Introduction
The bodies of living organisms within a unit area make up a standing crop of biomass. More specifically, biomass can be defined as the mass of organisms per unit area and is usually expressed in units of energy (e.g., joules m-2) or dry organic matter (e.g., tons ha -1 or grams m -2). Most of the biomass in a community is composed of plants, which are the primary producers of biomass because of their ability to fix carbon through photosynthesis. This chemical reaction can be described by the following simple formula:
6CO2 + 6H2O + light energy >>> C6H12O6 + 6O2
The product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released into the atmosphere (Figure 9l-1). All of the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combining of carbon dioxide and water with sunlight (Figure 9l-1). This chemical reaction is catalyzed by chlorophyll acting together with other pigment, lipid, sugar, protein, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage, or it can be combined with other sugar molecules to form specialized carbohydrates, such as cellulose. Sugars can also be combined with other nutrients such as nitrogen, phosphorus, and sulfur, to build complex molecules such as proteins and nucleic acids.
Figure 9l-1: Inputs and outputs of the photosynthetic process.
The primary productivity of a community is the amount of biomass produced through photosynthesis per unit area and time by plants, the primary producers. Primary productivity is usually expressed in units of energy (e.g., joules m -2 day -1) or in units of dry organic matter (e.g., kg m -2 year -1). Globally, primary production amounts to 243 billion metric tons of dry plant biomass per year. The total energy fixed by plants in a community through photosynthesis is referred to as gross primary productivity (GPP). Because all the energy fixed by the plant is converted into sugar, it is theoretically possible to determine a plant's energy uptake by measuring the amount of sugar produced. A proportion of the energy of gross primary productivity is used by plants in a process called respiration. Respiration provides a plant with the energy needed for various plant physiological and morphological activities. The general equation for respiration is:
C6H12O6 + 6O2 >>> 6CO2 + 6H2O + released energy
Subtracting respiration from gross primary production gives us net primary productivity (NPP), which represents the rate of production of biomass that is available for consumption (herbivory) by heterotrophic organisms (bacteria, fungi, and animals).
Globally, patterns of primary productivity vary both spatially and temporally. The least productive ecosystems are those limited by heat energy and water like the deserts and the polar tundra. The most productive ecosystems are systems with high temperatures, plenty of water and lots of available soil nitrogen. Table 9l-1 describes the approximate average net primary productivity for a variety of ecosystem types.
Table 9l-1: Average annual Net Primary Productivity of the Earth's major biomes.
| Ecosystem Type |
Net Primary Productivity (kilocalories/meter -2 /year) |
| Tropical Rain Forest |
9000 |
| Estuary |
9000 |
| Swamps and Marshes |
9000 |
| Savanna |
3000 |
| Deciduous Temperate Forest |
6000 |
| Boreal Forest |
3500 |
| Temperate Grassland |
2000 |
| Polar Tundra |
600 |
| Desert |
< 200 |