western hemlock seedlings exposed to an atmospheric CO2 concentration of nearly 1600 ppm (McDowell et al., 1999).

In a thorough review of these topics, Drake et al. (1999) concluded that, on average, a doubling of the atmospheric CO2 concentration reduces plant respiration rates by approximately 17%.  This finding contrasts strikingly with the much smaller effects reported by Amthor (2000), who found an average reduction in dark respiration of only 1.5% for nine deciduous trees species exposed to 800 ppm CO2.  The period of CO2 exposure in his much shorter experiments, however, was but a mere 15 minutes.  Hence, if the air's CO2 content doubles, plants will likely sequester something on the order of 17% more carbon than ambiently-grown plants, solely as a consequence of CO2-induced reductions in respiration.  And it is good to remember that this stored carbon is in addition to that sequestered as a result of CO2-induced increases in plant photosynthetic rates.

What is the fate of the extra carbon that is stored within plant tissues as a consequence of atmospheric CO2 enrichment?  Is it rapidly returned to the atmosphere following tissue senescence and decomposition?  Or is it somehow locked away for long periods of time?

To answer these questions, it is important to note that atmospheric CO2 enrichment typically reduces, or has no effect upon, decomposition rates of senesced plant material (see our Subject Index Summary on Decomposition).  De Angelis et al. (2000), for example, noted that when leaf litter from Mediterranean forest species exposed to 710 ppm CO2 for 3.5 years was collected and allowed to decompose at 710 ppm CO2 for approximately one year, it decomposed at a rate that was 4% less than that observed for leaf litter produced and incubated at ambient CO2 concentrations for one year.  Similarly, leaf litter collected from yellow-poplar (Liriodendron tulipifera) seedlings exposed to 700 ppm CO2 for four years contained 12% more biomass than leaf litter collected from seedlings grown at ambient CO2, following two years of decomposition at their respective CO2 growth concentrations (Scherzel et al., 1998).  However, Hirschel et al. (1997) found no significant CO2-induced effects on decomposition rates in tropical rainforest species, as Scherzel et al. (1998) also found for eastern white pine (Pinus strobes).  Thus, it would appear that elevated CO2 typically reduces or has no effect upon plant litter decomposition rates.  In addition, it is important to note that none of these decomposition studies looked at wood, which can sequester carbon for long periods of time, even for millennia (Chambers et al., 1998), provided it is not burned.

Based upon several different types of empirical data, a number of researchers have concluded that current rates of carbon sequestration are robust and that future rates will increase with increasing atmospheric CO2 concentrations.  In the study of Fan et al. (1998) based on atmospheric measurements, for example, the broad-leaved forested region of North America between 15 and 51°N latitude was calculated to possess a current carbon sink that can annually remove all the CO2 emitted into the air from fossil fuel combustion in both Canada and the United States.  On another large scale, Phillips et al. (1998) used data derived from tree basal area to show that average forest biomass in the tropics has increased substantially over the last 40 years and that growth in the Neotropics alone can account for 40% of the missing carbon of the entire globe.  And in looking to the future, White et al. (2000) have calculated that coniferous and mixed forests north of 50°N latitude will likely expand their northern and southern boundaries by about 50% due to the combined effects of increasing atmospheric CO2, rising temperature, and nitrogen deposition.

The latter of these factors is an important variable.  As indicated in the study of White et al. (2000), it can play an interactive role with increasing atmospheric CO2 to increase plant growth and carbon sequestration.  However, the magnitude of that role is still being debated.  Nadelhoffer et al. (1999), for example, concluded that nitrogen deposition from human activities is "unlikely to be a major contributor" to the large CO2 sink that exists in northern temperate forests.  Houghton et al. (1998), however, feel that nitrogen deposition holds equal weight with CO2 fertilization in the production of terrestrial carbon sinks; and Lloyd (1999) demonstrated that when CO2 and nitrogen increase together, modeled forest productivity is greater than that predicted by the sum of the individual contributions of these two variables.  Thus, anthropogenic nitrogen deposition can have anywhere from small to large positive effects on carbon sequestration, as well as everything in between.

In spite of these many positive findings, some people still worry that rising air temperatures will negatively impact carbon sequestration in forests.  However, Liski et al. (1999) showed that carbon storage in soils of both high- and low-productivity boreal forests in Finland actually increased with increasing temperature, thereby putting to rest the idea that rising temperatures will spur carbon losses from soils and trees and exacerbate global warming.  Similarly, King et al. (1999) showed that aspen seedlings increased their photosynthetic rates and biomass production as temperatures rose from 10 to 29°C, putting to rest the idea that high-temperature-induced increases in respiration rates would cause net losses in carbon fixation.  Moreover, White et al. (1999) showed that rising temperatures increased the growing season by about 15 days for 12 sites in deciduous forests located within the United States, causing a 1.6% increase in net ecosystem productivity per day.  Thus, rather than exerting a negative influence on forest carbon sequestration, if air temperatures rise in the future they will likely have a positive effect on carbon storage in forests and their associated soils.

