Proceedings of the UNESCO - University of Tsukuba International Seminar on Traditional Technology for Environmental Conservation and Sustainable Development in the Asian-Pacific Region, held in Tsukuba Science City, Japan, 11-14 December, 1995.
Editors: Kozo Ishizuka, D. Sc. , Shigeru Hisajima, D. Sc. , Darryl R.J. Macer, Ph.D.
Such human actions have been highly successful in providing food, fuel and fiber for an ever-growing human population which recently exceeded 5.5 billions in the world. Yet they have been achieved at considerable environmental cost. Debits include major depletions in climax vegetation cover, especially at the cost of natural forests in both temperate and tropical regions; the extinction of many thousands of plant and animal species; and a worldwide reduction in biodiversity.
As is well known, deforestation in tropical regions is one of the present serious global environmental problems. According to an FAO survey, an annual mean rate of the deforestation in topical regions was 150,000 km2 between 1981 and 1991. In addition to a huge amount of fossil fuel consumption, the deforestation contributes a steady increase of the atmospheric CO2 concentration which leads to global warming. Causes of deforestation are burning of forests to make farmland, felling of trees for making charcoals, conversion into pastures, and felling of trees for commercial purposes. Behind the deforestation in tropical regions are poverty and the rapid increase in human population.
A hypothesis concerning a major cause of the deforestation, that is, land use change from forests to herbaceous communities such as croplands and pastures will be discussed from the ecological point of view.
All life on land, including we humans, depends on photosynthate produced by green plants. Using light energy from the sun, carbon dioxide from the air, and water and nutrients from the soil, plants are able to make complex organic compounds that sustain the tremendous richness of terrestrial ecosystems.
Forests and grasslands are the two major types of ecosystem to function as areas of primary production on land. The ecological characteristics of these two types will be compared from the viewpoint of dry-matter production economy which was initiated by Boysen-Jensen (1932) and Monsi and Saeki (1953).
Biomass and primary productivity on the earth have been examined comprehensively through the International Biological Program (IBP), which was conducted over a ten year period from 1965 to 1974. As listed in Table 1, the biomass and primary productivity of each biome from tropical rain forest to wetland were compiled by Whittaker and Likens (1973). They estimated 827 Pg Carbon (=1590 Pg d.w.) for total terrestrial biomass and 743 Pg Carbon for total forest biomass, respectively. Forest ecosystems ranging from tropical rain forests to boreal coniferous forests, which occupy about 1/3 of the total land area, amount to about 87% of the total terrestrial biomass. On the other hand, herbaceous ecosystems including natural grasslands such as savanna, temperate grasslands and similar types, and arable lands constitute a tiny portion of the total biomass less than 10%, although they occupy about 48 million km2, which is comparable with forested areas.
Increasingly the products of photosynthesis are being diverted to support humans: we now harvest, directly or indirectly, plant-derived materials with agricultural crops of about 2000 species. It is estimated that 2.7 billion tons of food produced by these crops were consumed by the world population (exceeding 5 billions in 1985), accounting for around 10% of terrestrial primary production. In the case of the production statistics of the ten leading crops of the world (Figure 1), we should note that almost all primary products are derived from herbaceous crops. The three most important crops are wheat, rice and maize, followed by the potato. Half of the ten crops are cereals (wheat, rice, maize, barley and millet), and three are tubers (potato, cassava and sweet potato). Nine crops including soybean are herbaceous and only the ninth crop, the grape, is arboreal. In addition to the grape, perennial arboreal crops include banana, orange, apple, and coconut, but their production is negligibly small, compared with that of herbaceous crops.
Why do we humans utilize mainly herbaceous crops for food, even though forest ecosystems occupy an overwhelming majority of biomass in the world? We must distinguish biomass and primary productivity of a plant community to consider this question. As shown in Table 2, which lists higher records of net primary productivity (NPP) obtained from various plant populations or communities including crops, natural grasslands and forests, biomass and primary productivity do not always coincide with each other. Among the examples, a nepier grass population in Puerto Rico had the highest annual NPP of 96.4 ton/ha. NPP values higher than 70 ton/ha were restricted to crops or a natural grassland having a C4 photosynthetic mechanism. On the other hand, the highest NPP value of forest was 28.6 ton/ha from a tropical rain forest in Thailand. This is only 30% of the highest value of the nepier grass and lower than those of such C4 crops as sugar beet or alfalfa.
From the present overview, it can be concluded that the observed characteristics of carbon dynamics in a savanna community are caused by differences in production- reproduction parameters, particularly the high mortality of non-photosynthetic organs, compared with those in forest communities. Such ecologically significant characteristics must be common for all herbaceous communities, including temperate grasslands and crops, which have a tiny fraction of non-photosynthetic organs. I suggest that this may be the answer to the initial question mentioned as to why herbaceous crops are major food sources and arboreal crops are minor ones for humans. Deforestation and resulting land-use conversion to arable land is one of the most serious global environmental issues, and its ultimate cause may be the ever-growing human population, currently increasing by a million every five days. The hypothesis presented here should be confirmed by further studies using much more reliable data for the ecophysiological parameters of field- grown plants. Such an approach will deepen our understanding of causal relationships determining the productivity and biomass of different life forms peculiar to different climatic regions, and this will give us a means for mitigating food shortage, which will be more and more accelerated in the near future by ongoing global environmental changes.
Table 1: Biomass and primary productivity of each biome (Whitaker & Likens, 1973)
Table 2: High records of net primary productivity (NPP) obtained from various plant
populations or communities