Rouhier and Read (1998) grew Scots pine seedlings for four months in growth cabinets maintained at atmospheric CO2 concentrations of either 350 or 700 ppm. In addition, one third of the seedlings were inoculated with one species of mycorrhizal fungi, one third were inoculated with another species, and one third were not inoculated at all, in order to determine the effects of elevated CO2 on mycorrhizal fungi and their interactive effects on seedling growth. These procedures resulted in the doubled atmospheric CO2 content increasing seedling dry mass by an average of 45% regardless of fungal inoculation. In addition, the extra CO2 increased the number of hyphal tips associated with seedling roots by about 62% for both fungal species. Hyphal growth was also accelerated by elevated CO2; and after 55 days of treatment, the mycorrhizal network produced by one of the fungal symbionts occupied 444% more area than its counterpart exposed to ambient CO2.

These results suggest that as the air's CO2 content continues to rise, fungal symbionts of Scots pine will likely receive greater allocations of carbon from their host. This carbon can be used to increase their mycorrhizal networks, which would enable the fungi to explore greater volumes of soil in search of minerals and nutrients to benefit the growth of its host. In addition, by receiving greater allocations of carbon, fungal symbionts may keep photosynthetic down regulation from occurring, as they provide an additional sink for leaf-produced carbohydrates.

Janssens et al. (1998) grew three-year-old Scots pine seedlings in open-top chambers kept at ambient and 700 ppm atmospheric CO2 concentrations for six months, while they studied the effects of elevated CO2 on root growth and respiration. In doing so, they learned that the elevated CO2 treatment significantly increased total root length by 122% and dry mass by 135% relative to the roots of seedlings grown in ambient-CO2 air. In addition, although starch accumulation in the CO2-enriched roots was nearly 90% greater than that observed in the roots produced in the ambient-CO2 treatment, the carbon-to-nitrogen ratio of the CO2-enriched roots was significantly lower than that of the control-plant roots, indicative of the fact that they contained an even greater relative abundance of nitrogen. The most important implication of this study, therefore, was that Scots pine seedlings will likely be able to find the nitrogen they need to sustain large growth responses to atmospheric CO2 enrichment with the huge root systems they typically produce in CO2-enriched air.

Kainulainen et al. (1998) constructed open-top chambers around Scots pine trees that were about twenty years old and fumigated them with combinations of ambient or CO2-enriched air (645 ppm) and ambient or twice-ambient (20 to 40 ppb) ozone-enriched air for three growing seasons to study the interactive effects of these gases on starch and secondary metabolite production. In doing so, they determined that elevated CO2 and O3 (ozone) had no significant impact on starch production in Scots pine, even after two years of treatment exposure. However, near the end of the third year, the elevated CO2 alone significantly enhanced starch production in current-year needles, although neither extra CO2, extra O3, nor combinations thereof had any significant effects on the concentrations of secondary metabolites they investigated.

Kellomaki and Wang (1998) constructed closed-top chambers around 30-year-old Scots pine trees, which they fumigated with air containing either 350 or 700 ppm CO2 at ambient and elevated (ambient plus 4°C) air temperatures for one full year, after which they assessed tree water-use by measuring cumulative sap flow for 32 additional days. This protocol revealed that the CO2-enriched air reduced cumulative sap flow by 14% at ambient air temperatures, but that sap flow was unaffected by atmospheric CO2 concentration in the trees growing at the elevated air temperatures. These findings suggest that cumulative water-use by Scotts pine trees in a CO2-enriched world of the future will likely be less than or equal to -- but no more than -- what it is today.

Seven years later, however, Wang et al. (2005) published a report of a study in which they measured sap flow, crown structure and microclimatic parameters in order to calculate the transpiration rates of individual 30-year-old Scots pine trees that were maintained for a period of three years in ambient air and air enriched with an extra 350 ppm of CO2 and/or warmed by 2 to 6°C in closed-top chambers constructed within a naturally-seeded stand of the trees. As they describe it, the results of this experiment indicated that "(i) elevated CO2 significantly enhanced whole-tree transpiration rate during the first measuring year [by 14%] due to a large increase in whole-tree foliage area, 1998, but reduced it in the subsequent years of 1999 and 2000 [by 13% and 16%, respectively] as a consequence of a greater decrease in crown conductance which off-set the increase in foliage area per tree; (ii) trees growing in elevated temperature always had higher sap flow rates throughout three measuring years [by 54%, 45% and 57%, respectively]; and (iii) the response of sap flow to the combination of elevated temperature and CO2 was similar to that of elevated temperature alone, indicating a dominant role for temperature and a lack of interaction between elevated CO2 and temperature." These observations suggest that as the air's CO2 content continues to rise, we probably can expect to see a decrease in evaporative water loss rates from naturally-occurring stands of Scots pine trees ... unless there is a large concurrent increase in air temperature. As demonstrated in various places throughout our website, however, there is good reason to believe we will not see CO2-induced global temperature increases of the magnitude employed in this study during what yet remains of the current interglacial.

