An historic journey into the terrestrial carbon cycle


Beerling, D. J. & Woodward, F. I. (2001) Vegetation and the terrestrial carbon cycle. Modelling the first 400 million years. Cambridge University Press, Cambridge, UK. x + 405 pp., figs, tables, index. Hardback: price £100, US$150, ISBN 0-521-80196-6.

This book is, as admitted by the authors, an ‘idiosyncratic view of terrestrial plant life since its first appearance in the Silurian period’. Indeed, rather than step through each period and epoch with a reconstruction of the past based on proxies, this book presents an interesting perspective on the evolution of terrestrial vascular plants through the eyes of plant ecophysiology and the carbon cycle. The authors provide the reader with a rich and thorough primer on terrestrial carbon cycling, with particular emphases on the processes of photosynthesis and respiration. It is not, however, for the faint of carbon-heart. The reader should be relatively well schooled in the underlying ecophysiological paradigms of the terrestrial carbon cycle, which are provided in introductory chapters in great detail.

The general flow of this book is outstanding. The reader is first guided through first principles of terrestrial carbon cycling and the underlying assumptions that must be made in order to hindcast well into the geologic past. Introductory chapters provide background in global geochemical, morphological, marine and terrestrial events that probably directed the evolution and distribution of terrestrial vegetation. A thorough primer is provided on our current understanding of how plants process carbon, with regard to their biochemistry and sensitivity to climate and atmospheric CO2 concentrations, as well as how contemporary global models interpret and predict these processes. A discussion on how carbon and oxygen isotopes provide insight into the past is also a welcome piece of the puzzle.

Once a thorough introduction to the processes and how they probably relate to the geologic past is covered, the reader is guided through history in distinct chapters that describe, via model simulations, important events that are likely to have driven the evolution of plants and their global functional and structural distributions with regard to plate tectonics, catastrophic impacts and glacial/interglacial cycles. Specific in-depth analyses include the late Carboniferous, Jurassic, Cretaceous, Eocene, Quaternary and projections into the future. Simulations are often compared to available palaeo proxies including, but not limited to geochemical records, sediments, fossilized tree rings, pollen analyses, fossil plant records and geologic parent material to infer climate and plant functional type distributions at the global scale. The authors use a single model to interpret their results, providing consistency in their analyses throughout the entire text.

The reader should be cautioned, however, of a carbon dioxide-centric bias in the authors’ presentation. Here I give three examples. (1) During discussions on climatic impacts on vegetation in the Carboniferous, the authors argue that decomposition rates (particularly during periods of anaerobic (waterlogged) conditions), oxygen consumption and release of carbon dioxide by heterotrophic respiration would be reduced. In addition, reductions in atmospheric CO2 would have resulted from increased weathering rates, stimulated by vegetation activity. Their conclusion is that this scenario provides irrefutable evidence supporting the onset of glaciation, however, they neglect to mention how CH4 production by methanotrophs might have offset cooling and decreasing atmospheric CO2 concentrations. (2) The authors suggest that, during the late Silurian and early Devonian, when the land surface was being colonised by terrestrial plants, the role played by large Devonian vascular plants in the cycling of carbon was probably substantial, with deep root penetration into soils, the colonization of uplands and primary succession significantly enhancing rates of chemical weathering and atmospheric CO2 draw down. Their assumptions about primary succession on regolith overlook the importance of how nitrogen limitations might have stimulated associations of plants and nitrogen-fixing organisms. This would have provided an interesting discussion on how and when such associations might have evolved. (3) The authors discuss the consequences of high-impact events on climate in the Cretaceous and how increased dust would have decreased irradiance, thereby decreasing mean annual temperatures and increasing atmospheric CO2 concentrations, but do not include the possibility of increased drawdown of CO2 through fertilization of marine ecosystems. Again, a bias in the authors’ central tendencies, towards explaining the palaeo past through atmospheric CO2 concentrations and climate interactions, leaves the reader hanging with many unanswered questions. The authors do, however, acknowledge modelling weaknesses, particularly with regard to the lack of feedbacks between vegetation and climate.

In summary, this book provides an interesting and sometimes unexpected journey into the palaeo past through the eyes of the terrestrial carbon cycle and our current understanding of how climate and atmospheric CO2 concentrations influence the kinetic and enzymatic reactions of photosynthesis and its associated processes. It is well worth a read for an audience educated in basic principles of plant ecophysiology and provides insight into how the Earth System may have evolved into the present and might develop in the future.