Although life-sustaining photosynthesis occurs aboveground in the presence of light, what happens in the darkness beneath the surface of the soil is equally important to the welfare of earth's higher plants and the many benefits they provide for the rest of the biosphere. In this brief review of some of the scientific literature germane to this subject, we thus focus on root responses of deciduous trees to atmospheric CO2 enrichment.
Berntsen and Bazzaz (1998) removed intact chunks of soil from the Hardwood-White Pine-Hemlock forest region of New England and placed them in plastic containers within controlled environment glasshouses maintained at either 375 or 700 ppm CO2 for a period of two years in order to study the effects of elevated CO2 on the regeneration of plants from seeds and rhizomes present in the soil. At the conclusion of the study, total mesocosm plant biomass (more than 95% of which was supplied by yellow and white birch tree seedlings) was found to be 31% higher in the elevated CO2 treatment than in ambient air, with a mean enhancement of 23% aboveground and 62% belowground. The extra CO2 also increased the mycorrhizal colonization of root tips by 45% in white birch and 71% in yellow birch; and the CO2-enriched yellow birch seedlings exhibited 322% greater root length and 305% more root surface area than did the yellow birch seedlings growing in ambient air.
Kubiske et al. (1998) grew cuttings of four quaking aspen genotypes in open-top chambers for five months at atmospheric CO2 concentrations of either 380 or 720 ppm and low or high soil nitrogen concentrations. They found, surprisingly, that the cuttings grown in elevated CO2 displayed no discernible increases in aboveground growth. However, the extra CO2 significantly increased fine-root length and root turnover rates at high soil nitrogen by increasing fine-root production, which would logically be expected to produce benefits (not the least of which would be a larger belowground water- and nutrient-gathering system) that would eventually lead to enhanced aboveground growth as well. That this reasoning is indeed correct may be verified by perusing the many positive aboveground growth responses of quaking aspen trees to atmospheric CO2 enrichment that are listed in the biomass portion of the Plant Growth Data section of our website (see Populus tremuloides Michx.).
Expanding on this study, Pregitzer et al. (2000) grew six quaking aspen genotypes for 2.5 growing seasons in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm with both adequate and inadequate supplies of soil nitrogen. This work demonstrated that the trees exposed to elevated CO2 developed thicker and longer roots than the trees growing in ambient air, and that the fine-root biomass of the CO2-enriched trees was enhanced by 17% in the nitrogen-poor soils and by 65% in the nitrogen-rich soils.
Yet another study of quaking aspen conducted by King et al. (2001) demonstrated that trees exposed to an atmospheric CO2 concentration 560 ppm in a FACE experiment produced 133% more fine-root biomass than trees grown in ambient air of 360 ppm, which roughly equates to 233% more fine-root biomass for the degree of CO2 enrichment employed in the prior study of Pregitzer et al. And when simultaneously exposed to air of 1.5 times the normal ozone concentration, the degree of fine-root biomass stimulation produced by the extra CO2 was still as great as 66%, or roughly 115% when extrapolated to the greater CO2 enrichment employed by Pregitzer et al.
In a final quaking aspen study, King et al. (1999) grew four clones at two different temperature regimes (separated by 5°C) and two levels of soil nitrogen (N) availability (high and low) for 98 days, while measuring photosynthesis, growth, biomass allocation, and root production and mortality. They found that the higher of the two temperature regimes increased rates of photosynthesis by 65% and rates of whole-plant growth by 37%, while it simultaneously enhanced root production and turnover. It was thus their conclusion that "trembling aspen has the potential for substantially greater growth and root turnover under conditions of warmer soil at sites of both high and low N-availability" and that "an immediate consequence of this will be greater inputs of C and nutrients to forest soils."
In light of these several findings pertaining to quaking aspen trees, it is evident that increases in atmospheric CO2 concentration, air temperature and soil nitrogen content all enhance their belowground growth, which positively impacts their aboveground growth. And it is important to note that all of these environmental changes have been imputed to occur as a consequence of the burning of fossil fuels.
Turning our attention to other deciduous trees, we focus next on the study of Gleadow et al. (1998), who grew eucalyptus seedlings for six months in glasshouses maintained at atmospheric CO2 concentrations of either 400 or 800 ppm, fertilizing them twice daily with low or high nitrogen solutions. The elevated CO2 of their experiment increased total plant biomass by 98 and 134% relative to plants grown at ambient CO2 in the high and low nitrogen treatments, respectively. In addition, in the low nitrogen treatment, elevated CO2 stimulated greater root growth, as indicated by a 33% higher root:shoot ratio.
