Seeds (Trees) -- Summary
.. --> Article Subheading -->How does enriching the air with carbon dioxide impact the reproductive capacity of trees? LaDeau and Clark (2001) addressed this question in a major way when they determined the reproductive response of loblolly pine trees to atmospheric CO2 enrichment at Duke Forest in the Piedmont region of North Carolina, USA, where in August of 1996 three 30-m-diameter FACE rings began to enrich the air around the 13-year-old trees they encircled to 200 ppm above the atmosphere's normal background concentration, while three other FACE rings served as control plots. Because the trees were not mature at the start of the experiment they did not produce any cones until a few rare ones appeared in 1998. By the fall of 1999, however, the two scientists found that, compared to the trees growing in ambient air, the CO2-enriched trees were twice as likely to be reproductively mature, and they 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; and from August 1999 through July 2000, they collected three times as many seeds in the CO2-fertilized FACE rings as in the control rings.
Also working on this aspect of the Duke Forest FACE study were Hussain et al. (2001), who report that (1) seeds collected from the CO2-enriched trees were 91% heavier than those collected from the trees growing in ambient air, (2) the CO2-enriched seeds had a lipid content that was 265% greater than that of the seeds produced on the ambient-treatment trees, (3) the germination success for seeds developed under atmospheric CO2 enrichment was more than three times greater than that observed for control seeds developed at ambient CO2, regardless of germination CO2 concentration, (4) seeds from the CO2-enriched trees germinated approximately five days earlier than their ambiently-produced counterparts, again regardless of germination CO2 concentration, and (5) seedlings developing from seeds collected from CO2-enriched trees displayed significantly greater root lengths and needle numbers than seedlings developing from trees exposed to ambient air, also regardless of growth CO2 concentration.
What are the implications of these findings?
The propensity for elevated levels of atmospheric CO2 to hasten the production of more plentiful seeds on the trees of this valuable timber species bodes well for naturally-regenerating loblolly pine stands of the southeastern United States, where LaDeau and Clark report the trees "are profoundly seed-limited for at least 25 years." Hence, as the air's CO2 content continues to climb, they conclude that "this period of seed limitation may be reduced," which is good news indeed for this highly-prized tree. In addition, the observations of Hussain et al. suggest that loblolly pine trees in a CO2-enriched world of the future will likely display significant increases in their photosynthetic rates. Enhanced carbohydrate supplies resulting from this phenomenon will likely be used to increase seed weight and lipid content. Such seeds should consequently exhibit significant increases in germination success, and their enhanced lipid supplies will likely lead to greater root lengths and needle numbers in developing seedlings. Consequently, when CO2-enriched loblolly pine seedlings become photosynthetically-active, they will likely produce biomass at greater rates than those exhibited by seedlings growing under current CO2 concentrations.
Five years later, LaDeau and Clark (2006a) conducted a follow-up study that extended the work they had begun five years earlier. This latter effort revealed that "carbon dioxide enrichment affected mean cone production both through early maturation and increased fecundity," so that "trees in the elevated CO2 plots produced twice as many cones between 1998 and 2004 as trees in the ambient plots." They also determined that the trees grown in elevated CO2 "made the transition to reproductive maturation at smaller [trunk] diameters," and that they "not only reached reproductive maturation at smaller diameters, but also at younger ages." By 2004, for example, they say that "roughly 50% of ambient trees and 75% of fumigated trees [had] produced cones." In addition, they observed that "22% of the trees in high CO2 produced between 40 and 100 cones during the study, compared with only 9% of ambient trees."
"In this 8-year study," in the words of the two researchers, "we find that previous short-term responses indeed persist," in contradiction of the opinions of biological pessimists who downplay the immense biological benefits of atmospheric CO2 enrichment. In addition, they note that "P. taeda trees that produce large seed crops early in their life span tend to continue to be prolific producers (Schutlz, 1997)," and they conclude that this fact, together with their findings, suggests that "individual responses seen in this young forest may be sustained over their life span."
In a concurrent report, LaDeau and Clark (2006b) additionally analyzed the seed and pollen responses of the loblolly pines to atmospheric CO2 enrichment, finding that the "trees grown in high-CO2 plots first began producing pollen while younger and at smaller sizes relative to ambient-grown trees," and that cone pollen and airborne pollen grain abundances were significantly greater in the CO2-enriched stands. More specifically, they found that "by spring 2005, 63% of all trees growing in high CO2 had produced both pollen and seeds vs. only 36% of trees in the ambient plots."
This propensity for elevated concentrations of atmospheric CO2 to both hasten and increase the production of pollen by this valuable timber species bodes well for naturally-regenerated loblolly pine stands that have a continuous range from Maryland south to Florida and west to Texas, where they currently are profoundly seed-limited for at least 25 years. In addition, the researchers say that precocious pollen production "could enhance the production of viable seeds by increasing the percentage of fertilized ovules," and that "more pollen disseminated from multiple-source trees may also increase rates of gene flow among stands, and could further reduce rates of self-pollination, indirectly enhancing the production of viable seeds." Also of importance, in view of the negative twists that climate alarmists attempt to put on even overwhelmingly positive research findings, they say that "pine pollen is not a dangerous allergen for the public at large."
