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Long-Term Studies (Woody Plants - Pine Trees: Scots) -- Summary
How long-lived Scots pines will likely respond to rising atmospheric CO2 concentrations has been assessed by how they have responded to (1) past increases in the atmosphere's CO2 concentration and (2) experimentally-enhanced CO2 concentrations; and in this summary we review what those studies have revealed.

In a study conducted in Europe, three-year-old pot-grown Scots pine (Pinus sylvestris L.) seedlings were planted in the ground in four open-top chambers at the University of Antwerp in Belgium on 21 March 1996, where they were continuously maintained at atmospheric CO2 concentrations of either 350 or 750 ppm in order to determine the long-term effects of elevated CO2 on various aspects of the growth and development of this important timber species, as described by Jach and Ceulemans (1999). In addition, in order to make the experimental results as representative as possible of the natural world, no nutrients or irrigation waters were applied to the soils during the entire period of the investigation.

During the second year of the study, Jach and Ceulemans (2000a) discovered that the photosynthetic rates of current and one-year-old CO2-enriched needles were 62 and 65% greater, respectively, than the photosynthetic rates of comparable needles on seedlings that were growing in ambient air. Simultaneously, Jach and Ceulemans (2000b) found that dark respiration rates expressed on a needle-mass basis were 27 and 33% lower in current-year and one-year-old needless, respectively, on the CO2-enriched trees than on the ambient-treatment trees. And after three years of differential CO2 exposure, Jach et al. (2000) determined that the extra CO2 of the study had increased total seedling biomass production by 55%, in spite of the fact that the experimental soils were relatively nutrient-poor.

To possibly compensate for this deficiency of nutrients, Jach et al. found that the elevated CO2 had increased root biomass by more than 150%, which would likely have enhanced the ability of the CO2-enriched seedlings to explore a greater volume of soil for the nutrients they required to sustain their augmented growth and development. Indeed, the three researchers concluded that "it is likely that on nutrient-poor forest sites valuable gains to the timber industry may be achieved under future climatic conditions, since increased root production may enhance both nutrient availability, and hence timber production, as well as increase wind stability."

At the three-year point of the study, Gielen et al. (2000) determined that elevated CO2 did not significantly impact the photochemical quantum efficiency of photosystem II, nor did it affect any parameters associated with chlorophyll fluorescence, indicative of the likelihood that atmospheric CO2 enrichment did not modify the light-dependent reactions of photosynthesis. They did find, however, that elevated CO2 reduced needle nitrogen and chlorophyll contents by 33 and 26%, respectively, although these reductions were statistically insignificant. Nonetheless, these latter observations suggest that the light-independent reactions of photosynthesis were being modified by long-term exposure to elevated CO2 in a manner indicative of photosynthetic acclimation, which phenomenon allows for the redistribution of limiting resources, such as nitrogen, to other areas of the tree where they may be more needed.

At the four-year point of the study, Lin et al. (2001) found that elevated CO2 reduced needle stomatal density by an average of 7.4%, while increasing needle thickness, mesophyll tissue area, and total cross-sectional area by 6.4, 5.7 and 10.4%, respectively. In addition, atmospheric CO2 enrichment increased the average relative area occupied by phloem cells by 4.4%. The first of these observations suggests that Scots pine trees will be better able to conserve water and cope with periods of drought in a future high-CO2 world; while the increase in mesophyll tissue portends an increase in photosynthetic rates, and the increase in phloem cell area suggests a greater capacity for transport of photosynthetic sugars from needles to actively growing sink tissues.

Last of all, in a separate study, Waterhouse et al. (2004) determined the intrinsic water use efficiency response of Scots pines growing in South Bedfordshire in England to the increase in the air's CO2 content experienced between 1895 and 1994, using parameters derived from measurements of stable carbon isotope ratios of trunk cellulose. This work revealed the existence of a long-term increase in intrinsic water use efficiency during the prior century; and they state that the main cause of this behavior was likely "the increase in atmospheric CO2 concentration."

Linearly extrapolating the response (which occurred over a period of time when the air's CO2 concentration rose by approximately 65 ppm) to what would be expected for a 300-ppm increase, the intrinsic water use efficiency increase they derived amounts to 195%, as best we can determine from the graphs of their results. This response is huge, and it is probably not due to the rising CO2 alone, but to the positive synergism that occurs when atmospheric CO2 and temperature rise together, about which more can be read in our Subject Index under the general heading of Interactive Effects of CO2 and Temperature on Plant Growth (Trees).

In conclusion, the evidence gained from the last of these multi-year studies suggests that the historical rise in the atmosphere's CO2 concentration has significantly enhanced the growth and well-being of earth's Scots pine trees over the past century or more; while the evidence gained from the other studies suggests that the ongoing rise in atmospheric CO2 (and possibly temperature as well) will likely do the same for them for decades yet to come.

Gielen, B., Jach, M.E. and Ceulemans, R. 2000. Effects of season, needle age and elevated atmospheric CO2 on chlorophyll fluorescence parameters and needle nitrogen concentration in (Pinus sylvestris L.). Photosynthetica 38: 13-21.

Jach, M. E. and R. Ceulemans. 1999. Effects of elevated atmospheric CO2 on phenology, growth and crown structure of Scots pine (Pinus sylvestris L.) seedlings after two years of exposure in the field. Tree Physiology 19:289-300.

Jach, M.E. and Ceulemans, R. 2000a. Effects of season, needle age and elevated atmospheric CO2 on photosynthesis in Scots pine (Pinus sylvestris L.). Tree Physiology 20: 145-157.

Jach, M.E. and Ceulemans, R. 2000b. Short- versus long-term effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.). Photosynthetica 38: 57-67.

Jach, M.E., Laureysens, I. and Ceulemans, R. 2000. Above- and below-ground production of young Scots pine (Pinus sylvestris L.) trees after three years of growth in the field under elevated CO2. Annals of Botany 85: 789-798.

Lin, J., Jach, M.E. and Ceulemans, R. 2001. Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2. New Phytologist 150: 665-674.

Waterhouse, J.S., Switsur, V.R., Barker, A.C., Carter, A.H.C., Hemming, D.L., Loader, N.J. and Robertson, I. 2004. Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations. Quaternary Science Reviews 23: 803-810.

Last updated 17 February 2010