Rogers et al. (1983) observed no difference in the wood density of loblolly pine (Pinus taeda) trees grown at 340 and 718 ppm CO2 for ten weeks; but they found a 33% CO2-induced increase in the wood density of sweetgum (Liquidambar styraciflua) trees that were grown at these concentrations for only eight weeks. Doyle (1987) and Telewski and Strain (1987) studied the same two tree species over three growing seasons in air of 350 and 650 ppm CO2, finding no effect of atmospheric CO2 enrichment on the stem density of sweetgum, but a mean increase of 9% in the stem density of loblolly pine.

Conroy et al. (1990) grew seedlings of two Pinus radiata families at 340 and 660 ppm CO2 for 114 weeks, finding CO2-induced trunk density increases for the two families of 5.4 and 5.6% when soil phosphorus was less than adequate and increases of 5.6 and 1.2% when it was non-limiting. In a similar study, Hattenschwiler et al. (1996) grew six genotypes of clonally-propagated four-year-old Norway spruce (Picea abies) for three years at CO2 concentrations of 280, 420 and 560 ppm at three different rates of wet nitrogen deposition. On average, they found that wood density was 12% greater in the trees grown at the two higher CO2 concentrations than it was in the trees grown at 280 ppm.

Norby et al. (1996) grew yellow poplar or "tulip" trees (Liriodendron tulipifera) at ambient and ambient plus 300 ppm CO2 for three years, during which time the wood density of the trees increased by approximately 7%. Tognetti et al. (1998) studied two species of oak tree - one deciduous (Quercus pubescens) and one evergreen (Quercus ilex) - growing in the vicinity of CO2 springs in central Italy that raised the CO2 concentration of the surrounding air by approximately 385 ppm. This increase in the air's CO2 content increased the wood density of the deciduous oaks by 4.2% and that of the evergreen oaks by 6.4%.

Telewski et al. (1999) grew loblolly pine trees for four years at ambient and ambient plus 300 ppm CO2. In their study, wood density determined directly from mass and volume measurements was increased by 15% by the extra CO2; while average ring density determined by X-ray densitometry was increased by 4.5%.

Beismann et al. (2002) grew different genotypes of spruce and beech (Fagus sylvatica) seedlings for four years in open-top chambers maintained at atmospheric CO2 concentrations of 370 and 590 ppm in combination with low and high levels of wet nitrogen application on both rich calcareous and poor acidic soils to study the effects of these factors on seedling toughness (fracture characteristics) and rigidity (bending characteristics such as modulus of elasticity). They found that some genotypes of each species were sensitive to elevated CO2, while others were not. Similarly, some were responsive to elevated nitrogen deposition, while others were not. Moreover, such responses were often dependent upon soil type. Averaged across all tested genotypes, however, atmospheric CO2 enrichment increased wood toughness in spruce seedlings grown on acidic soils by 12 and 18% at low and high levels of nitrogen deposition, respectively. In addition, atmospheric CO2 enrichment increased this same wood property in spruce seedlings grown on calcareous soils by about 17 and 14% with low and high levels of nitrogen deposition, respectively. In contrast, elevated CO2 had no significant effects on the mechanical wood properties of beech seedlings, regardless of soil type.

In another study, Kilpelainen et al. (2003) erected 16 open-top chambers within a 15-year-old stand of Scots pines growing on a nutrient-poor sandy soil of low nitrogen content near the Mekrijarvi Research Station of the University of Joensuu, Finland. Over the next three years they maintained the trees within these chambers in a well-watered condition, while they enriched the air in half of the chambers to a mean daytime CO2 concentration of approximately 580 ppm and maintained the air in half of each of the two CO2 treatments at 2°C above ambient. In the ambient temperature treatment the 60% increase in the air's CO2 concentration significantly increased latewood density by 27% and maximum wood density by 11%, while in the elevated-temperature treatment it significantly increased latewood density by 25% and maximum wood density by 15%. These changes led to mean overall CO2-induced wood density increases of 2.8% in the ambient-temperature treatment and 5.6% in the elevated-temperature treatment.

Kostiainen et al. (2004) investigated the effects of elevated CO2 (doubled concentration: 720 ppm vs. 360 ppm) and elevated nutrient input to soil (described as "heavy fertilization," i.e., "higher than used in forestry in practice") on a number of wood properties of 40-year-old Norway spruce (Picea abies L. Karst.) trees that were enclosed by open-top chambers for a period of three years. In discussing their findings, they report that previous data from this long-term study "showed that fertilization decreased wood density (Makinen et al., 2002)," and in the presence of elevated CO2, such was still found to be the case in the new study, but only for earlywood density (a mean decrease of 3.8% over the three years of the study). In the case of latewood density, the extra CO2 supplied to the trees overrode the negative effect of heavy fertilization and increased mean wood density by 4.6%. Moreover, in the treatment where no extra nutrients were supplied to the trees, both earlywood and latewood density were increased by the doubling of the air's CO2 concentration: by 4.8% in the case of earlywood density and by 2.0% in the case of latewood density. Thus, under normal growing conditions, a doubling of the air's CO2 concentration would likely increase the wood density of Norway spruce trees by something on the order of 2-5%.

