Harvesting the Biosphere: What We Have Taken from Nature

1. Biomass: Definitions and Compositions

1 Biomass: Definitions and Compositions The classic life sciences—botany, zoology, plant and animal anatomy and physiology— were for centuries preoccupied with classification. This concern was later extended to life’s assemblages, that is (in ascending order), communities, ecosystems, and biomes. Such a focus is now seen as antiquated: the preoccupation has shifted to the intricacies of genetic makeup and metabolism and to the dynamic processes of evolution and adaptation. An undesirable side effect of this shift has been a lack of attention to the precise meanings of many variables used to describe organisms and their evolution. This declining rigor is not a trivial matter, as even apparently straightforward definitions hide some complications; settling these matters (or explaining clearly how unsettled they must remain) is an essential precondition for using the variables with the least possible ambiguity. This clarification must proceed along two tracks because biomass stores and productivities are not expressed in mass terms only but are also quantified as reservoirs and fluxes of carbon, a choice that requires reliable data on the element’s content of major biopolymers. Key Variables Biomass is the mass of any living organism. The term can be applied on scales ranging from that of tissues and organs (the biomass of leaves, of muscles) through that of individual species (the peak biomass of corn to be harvested for silage, the biomass of migrating wildebeests) and narrowly circumscribed ecosystems (such as the total plankton biomass of a small pond) all the way to that of the aggregates of a biome (for example, the biomass of boreal forests), and on to the planetary level (the biomass of all photosynthesizing organisms). Complications and uncertainties arise as soon as we start analyzing the term, be it according to the two major classes of life forms that it embraces or according to the physical or chemical composition of biomass. 6 Chapter 1 Most of the world’s biomass is in forests, and most of the forest biomass is in the dead wood of large tree trunks. This photograph shows a massive trunk of a giant sequoia (Sequoiadendron giganteum) in the Tuolumne Grove in Yosemite National Park. Photograph by V. Smil. Biomass 7 Life’s great dichotomy is between autotrophs, organisms that can nourish themselves , and heterotrophs, or life forms that must feed on other organisms. And for comparisons among different organisms, their biomasses must be adjusted for widely differing water content and expressed in absolutely dry terms or, alternatively , their carbon content may be calculated using specific conversion rates. Autotrophs (whose sizes span nearly ten orders of magnitude, from the smallest bacteria at 0.01 μm to the tallest trees at more than 100 m) can transform inorganic carbon (drawn overwhelmingly from atmospheric CO2), hydrogen (mostly from H2O), and smaller amounts of nitrogen and mineral nutrients into complex organic compounds. Most autotrophs are phototrophs, organisms that rely on solar radiation to power complex photosynthetic processes, but some bacteria are chemoautotrophs, which derive the needed energy by oxidation of sulfur or H2S, and an even smaller group of facultative autotrophs (most notably methanogenic bacteria) can switch to a heterotrophic existence in the presence of suitable organic substrates. Autotrophic biomass is the correct term for all tissues produced by photosynthesizers and chemotrophs, but I will use a slightly narrower term, phytomass, which refers to both planktonic organisms (cyanobacteria, coccolithophorids, silicoflagellates, diatoms) and macroscopic aquatic autotrophs (algae, sea grasses), the two categories whose direct harvests by humans are rather limited, as well as to all terrestrial autotrophs (that is, mostly to herbs, trees, and shrubs), whose harvests dominate the photosynthate that is used by humans. Because of substantial differences in the moisture content of fresh biomass— phytoplankton cells or young plant shoots are more than 95% water, freshly cut wood trunks contain about 50%, mature cereal grain and dry straw have only about 15% water, and some dry seeds have less than 5% moisture—a comparative accounting should be done only in terms of absolutely dry biomass. Its values are obtained after desiccating the fresh tissues at 104°C–105°C to constant weight. Dry biomass has the density range of 0.2–0.9 g/cm3 , and about 95% of it is organic matter (although minerals can make up as much as 70% of dry phytomass in some phytoplankton species). Conversions to absolutely dry biomass introduce two possibly significant errors. First, no conversions done on a scale larger than...