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. Thus, we will briefly review the content of each of these chapters, beginning with the first seven in this current book review.
The first chapter chronicles plant responses to CO2 enrichment. It reviews how molecular, physiological, and ecological processes can impact a plant's ability to sequester additional carbon during exposure to elevated levels of CO2. The authors of this chapter also propose research avenues that would likely increase our understanding of how earth's ecosystems would respond to atmospheric CO2 enrichment.
The next two chapters deal with pine and oak tree photosynthetic responses to CO2 enrichment. In particular, they address how growth in elevated CO2 results in the down regulation or acclimation of photosynthesis in these species. Although photosynthetic acclimation occurred in ponderosa pine, as evidenced by a 38% decrease in leaf rubisco content and a 36% decrease in rubisco activity, net photosynthesis was still 53% greater in trees grown in elevated CO2 (700 ppm), compared to trees grown at 350 ppm, after six years of CO2 enrichment. In addition, when exposed to twice-ambient CO2 concentrations for approximately three months, two oak species exhibited rates of photosynthesis that were 73 and 51% greater than their respective counterparts grown in ambient CO2. For these two species, that which exhibited the greatest photosynthetic enhancement also displayed a 40% reduction in rubisco activity when grown at elevated CO2, while that which had a 51% increase in photosynthesis displayed no significant change in the activity of this enzyme. Thus, these two chapters show that species can acclimate to elevated CO2 by various degrees, and that acclimation itself does not need to negate the photosynthetic enhancement that almost universally results from atmospheric CO2 enrichment.
The next chapter explores the phenomenon of photosynthetic acclimation by investigating the influence of sink strength on this process. Researchers removed several actively growing shoot tips from branches of loblolly pine, and girdled some of them as well, to decrease sink strength and photosynthate transport from these branches, respectively. It was hypothesized that these physical manipulations would induce photosynthetic acclimation due to the accumulation of sugars in branch needles that otherwise might have been transported to intact terminal regions on the same branch or exported to sinks located in other areas of the tree. This hypothesis was supported by an observed accumulation in photosynthetically-derived sugars within manipulated branch needles. This phenomenon most likely led to end-product inhibition of photosynthesis, which was manifested by a significant decrease in rubisco activity. Although this study was conducted under ambient CO2 concentration, it revealed that atmospheric CO2 enrichment could lead to photosynthetic down regulation in this tree species if the excess carbohydrate produced under elevated CO2 conditions is not mobilized from source needles to actively growing plant sinks.
The next chapter investigates the photosynthetic response of Douglas-fir seedlings to combinations of elevated CO2 and temperature. In this particular study, elevated CO2 did not have a significant effect on the light compensation point or the rates of photosynthesis and respiration. Thus, carbon uptake at a given irradiance was similar regardless of CO2 concentration. In addition, there were no significant interactions between CO2 and temperature, in contrast to the many beneficial interactive effects these variables often have on photosynthesis and growth. Elevated temperature, however, significantly increased the light compensation point and the rates of photosynthesis and respiration, indicating that rising temperature alone may substantially stimulate daily rates of net carbon uptake in this tree species.
The next chapter analyses numerous published results in an attempt to determine whether observed CO2-induced decreases in respiration are the result of a CO2-dependent suppression of important respiratory enzyme activities. After reviewing the literature, the authors concluded that the CO2-induced suppression of key respiratory enzymes is not a primary mechanism responsible for causing decreased respiration, although it may contribute to this phenomenon to a lesser degree.
The last of the first seven chapters of the book reports on the effects of elevated CO2 on water relations in an oak species growing near a CO2-emitting spring located in central Italy. Trees growing near the CO2 spring exhibited lower rates of maximum stomatal conductance during both the wet and dry periods of the study, relative to trees growing away from the spring in ambient air. In addition, sap flow measurements indicated that water fluxes from naturally CO2-enriched trees were less than those from trees growing at control sites, suggesting that elevated CO2 reduced their transpiration rates and overall water use, which is consistent with the findings of most papers that study the effects of CO2 on plant water relations.
Reviewer: Dr. Keith E. Idso, Vice President
Last updated 15 April 1999