DeLucia, E.H., Thomas, R.B. and Ward, J.K. (Eds.) 1999. Critical Assessment of the Response of Forest Ecosystems to Elevated Atmospheric Carbon Dioxide. Published as Tree Physiology, Volume 19.
Literally thousands of scientific studies on individual plants exposed to elevated levels of carbon dioxide show that atmospheric CO2 enrichment nearly always leads to increased plant growth and development, even when conditions for growth are less than optimal due to resource limitations or environmental stresses. More recently, scientific studies have tried to elucidate ecosystem responses to atmospheric CO2 enrichment in an attempt to determine how entire biomes may function as the CO2 content of the air continues to rise. In October 1997, Duke University held a workshop that highlighted research projects investigating forest responses to elevated CO2 concentrations. The results of this workshop were peer-reviewed and subsequently published in a special edition of Tree Physiology, which essentially makes a nice "book" with fourteen "chapters" on this topic. In our Vol.2, No.8 issue, we reviewed the contents of the first seven of these articles; and in this issue (Vol.2, No.9), we review the last seven.
Chapter eight reports the effects of elevated CO2 on xylem embolism in five different Mediterranean tree species growing near a CO2-emitting spring located in central Italy. The authors determined that although the degree of xylem embolism differed between species, it was very small between trees growing near the CO2 springs and those growing some distance away in ambient CO2 concentrations. Hence, although elevated CO2 can sometimes nudge the degree of xylem embolism in either direction, the magnitude of this phenomenon is relatively small and species-dependent.
The next chapter examines loblolly pine ecosystem responses to elevated CO2 within the constraints of a computer model that simulates forest growth dynamics. According to the model, CO2 enrichment stimulated annual net primary productivity by 13, 10, and 7.5% after one, two, and ten years of CO2 exposure, respectively. In addition, after 10 simulated years of treatment, CO2-enriched ecosystems sequestered 4 and 9.2% more carbon in their trees and soil than that sequestered within ecosystems subjected to ambient CO2 levels. The nominal results of this computer assessment caused the authors to conclude that photosynthetic stimulation and carbon sequestration resulting from atmospheric CO2 enrichment would merely elicit a transient response in loblolly pine ecosystems. However, actual long-term experiments will be needed to determine whether these model-derived predictions are correct.
Chapter 10 investigates the response of Scots pine to elevated CO2 after two years of growth in open-top chambers. The authors determined that CO2 enrichment significantly advanced the date of bud burst in both years of the experiment, thereby lengthening the growing season and increasing the amount of time for carbon sequestration by seedlings. Elevated CO2 also increased total needle area in both years by stimulating shoot number, length, and individual needle area. After analyzing needle carbohydrate contents, they reported that CO2-enriched needles contained 10 and 25% more sugars and starch, respectively, than needles produced under ambient CO2 concentrations. Relative growth rates of CO2-enriched seedlings were significantly higher than those of ambiently-grown seedlings during the first year of the experiment. However, this initial stimulation decreased during the second year of the study.
The following chapter summarizes what is know about elevated CO2 effects on leaf litter quality and decomposition rates. After reviewing the literature, the authors concluded that in short-term CO2 enrichment studies, elevated CO2 significantly decreases leaf nitrogen levels in a species-dependent manner. However, in their evaluation of long-term natural studies, they found that elevated CO2 did not decrease leaf nitrogen contents in trees growing near a CO2 spring. With respect to leaf litter decomposition rates, the published literature revealed no apparent trend in how elevated CO2 affects this process; about 25% of the studies showed increased rates of decomposition, 25% showed decreased rates, and the remaining 50% of the studies displayed no change in decomposition rates.
In chapter 12, the effects of elevated CO2 on rhizosphere processes are reviewed by analyzing the peer-reviewed literature addressing this topic. It was determined that atmospheric CO2 enrichment generally stimulates photosynthetic rates in plants, which ultimately leads to greater carbon exudation into the soil. Depending upon the plant species and certain soil conditions, increased carbon deposition in the rhizosphere can cause an increase, decrease, or have no effect on the amount of organic matter and nutrient mineralization within soils. Due to these varied responses, the author proposed various feedback mechanisms, which may account for the broad range of empirical observations.
Chapter 13 investigates simulated nitrogen cycling responses to elevated CO2 in an evergreen and mixed deciduous forest using a nutrient cycling model. According to the model, nitrogen limitation precluded growth stimulation in an evergreen forest, while slightly increasing it in a deciduous one. In light of published empirical data, however, it is clear that lack of nitrogen need not preclude CO2-induced increases in plant growth.
In the final chapter of the "book," effects of elevated CO2 and water stress on longleaf pine are documented. After growing seedlings for 20 months in open-top chambers with atmospheric CO2 concentrations of 365 or 730 ppm, it was determined that elevated CO2 significantly increased the biomass of needles, stems, taproots, lateral roots, and fine roots under well-watered conditions. Moreover, when seedlings were water-stressed, atmospheric CO2 enrichment caused even greater biomass increases for each of these measured parameters. When water was not limiting growth, for example, needle biomass increased by about 18% with elevated CO2, while it increased by 75% when seedlings were water-stressed.
In conclusion, this book describes many important aspects of forest response to atmospheric CO2 enrichment and defines several areas that deserve greater research attention. As its suggestions are implemented, we can anticipate reading many new papers in the years to come that will help us understand why most of earth's forests are responding so favorably to the increasing CO2 content of the air.
Reviewer: Dr. Keith E. Idso, Vice President
Last updated 1 May 1999