Another major study of the reproductive responses of trees to elevated levels of atmospheric CO2 was conducted at the Kennedy Space Center, Florida, USA, where in 1996 three species of scrub-oak (Quercus myrtifolia, Q. chapmanii, and Q. geminata) were enclosed within sixteen open-top chambers, half of which were maintained at 379 ppm CO2 and half at 704 ppm. Five years later -- in August, September and October of 2001 - Stiling et al. (2004) counted the numbers of acorns on randomly selected twigs of each species, while in November of that year they counted the numbers of fallen acorns of each species within equal-size quadrates of ground area, additionally evaluating mean acorn weight, acorn germination rate, and degree of acorn infestation by weevils.
So what did they find?
Acorn germination rate and degree of predation by weevils were unaffected by elevated CO2, while acorn size was enhanced by a small amount: 3.6% for Q. myrtifolia, 7.0% for Q. chapmanii, and 7.7% for Q. geminata. Acorn number responses, on the other hand, were enormous, but for only two of the three species, as Q. geminata did not register any CO2-induced increase in reproductive output, in harmony with its unresponsive overall growth rate. For Q. myrtifolia, however, Stiling et al. report "there were four times as many acorns per 100 twigs in elevated CO2 as in ambient CO2 and for Q. chapmanii the increase was over threefold." On the ground, the enhancement was greater still, with the researchers reporting that "the number of Q. myrtifolia acorns per meter squared in elevated CO2 was over seven times greater than in ambient CO2 and for Q. chapmanii, the increase was nearly sixfold."
Stiling et al. say these results lead them to believe "there will be large increases in seedling production in scrub-oak forests in an atmosphere of elevated CO2," noting that "this is important because many forest systems are 'recruitment-limited' (Ribbens et al., 1994; Hubbell et al., 1999)," which conclusion echoes that of LaDeau and Clark with respect to loblolly pines. Therefore, and if other trees behave similarly, it would appear that the rising CO2 content of earth's atmosphere will be a great boon to the regenerative prowess of the planet's forests.
A third major study of CO2 effects on seed production in trees has been conducted at the FACE facility near Rhinelander, Wisconsin (USA), where young paper birch (Betula papyrifera Marsh.) seedlings were planted in 1997 and have been growing since 1998 in open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 560 ppm, as well as at atmospheric ozone (O3) concentrations of either ambient or 1.5 times ambient. There, Darbah et al. (2007) collected many types of data pertaining to flowering, seed production, seed germination and new seedling growth and development over the 2004-2006 growing seasons; and as they describe it, "elevated CO2 had significant positive effect on birch catkin size, weight, and germination success rate." More specifically, they note that "elevated CO2 increased germination rate of birch by 110%, compared to ambient CO2 concentrations, decreased seedling mortality by 73%, increased seed weight by 17% [and] increased [new seedling] root length by 59%."
Conversely, the six researchers found that "the opposite was true of elevated O3," as it "decreased the germination rate of birch by 62%, decreased seed weight by 25%, and increased [new seedling] root length by [only] 15%." In addition, they note that "the seeds produced under elevated O3 had much less stored carbohydrate, lipids, and proteins for the newly developing seedling to depend on and, hence, the slow growth rate." They also report that "the total number of trees that flowered increased by 139% under elevated CO2 [but only] 40% under elevated O3." Likewise, they say that "with respect to the quantity of flowers produced, elevated CO2 had 262% increase, while that of elevated O3 had [only a] 75% increase compared to the control treatment."
In discussing their results, Darbah et al. say their findings imply that seedling recruitment in paper birch "will be enhanced under elevated CO2 but reduced under elevated O3," which is another important reason to hope that the air's CO2 content continues to climb as long as the atmosphere's ozone concentration is in a significantly ascending mode, which hope is also justified by the several findings of the researchers who have studied aspects of tree reproduction at the Duke Forest and Kennedy Space Center.
Darbah, J.N.T., Kubiske, M.E., Nelson, N., Oksanen, E., Vaapavuori, E. and Karnosky, D.F. 2007. Impacts of elevated atmospheric CO2 and O3 on paper Birch (Betula papyrifera): Reproductive fitness. The Scientific World JOURNAL 7(S1): 240-246.
Hubbell, S.P., Foster, R.B., O'Brien, S.T., Harms, K.E., Condit, R., Wechsler, B., Wright, S.J. and Loo de Lao, S. 1999. Light-gap disturbances, recruitment limitation, and tree diversity in a neotropical forest. Science 283: 554-557.
Hussain, M., Kubiske, M.E. and Connor, K.F. 2001. Germination of CO2-enriched Pinus taeda L. seeds and subsequent seedling growth responses to CO2 enrichment. Functional Ecology 15: 344-350.
LaDeau, S.L. and Clark, J.S. 2001. Rising CO2 levels and the fecundity of forest trees. Science 292: 95-98.
LaDeau, S.L. and Clark, J.S. 2006a. Elevated CO2 and tree fecundity: the role of tree size, interannual variability, and population heterogeneity. Global Change Biology 12: 822-833.
LaDeau, S.L. and Clark, J.S. 2006b. Pollen production by Pinus taeda growing in elevated atmospheric CO2. Functional Ecology 20: 541-547.
Ribbens, E., Silander, J.A. and Pacala, S.W. 1994. Seedling recruitment in forests: calibrating models to predict patterns of tree seedling dispersion. Ecology 75: 1794-1806.
Schutlz, R.P. 1997. Loblolly Pine - The Ecology and Culture of Loblolly Pine (Pinus taeda L.). USDA Forest Service Agricultural Handbook 713. USDA Forest Service, Washington, DC, USA.
Stiling, P., Moon, D., Hymus, G. and Drake, B. 2004. Differential effects of elevated CO2 on acorn density, weight, germination, and predation among three oak species in a scrub-oak forest. Global Change Biology 10: 228-232.