Michael Pidwirny
LABORATORY 10: PLANT PRODUCTIVITY AND STANDING BIOMASS
LEARNING GOALS
The objectives of this laboratory are to familiarize you with the biomass productivity of the major terrestrial and marine biomes found on planet Earth. We will also examine how the productivity of plants in terrestrial biomes is influenced by air temperature and the availability of water supplied mainly through precipitation.
Upon completion of this laboratory you will be able to:
- Describe the productivity differences that exist between the major terrestrial and marine biomes as measure by net primary production and standing biomass.
- Explain how the productivity of terrestrial biomes varies with latitude.
- Understand how variations in temperature and precipitation influence the productivity of the various terrestrial biomes.
PRODUCTIVITY OF PLANTS
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 living biomass in an ecosystem is composed of plants. Plants produce biomass through their ability to fix carbon through photosynthesis. The chemical process behind photosynthesis 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 the byproduct oxygen. Another product of photosynthesis is oxygen which is released into the atmosphere. All the sugar produced in the photosynthetic cells of plants and other organisms is derived from the initial chemical combination of carbon dioxide and water with sunlight. This chemical reaction is catalyzed by chlorophyll acting together with other pigments, lipids, sugars, proteins, and nucleic acid molecules. Sugars created in photosynthesis can be later converted by the plant to starch for storage or 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.
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 about 250 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