Several groups of scientists have been documenting a variety of responses of different species of pine trees to atmospheric CO2 enrichment for a number of years. One of the longer of these studies is that of Walker et al. (2000), who grew ponderosa pine (Pinus ponderosa Dougl.) seedlings for five years in open-top chambers at atmospheric CO2 concentrations of 350 and 700 ppm on soils of low, medium and high nitrogen content to determine the interactive effects of these variables on the long-term growth response of the trees. What they found at final harvest was that the trees exposed to twice-ambient levels of atmospheric CO2 were 43, 64 and 25% taller than the trees exposed to ambient air at high, medium and low levels of soil nitrogen content, respectively. Likewise, the trunk diameters of the trees fumigated with 700 ppm CO2 were 24, 73 and 20% greater than the trunk diameters of ambiently-grown trees exposed to high, medium and low levels of soil nitrogen content, respectively.
Seedlings of a second species of pine -- loblolly pine (Pinus taeda L.) -- were grown by Tissue et al. (1997) for four years in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 650 ppm. Throughout the summers of this experiment, the seedlings in the CO2-enriched chambers displayed photosynthetic rates that were 60-130% greater than those of the seedlings growing in ambient air; while during the colder winter months, they exhibited photosynthetic rates that were 14 to 44% greater. These persistent increases in the rate of net carbon uptake increased biomass accumulation rates in the CO2-enriched seedlings by fully 90%, prompting the scientists conducting the study to declare that loblolly pines growing in a CO2-enriched world of the future "could be a large sink for fossil fuel carbon emitted to the atmosphere."
In another study of the same trees, Telewski et al. (1999) determined that elevated CO2 did not significantly affect anatomical features of xylem cells, including their cell wall to cell interior ratio, resin canal area, and resin canal density, but that it did significantly increase annual growth-ring widths by 93, 29, 15 and 37% during the four consecutive years of the study. Also, although not significantly so, the extra CO2 increased average ring density in the same four years by 60, 4, 3 and 5%, leading the researchers to state that "projected increases in the atmospheric content of CO2 may result in increased wood production without a loss in structural strength." Indeed, the tendency for wood density to increase in CO2-enriched air portends the possibility of increased structural strength in the years ahead.
Loblolly pines have also been studied in a still-ongoing free-air CO2 enrichment (FACE) experiment at Duke Forest in the Piedmont region of North Carolina, USA, where in August of 1996 three 30-m-diameter CO2 delivery rings began to enrich the air around the 13-year-old trees they encircled to 200 ppm above the atmosphere's ambient CO2 concentration, while three other FACE rings served as ambient-air control plots. Finzi et al. (2002) report that over the first four years of differential CO2 exposure in this study, the trees in the CO2-enriched plots maintained average yearly rates of dry matter production that were 32% greater than those of the trees growing in ambient air. The CO2-enriched trees were also found to contain more nitrogen in their biomass, and they displayed a 10% increase in nitrogen-use efficiency.
In the same experiment, LaDeau and Clark (2001) found that by the fall of 1999, the CO2-enriched trees "were twice as likely to be reproductively mature and produced three times more cones per tree." Similarly, the trees growing in the CO2-enriched air produced 2.4 times more cones in the fall of 2000. From August 1999 through July 2000, the two scientists also collected three times as many seeds in the CO2-enriched FACE rings as they did in the ambient-air control rings. These findings are particularly important, for LaDeau and Clark report that naturally-regenerated loblolly pine stands of the southeastern United States "are profoundly seed-limited for at least 25 years."
Following the initial planting of the loblolly pine stand, a deciduous understory developed as a consequence of seed dispersal from nearby hardwood forests. Finzi and Schlesinger (2002) thus studied the effects of atmospheric CO2 enrichment on total plantation leaf litter chemistry and decomposition following two complete years of differential CO2 exposure, finding that elevated CO2 had little effect on green leaf and leaf litter nitrogen contents. In addition, elevated CO2 did not significantly affect the efficiency of nitrogen retranslocation prior to leaf senescence, nor did lignin and total nonstructural carbohydrate contents of the leaf litter change much. Thus, in general, elevated CO2 did not alter leaf litter chemistry; and, consequently, decomposition of leaf litter remained unaffected by elevated CO2.
A third type of pine that has been studied in some detail within this context is the Scots pine (Pinus sylvestris L.), three-year-old seedlings of which were rooted in the ground and grown in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 750 ppm for several years to determine the long-term effects of elevated CO2 on this important European timber species. In addition, in order to make the experimental results more representative of the natural world, no nutrients or irrigation waters were applied to the soils over the course of the experiment.
During the second year of this study, Jach and Ceulemans (2000a) learned 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 expressed on a needle-mass basis was 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 this study 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, elevated CO2 increased root biomass by more than 150%, which would likely enhance the abilities of CO2-enriched seedlings to explore greater soil volumes for nutrients required to sustain their augmented growth and development. Indeed, the 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."
Also 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. However, elevated CO2 did reduce needle nitrogen and chlorophyll contents by 33 and 26%, respectively; but 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 are needed more.Last of all, 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.
In conclusion, the balance of evidence obtained from the several multi-year experiments that have been conducted on ponderosa, loblolly and Scots pine trees suggests that the ongoing rise in the air's CO2 content will significantly enhance the growth and well-being of these important sources of timber and wildlife habitat in the years and decades to come.
References
Finzi, A.C., DeLucia, E.H., Hamilton, J.G., Richter, D.D. and Schlesinger, W.H. 2002. The nitrogen budget of a pine forest under free air CO2 enrichment. Oecologia 132: 567-578.
Finzi, A.C. and Schlesinger, W.H. 2002. Species control variation in litter decomposition in a pine forest exposed to elevated CO2. Global Change Biology 8: 1217-1229.
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 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.
LaDeau, S.L. and Clark, J.S. 2001. Rising CO2 levels and the fecundity of forest trees. Science 292: 95-98.
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.
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.
Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134.
Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T. 2000. Growth, nutrition, and water relations of ponderosa pine in a field soil as influenced by long-term exposure to elevated atmospheric CO2. Forest Ecology and Management 137: 1-11.