Harvesting the Biosphere: What We Have Taken from Nature

2. Biomass Stores: Means and Extremes

2 Biomass Stores: Means and Extremes In contrast to regularly conducted inventories of wood volumes in commercially exploited forests and the frequent (now satellite-aided) monitoring of crop productivity , accurate quantifications of phytomass in natural ecosystems remain relatively uncommon. But they do support some easily defensible generalizations, illustrate some important exceptions, and help correct some stereotypes. These assessments are usually presented in units of dry matter per unit area (g/m2 or t/ha) or as the total mass of carbon (g C/m2 or t C/ha). Our knowledge also remains highly uneven in spatial terms as temperate ecosystems have been studied much more intensively than the tropics. Integrations on biome, continental, and planetary scales have the highest margins of error, and the quantification of heterotrophic biomass is more tenuous than that of phytomass. Phytomass and Zoomass Phytomass stores vary greatly among biomes, large areas with similar climates whose global distribution shows distinct latitudinal regularities. The poleward sequence of biomes starts with tropical rain forests, the most diverse terrestrial biome: they fill the entire basin of the Amazon and the Congo, as well as most of Central America and West Africa, and spread across Southeast Asia and Indonesia. Tropical forests give way to tropical grasslands, in Africa from the Atlantic to Sudan and from Angola to Mozambique, in South America both south and north of Amazonia, and in Australia in the northern third of the continent. In turn, tropical grasslands blend into deserts, a biome that is relatively restricted in the Americas but spreads across Africa from Mali to Sudan and then continues across the Arabian Peninsula and Iran to western India and north of the Hindu Kush and Tibetan plateau into the heart of Asia, then all the way to northern China; desert also fills most of Australia’s interior. 16 Chapter 2 The tropical rain forests of South America, Africa, and Southeast Asia are the biosphere’s largest depositories of phytomass, with the richest plant communities, arrayed in several vertical layers, storing as much as 1,000 t of dry phytomass per hectare. This photograph of Amazonian forest in Peru is available at http://upload.wikimedia.org/wikipedia/commons/f/ f6/Amazonian_rainforest_2.JPG. Biomass Stores 17 Temperate grasslands fill the semiarid and arid centers of North America and Asia, while temperate deciduous forests were the natural climax vegetation across most of Atlantic and Eastern Europe, southeastern North America, and parts of northeastern Asia. Extensive boreal forest (taiga) spreads across North America from the Pacific coast to the Atlantic and in Europe from Scandinavia to the Sea of Okhotsk. Treeless tundra is the least biodiverse biome found along the northern fringes of North America and Eurasia. The perpetually dark and cold abyssal, nutrient-poor waters of the open ocean are the aquatic counterpart of desert. Highly diverse marine biomes are limited to relatively small areas of coral reefs and kelp forests, to the regions of upwelling (where rising cold water replenishes surface nutrients) along the western coast of the Americas and Africa, and to estuaries of major rivers. Before citing any numbers for phytomass densities I must stress several important yet often ignored facts, none of them more critical than our surprisingly limited basis for accurate assessments of global phytomass. Houghton, Hall, and Goetz (2009, 1) put it best: “Our knowledge of the distribution and amount of terrestrial biomass is based almost entirely on ground measurements over an extremely small, and possibly biased sample, with many regions still unmeasured. Our understanding of changes of terrestrial biomass is even more rudimentary.” This conclusion is true even when we take into account the latest advances in quantifying carbon in tropical forests (Saatchi et al. 2011), although the creators of many global vegetation models and the even more complex simulations of biosphere-atmosphere interactions want us to believe that their understanding has become deep enough to recreate faithful virtual versions of Earth’s biomass in silico Besides the limited basis of actually measured phytomass stocks, we have to reckon with a large variability (spatial and temporal) of phytomass densities. The possibilities of bias resulting from destructive sampling of small plots were noted in the last chapter, but even the best available large-scale mapping of forest ecosystems frequently ends with very different totals (Houghton et al. 2001). The range of phytomass densities spans three orders of magnitude (the difference between the northernmost tundras and giant trees in the rain...