In one final study, Buitenwerf et al. (2012) analyzed changes in woody-plant density at three sites in South African savannas where the normal disturbance regime (fire and herbivores) was kept constant for either 30 or 50 years, noting that "if global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime."

In describing their findings, the four South African scientists report that for the more arid savannas they analyzed, there was no indication of global drivers promoting an increase in wood density over the period of their study. However, they found that wood density actually tripled in a mesic savanna between the 1970s and 1990s, and that in another mesic savanna it doubled from the mid-1990s to 2010, while "aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover." Commenting on these results, Buitenwerf et al. say their analysis "has shown significant increase in tree densities and stature that are consistent with global drivers promoting woody thickening." And they conclude that "the only plausible candidate in the experimental areas is increasing CO2 since there were no significant temperature or rainfall trends over the last 50 years."

In light of these several observations, it is clear that different species of trees sometimes respond differently to atmospheric CO2 enrichment, and that they respond with still greater variety under different sets of environmental conditions. In general, however, atmospheric CO2 enrichment tends to increase wood density in both seedlings and mature trees more often than not, thereby also increasing a number of strength properties of their branches and trunks.

References
Beismann, H., Schweingruber, F., Speck, T. and Korner, C. 2002. Mechanical properties of spruce and beech wood grown in elevated CO2. Trees 16: 511-518.

Buitenwerf, R., Bond, W.J., Stevens, N. and Trollope, W.S.W. 2012. Increased tree densities in South African savannas: >50 years of data suggests CO2 as a driver. Global Change Biology 18: 675-684.

Conroy, J.P., Milham, P.J., Mazur, M., Barlow, E.W.R. 1990. Growth, dry weight partitioning and wood properties of Pinus radiata D. Don after 2 years of CO2 enrichment. Plant, Cell and Environment 13: 329-337.

Doyle, T.W. 1987. Seedling response to CO2 enrichment under stressed and non-stressed conditions. In: Jacoby Jr., G.C. and Hornbeck, J.W. (Editors), Proceedings of the International Symposium on Ecological Aspects of Tree-Ring Analysis. National Technical Information Service, Springfield, VA, pp. 501-510.

Hattenschwiler, S., Schweingruber, F.H., Korner, C. 1996. Tree ring responses to elevated CO2 and increased N deposition in Picea abies. Plant, Cell and Environment 19: 1369-1378.

Kilpelainen A., Peltola, H., Ryyppo, A., Sauvala, K., Laitinen, K. and Kellomaki, S. 2003. Wood properties of Scots pines (Pinus sylvestris) grown at elevated temperature and carbon dioxide concentration. Tree Physiology 23: 889-897.

Kostiainen, K., Kaakinen, S., Saranpaa, P., Sigurdsson, B.D., Linder, S. and Vapaavuori, E. 2004. Effect of elevated [CO2] on stem wood properties of mature Norway spruce grown at different soil nutrient availability. Global Change Biology 10: 1526-1538.

Makinen, H., Saranpaa, P. and Linder, S. 2002. Wood-density variation of Norway spruce in relation to nutrient optimization and fiber dimensions. Canadian Journal of Forest Research 32: 185-194.

Norby, R.J., Wullschleger, S.D., Gunderson, C.A. 1996. Tree responses to elevated CO2 and implications for forests. In: Koch, G.W. and Mooney, H.A. (Editors), Carbon Dioxide and Terrestrial Ecosystems. Academic Press, New York, NY, pp. 1-21.

Rogers, H.H., Bingham, G.E., Cure, J.D., Smith, J.M., Surano, K.A. 1983. Responses of selected plant species to elevated carbon dioxide in the field. Journal of Environmental Quality 12: 569-574.

Telewski, F.W., Strain, B.R. 1987. Densitometric and ring width analysis of 3-year-old Pinus taeda L. and Liquidambar styraciflua L. grown under three levels of CO2 and two water regimes. In: Jacoby Jr., G.C. and Hornbeck, J.W. (Editors), Proceedings of the International Symposium on Ecological Aspects of Tree-Ring Analysis. National Technical Information Service, Springfield, VA, pp. 494-500.

Telewski, F.W., Swanson, R.T., Strain, B.R. and Burns, J.M. 1999. Wood properties and ring width responses to long-term atmospheric CO2 enrichment in field-grown loblolly pine (Pinus taeda L.). Plant, Cell and Environment 22: 213-219.

Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245.