In conclusion, as the air's CO2 content continues to rise, the ability of earth's forests to sequester carbon should also rise.  With more CO2 in the atmosphere, trees will likely exhibit greater rates of photosynthesis and reduced rates of respiration.  Together, these observations - along with the finding that atmospheric CO2 enrichment has little or no effect on plant tissue decomposition rate - suggest that biologically-fixed carbon will experience greater residency times within plant tissues.  And if this carbon is directed into wood production, which increases substantially with atmospheric CO2 enrichment, some of it can be kept out of circulation for a very long time, possibly even a millennium or more.

References
Amthor, J.S.  2000.  Direct effect of elevated CO2 on nocturnal in situ leaf respiration in nine temperate deciduous tree species is small.  Tree Physiology 20: 139-144.

Barton, C.V.M. and Jarvis, P.G.  1999.  Growth response of branches of Picea sitchensis to four years exposure to elevated atmospheric carbon dioxide concentration.  New Phytologist 144: 233-243.

Chambers, J.Q., Higuchi, N. and Schimel, J.P.  1998.  Ancient trees in Amazonia.  Nature 391: 135-136.

De Angelis, P., Chigwerewe, K.S. and Mugnozza, G.E.S.  2000.  Litter quality and decomposition in a CO2-enriched Mediterranean forest ecosystem.  Plant and Soil 224: 31-41.

Drake, B.G., Azcon-Bieto, J., Berry, J., Bunce, J., Dijkstra, P., Farrar, J., Gifford, R.M., Gonzalez-Meler, M.A., Koch, G., Lambers, H., Siedow, J. and Wullschleger, S.  1999.  Does elevated atmospheric CO2 inhibit mitochondrial respiration in green plants?  Plant, Cell and Environment 22: 649-657.

Fan, S., Gloor, M., Mahlman, J., Pacala, S., Sarmiento, J., Takahashi, T. and Tans, P.  1998.  A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models.  Science 282: 442-446.

Hirschel, G., Korner, C. and Arnone III, J.A.  1997.  Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities?  Oecologia 110: 387-392.

Houghton, R.A., Davidson, E.A. and Woodwell, G.M.  1998.  Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance.  Global Biogeochemical Cycles 12: 25-34.

Houpis, J.L.J., Anderson, P.D., Pushnik, J.C. and Anschel, D.J.  1999.  Among-provenance variability of gas exchange and growth in response to long-term elevated CO2 exposure.  Water, Air, and Soil Pollution 116: 403-412.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

King, J.S., Pregitzer, K.S. and Zak, D.R.  1999.  Clonal variation in above- and below-ground responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability.  Plant and Soil 217: 119-130.

Liski, J., Ilvesniemi, H., Makela, A. and Westman, C.J.  1999.  CO2 emissions from soil in response to climatic warming are overestimated - The decomposition of old soil organic matter is tolerant of temperature.  Ambio 28: 171-174.

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.

McDowell, N.G., Marshall, J.D., Qi, J. and Mattson, K.  1999.  Direct inhibition of maintenance respiration in western hemlock roots exposed to ambient soil carbon dioxide concentrations.  Tree Physiology 19: 599-605.

Nadelhoffer, K.J., Emmett, B.A., Gundersen, P., Kjonaas, O.J., Koopmans, C.J., Schleppi, P., Tietema, A. and Wright, R.F.  1999.  Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests.  Nature 398: 145-148.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J.  1998.  Changes in the carbon balance of tropical forests: Evidence from long-term plots.  Science 282: 439-442.

Pregitzer, K.S., Zak, D.R., Maziaasz, J., DeForest, J., Curtis, P.S. and Lussenhop, J.  2000.  Interactive effects of atmospheric CO2 and soil-N availability on fine roots of Populus tremuloides.  Ecological Applications 10: 18-33.

Scherzel, A.J., Rebbeck, J. and Boerner, R.E.J.  1998.  Foliar nitrogen dynamics and decomposition of yellow-poplar and eastern white pine during four seasons of exposure to elevated ozone and carbon dioxide.  Forest Ecology and Management 109: 355-366.

White, A., Cannell, M.G.R. and Friend, A.D.  2000.  The high-latitude terrestrial carbon sink: a model analysis.  Global Change Biology 6: 227-245.

White, M.A., Running, S.W. and Thornton, P.E.  1999.  The impact of growing-season length variability on carbon assimilation and evapotranspiration over 88 years in the eastern US deciduous forest.  International Journal of Biometeorology 42: 139-145.

Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J.  2000.  Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides.  Ecological Applications 10: 34-46.