How Productive Is Your Planet?


Terrestrial Global Productivity. Roy, J., B. Saugier, and H. A. Mooney , editors . 2001 . Academic Press , San Diego, CA . 573 pp. $99.95 (hardcover) . ISBN 0-12505-290-1 .

Justus von Liebig (1862) measured the productivity of part of his garden in the 1860s and estimated that the net production of the world's terrestrial plants should be on the order of 125 Pg of biomass (62 Pg of carbon) per year (1 Pg = 1015 g). One-hundred ten years later, Whittaker and Likens (1973) estimated that the terrestrial part of the planet fixed about 55 Pg of carbon per year, based on a synthesis of reported rates of production from a few well-studied sites around the world, extrapolated to the area covered by each type. As more information accumulated after these pioneering estimates, it became clear that (1) ecologists often choose sites to sample subjectively rather than with random samples, and (2) issues of scaling from plots to the planet are intricate and problematic.

Terrestrial Global Productivity presents the current best estimates of the productivity of terrestrial ecosystems. Detailed pictures of production at specific locations are developed, along with global-scale stories. The first part of the book devotes seven chapters to examining the processes that influence the growth of plants, including rates of respiration, controls on nutrient cycling, and the importance of grazing and browsing animals. The second part divides the terrestrial portions of the planet into nine major types of ecosystems, and the third part examines the many ways that scientists extrapolate rates from small areas to the global total.

Current work in the area of global assessment of production has moved beyond summing the anecdotal examples of production from intensive research sites to embrace geographically explicit models of production and broad-scale remote sensing. A key issue in these newer approaches deals with validation and uncertainty. One can easily test the accuracy of a production estimate for a 100-m2 plot, but how can one determine the level of confidence warranted in a global estimate that results from models and remotely sensed data? This question is considered carefully throughout the book. Chapter 19 ( by P. Ciais and collaborators) focuses on combining approaches to estimating global-scale production through cross-comparisons from modeling, remote sensing, seasonal (and latitudinal) dynamics in carbon dioxide concentrations, and natural abundance ratios of several isotopes in the atmosphere.

So how productive is our planet? Figure 1 provides the final estimates from the book. Tropical rainforests are the most productive ecosystems, at a scale of 1 m2 and for the entire globe. Production across all these areas sums to a total net primary production of 62.6 Pg of carbon per year. This is strikingly similar to von Liebig's extrapolation from his garden, but the book notes that current production of our planet may be notably greater than it was in von Liebig's time, perhaps by 10–20%, as a result of human-related changes in atmospheric carbon dioxide and land use. But then again, von Liebig used a garden as his basis for extrapolation, which may be increasingly appropriate for our changing planet.

Figure 1.

The best current estimate of rates of productivity for the major terrestrial ecosystems types on Earth. The upper figure is grams of carbon per square meter, the lower figure integrates across area for a global estimate. The global total is 62.6 Pg of carbon per year (     from data in this book).