In a more complex study, Day et al. (1996) studied the effects of elevated CO2 on fine-root production in open-top chambers erected over a regenerating oak-palmetto scrub ecosystem in Florida, USA, determining that a 350-ppm increase in the atmosphere's CO2 concentration increased fine-root length densities by 63% while enhancing the distribution of fine roots at both the soil surface (0-12 cm) and at a depth of 50-60 cm. These findings suggest that the ongoing rise in the atmosphere's CO2 concentration will likely increase the distribution of fine roots near the soil surface, where the greatest concentrations of nutrients are located, and at a depth that coincides with the upper level of the site's water table, both of which phenomena should increase the trees' ability to acquire the nutrients and water they will need to support CO2-enhanced biomass production in the years and decades ahead.
In another study that employed CO2, temperature and nitrogen as treatments, Uselman et al. (2000) grew seedlings of the nitrogen-fixing black locust tree for 100 days in controlled environments maintained at atmospheric CO2 concentrations of 350 and 700 ppm and air temperatures of 26°C (ambient) and 30°C, with either some or no additional nitrogen fertilization, finding that the extra CO2 increased total seedling biomass by 14%, that the elevated temperature increased it by 55%, and that nitrogen fertilization increased it by 157%. With respect to root exudation, a similar pattern was seen. Plants grown in elevated CO2 exuded 20% more organic carbon compounds than plants grown in ambient air, while elevated temperature and fertilization increased root exudation by 71 and 55%, respectively. Hence, as the air's CO2 content continues to rise, black locust trees will likely exhibit enhanced rates of biomass production and exudation of dissolved organic compounds from their roots. Moreover, if air temperature also rises, even by as much as 4°C, its positive effect on biomass production and root exudation will likely be even greater than that resulting from the increasing atmospheric CO2 concentration; and the same would appear to hold true for anthropogenic nitrogen deposition, reinforcing what was learned about the impacts of these three environmental factors on the growth of quaking aspen trees.
In a somewhat different type of study, McDowell et al. (1999) grew five-month-old seedlings of western hemlock in root boxes, subjecting them for several hours to various root-space CO2 concentrations, ranging from approximately 90 to 7000 ppm, in order to determine the effect of soil CO2 concentration on growth, maintenance and total root respiration. Although they could detect no effect of atmospheric CO2 enrichment on growth respiration, it significantly impacted maintenance and total respiration rates. At a soil CO2 concentration of 1585 ppm, for example, total and maintenance respiration rates were 55 and 60% lower, respectively, than they were at a soil CO2 concentration of 395 ppm. In fact, the impact of elevated soil CO2 on maintenance respiration (which comprised 85% of the total respiration in this study) was so strong that it exhibited an exponential decline of about 37% for every doubling of the soil CO2 concentration.
In light of these several experimental findings, it can confidently be concluded that the ongoing rise in the air's CO2 content, together with possible concurrent increases in air temperature and nitrogen deposition, will likely help earth's woody plants to become ever more robust and productive as atmospheric CO2 concentrations rise higher and higher.
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.
Day, F.P., Weber, E.P., Hinkle, C.R. and Drake, B.G. 1996. Effects of elevated atmospheric CO2 on fine root length and distribution in an oak-palmetto scrub ecosystem in central Florida. Global Change Biology 2: 143-148.
Gleadow, R.M., Foley, W.J. and Woodrow, I.E. 1998. Enhanced CO2 alters the relationship between photosynthesis and defense in cyanogenic Eucalyptus cladocalyx F. Muell. Plant, Cell and Environment : 12-22.
King, J.S., Pregitzer, K.S. and Zak, D.R. 1999. Clonal variation in above- and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability. Plant and Soil 217: 119-130.
King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F. 2001. Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3. Oecologia 128: 237-250.
Kubiske, M.E., Pregitzer, K.S., Zak, D.R. and Mikan, C.J. 1998. Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil N availability. New Phytologist 140: 251-260.
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.
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.
Uselman, S.M., Qualls, R.G. and Thomas, R.B. 2000. Effects of increased atmospheric CO2, temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.). Plant and Soil 222: 191-202.