Redwood Reader

36.Effects of Elevated CO2 on Tree Seeds
http://www.co2science.org/print.php3

Seeds (Trees) -- Summary

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 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-regenerated loblolly pine stands of the southeastern United States, where LaDeau and Clark report these 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 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 photosynthesize and produce biomass at greater rates than those exhibited by seedlings growing under current ambient CO2 concentrations.
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 that 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. If other trees behave similarly, it would appear that the rising CO2 content of earth's atmosphere will be a boon indeed to the regenerative prowess of the planet's forests. More research results from other tree species are thus anxiously awaited.
References
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.
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.
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.

Page printed from: http://www.co2science.org/subject/s/summaries/seedstrees.htm Copyright © 2004. Center for the Study of Carbon Dioxide and Global Change

Commentary: Here again we have evidence showing the advantages of heightened carbon dioxide levels for recovering forests. The trees are setting seeds earlier, the seeds are more viable and they have higher survival rates. CO2 Science Magazine does an excellent job attacking the science behind climate change arguments but it fails to recognize the critical issue of fungi storing carbon in the soil and its global impacts, or the consequences when this important relationship is ignored or neglected.
Opportunities to repeat these experiments with local trees present themselves. The concept of FACE experiments would seem like a good idea. One also wonders if enriching selected seed trees with CO2 for a boost to create more viable seedlings and give better results in planting out, especially in situations where the object is getting trees established, for instance, in erosion control or slide stabilization plantings.
This brings us back to our illusory Institute of Mountain Agriculture. What is the effect of elevated on grafted material? Will elevated CO2 provide better survival of hardwood grafts in forest trees? Will selected and grafted forest trees ever become a large scale reality? Will the growing power of hardwoods be harnessed by annual harvesting? What is the mycorhizzial relation in mixed forests? Plenty of questions, but slowly they are being refined.