Plant Responses to Elevated CO2: Evidence from Natural Springs - Review

Ecology, Jan, 1999 by Yiqi Luo

Raschi, A., E Milglietta, R. Tognetti, and Paul Richard van Gardingen, editors. 1997. Cambridge University Press, New York. xiv 272 p. $69.95 ISBN: 0-521-58203-2.

Successful research often involves a search for the proper material and/or settings to address critical scientific issues. The field of molecular genetics, for instance, has used Escherichia coli as a model system to illustrate principles of DNA and RNA transcription. In ecology, different sizes of islands off seashore have been employed to understand relationships between species migration and population/community structure, resulting in the subdiscipline of island biogeography. In order to examine long-term plant and ecosystem responses to rising atmospheric C[O.sub.2] concentration in the natural world, global change scientists have also searched for new approaches and natural settings. Plant responses to elevated C[O.sub.2] was born out of such a search and presents research results from natural CO: springs. These natural springs provide a unique opportunity for global change research, particularly in relation to long-term issues.

Plant responses to elevated C[O.sub.2] have been extensively studied during the past two decades, primarily using growth chambers and greenhouses as controlled environments. More recently open-top chambers and free-air C[O.sub.2] enrichment facilities in natural ecosystems have been employed. Although past research has greatly improved our understanding of short-term responses of physiological processes to elevated C[O.sub.2], our capability to address long-term issues has been extremely limited because most of the experiments last only from several weeks to a few years. Salient long-term issues, which potentially can be addressed through the use of natural springs, include: 1) regulation of physiological acclimation through slow plant and soil processes, 2) genetic adaptation to a gradual increase in atmospheric C[O.sub.2], 3) changes in population structure and species composition of plant communities, and 4) dynamics of soil organic matter and nutrient supply.

Plant responses to elevated C[O.sub.2] devotes the majority of its 18 chapters to physiological acclimation. Two basic approaches are presented: one is to measure physiological activities of plants along C[O.sub.2] gradients, and the other is to conduct transplant experiments. In the latter case, seeds or rhizomes were collected from sites with different C[O.sub.2] exposure and then grown in controlled environments for physiological studies. While data are minimal in most of the chapters, measurements along C[O.sub.2] gradients suggest that a few consistent patterns occur in elevated C[O.sub.2]. These include 1) reduction in stomatal conductance and density, 2) increase in water use efficiency, and 3) increase in plant biomass growth. The chapter by M. C. Fordham et al. presents a rigorous study using the transplant approach. The authors found that the faster growth of plants adapted to high C[O.sub.2] sites is strongly correlated with seed weights. Unfortunately, none of the chapters examines adaptation using genetic and/or evolutionary approaches.

Although several chapters describe the vegetation around C[O.sub.2]-enriched springs, only E Selvi addresses the question of whether or not elevated C[O.sub.2] would induce changes in plant community structure. Selvi's survey of herbaceous communities surrounding the gas vents of six springs in Italy showed that these local grass communities have a consistent and homogeneous species composition, dominated by dense stands of Agrostis canina subsp. Monteluccii. Since this species is usually a successful colonizer in hostile environments (e.g., peatlike soils with pH ranging between 2.4 and 3.7), the homogeneous community structure surrounding all the springs may be evidence of adaptation to local and peculiar edaphic conditions, rather than adaptation to enriched C[O.sub.2] environments. Selvi is not optimistic about using natural springs to study community structure in elevated C[O.sub.2], at least in those areas where the survey was conducted.

Plant responses to elevated C[O.sub.2] strongly depend on other ecosystem processes (e.g., soil nutrient dynamics and supply). The chapter by P. Ineson and M. F. Cotrufo demonstrates that litter nitrogen concentration of trees from C[O.sub.2] springs is lower than that from a control site, leading to slower decomposition rates. A commendable aspect of that chapter is the authors' review on decomposition studies in elevated C[O.sub.2], which places their springs study in perspective. Other ecosystem processes (including soil organic matter dynamics, soil chemistry, and physical properties) may be beyond the scope of the book but certainly warrant deliberate investigation in order to understand ecosystem feedback in regulating plant responses to rising atmospheric C[O.sub.2].

Plant responses to elevated C[O.sub.2] presents ample background information on natural springs which is particularly useful to those scientists who may utilize such springs in their own research. Three types of C[O.sub.2]-enriched environments in the natural world are described. They are degassing of lakes, seams of burning coal deposits, and mineral springs where C[O.sub.2] gas is leaked usually from fractures. The chapters by G. Etiope and by G. Etiope and S. Lombardi provided useful guidelines for identifying and monitoring leakage of C[O.sub.2] and other volatile contaminants based on soil-gas, exhalation, and groundwater surveys.