Also working with closed-top chambers that were constructed around 20-year-old Scots pines and fumigated with air containing 350 and 700 ppm CO2 at ambient and elevated (ambient plus 4°C) air temperatures for a period of three years were Peltola et al. (2002), who studied the effects of elevated CO2 and air temperature on stem growth in this coniferous species when it was growing on a soil low in nitrogen. After three years of treatment, they found that cumulative stem diameter growth in the CO2-enriched trees growing at ambient air temperatures was 57% greater than that displayed by control trees growing at ambient CO2 and ambient air temperatures, while the trees exposed to elevated CO2 and elevated air temperature exhibited cumulative stem-diameter growth that was 67% greater than that displayed by trees exposed to ambient-CO2 air and ambient air temperatures. Consequently, as the air's CO2 content continues to rise, Scots pine trees will likely respond by increasing stem-diameter growth, even if growing on soils low in nitrogen, and even if air temperatures rise by as much as 4°C.

In a somewhat different type of study, Kainulainen et al. (2003) collected needle litter beneath 22-year-old Scots pines that had been growing for the prior three years in open-top chambers that had been maintained at atmospheric CO2 concentrations of 350 and 600 ppm in combination with ambient and elevated (approximately 1.4 x ambient) ozone concentrations to determine the impacts of these variables on the subsequent decomposition of senesced needles. This they did by enclosing the needles in litterbags and placing the bags within a native litter layer in a Scots pine forest, where decomposition rates were assessed by measuring accumulated litterbag mass loss over a period of 19 months. Interestingly, the three researchers found that exposure to elevated CO2 during growth did not affect subsequent rates of needle decomposition, nor did elevated O3 exposure affect decomposition, nor did exposure to elevated concentrations of the two gases together affect it.

Finally, Bergh et al. (2003) used a boreal version of the process-based BIOMASS simulation model to quantify the individual and combined effects of elevated air temperature (2 and 4°C above ambient) and CO2 concentration (350 ppm above ambient) on the net primary production (NPP) of Scots pine forests growing in Denmark, Finland, Iceland, Norway and Sweden. This work revealed that air temperature increases of 2 and 4°C led to mean NPP increases of 11 and 20%, respectively. However, when the air's CO2 concentration was simultaneously increased from 350 to 700 ppm, the corresponding mean NPP increases rose to 41 and 55%. Last of all, when the air's CO2 content was doubled at the prevailing ambient temperature, the mean value of the NPP rose by 27%. Consequently, as the air's CO2 content continues to rise, Ponderosa pines of Denmark, Finland, Iceland, Norway and Sweden should grow ever more productively; and if air temperature also rises, they will likely grow better still.

In summary, as the air's CO2 content continues to rise, we can expect to see the root systems of Scots pines significantly enhanced, together with the mycorrhizal fungal networks that live in close association with them and help secure the nutrients the trees need to sustain large CO2-induced increases in biomass production. Concurrently, we can expect to see much smaller changes in total evaporative water loss, which means that whole-tree water use efficiency should also be significantly enhanced.

References
Bergh, J., Freeman, M., Sigurdsson, B., Kellomaki, S., Laitinen, K., Niinisto, S., Peltola, H. and Linder, S. 2003. Modelling the short-term effects of climate change on the productivity of selected tree species in Nordic countries. Forest Ecology and Management 183: 327-340.

Janssens, I.A., Crookshanks, M., Taylor, G. and Ceulemans, R. 1998. Elevated atmospheric CO2 increases fine root production, respiration, rhizosphere respiration and soil CO2 efflux in Scots pine seedlings. Global Change Biology 4: 871-878.

Kainulainen, P., Holopainen, J.K. and Holopainen, T. 1998. The influence of elevated CO2 and O3 concentrations on Scots pine needles: Changes in starch and secondary metabolites over three exposure years. Oecologia 114: 455-460.

Kainulainen, P., Holopainen, T. and Holopainen, J.K. 2003. Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3. Global Change Biology 9: 295-304.

Kellomaki, S. and Wang, K.-Y. 1998. Sap flow in Scots pines growing under conditions of year-round carbon dioxide enrichment and temperature elevation. Plant, Cell and Environment 21: 969-981.

Peltola, H., Kilpelainen, A. and Kellomaki, S. 2002. Diameter growth of Scots pine (Pinus sylvestris) trees grown at elevated temperature and carbon dioxide concentration under boreal conditions. Tree Physiology 22: 963-972.

Rouhier, H. and Read, D.J. 1998. Plant and fungal responses to elevated atmospheric carbon dioxide in mycorrhizal seedlings of Pinus sylvestris. Environmental and Experimental Botany 40: 237-246.

Wang, K.-Y., Kellomaki, S., Zha, T. and Peltola, H. 2005. Annual and seasonal variation of sap flow and conductance of pine trees grown in elevated carbon dioxide and temperature. Journal of Experimental Botany 56: 155-165.