Appendix Environmental Controls on Net Primary Productivity

Net primary productivity is equal to the product of the rate of photosynthesis per unit leaf area and the total surface area of the active leaves per unit area of land, minus the rate of plant respiration per unit area of land. Given sufficient plant nutrients and substrates, temperature and moisture control the rate of photosynthesis.

Extremely cold and hot temperature limit the rate of photosynthesis. Within the range of temperatures that are tolerated, the rate of photosynthesis generally rises with temperature. Most biological metabolic activity takes place between 0 and 50°C. The optimal temperatures for plant productivity coincide with the 15-25°C optimum temperature range of photosynthesis.

A growing season is the period when temperatures are sufficiently warm to support synthesis and a positive net primary production. Warmer temperatures support both higher rates of photosynthesis and a longer growing season, resulting in a higher net primary production - if there are sufficient water and nutrients. The amount of water available to the plant will therefore limit both the rate of photosynthesis and the area of leaves that can be supported.

The influence of temperature and water availability is interrelated. It is the combination of warm temperature and water supply adequate to meet the demands of transpiration that results in the highest values of primary productivity. Net primary production in ecosystems varies widely, cf. Fig. 2.7 in Cramer et al. (1995) and Table 2.6:

1. The most productive terrestrial ecosystem are tropical evergreen rainforests with high rainfall and warm temperatures. Their net primary productivity ranges from 700 to 1400 gCm-2 yr-1.

2. Temperate mixed forests produce between 400 and 1000 gCm-2 yr—1.

3. Temperate grassland productivity is between 200 and 500 gCm-2 yr-1.

Table 2.6 Average net primary productivity of ecosystems

Ecosystem

Value" gCm 2 yr 1

Value6 gCm 2 yr 1

Swamp and marsh

1130

2500

Algal bed and reef

900

2000

Tropical forest

830

1800

Estuary

810

1800

Temperate forest

560

1250

Boreal forest

360

800

Savanna

320

700

Cultivated land

290

650

Woodland and shrubland

270

600

Grassland

230

500

Lake and stream

230

500

Upwelling zone

230

-

Continental shelf

160

360

Tundra and alpine meadow

65

140

Open ocean

57

125

Desert scrub

32

70

Rock, ice, and sand

15

-

a www.vendian.org/envelope/Temporary.URL/draft-npp.html b (Ricklefs, 1990). Note that Column 2 is ~Column 1 x 2.2, corresponding to the mean molecular weight of dry biomass of 26 g/mol per 1 carbon atom, a little less than 27 g/mol in glucose starch, CH2O — 1/6H2O. A typical molecular composition of dry woody biomass is CH14O0 6, MW = 23 g/mol.

a www.vendian.org/envelope/Temporary.URL/draft-npp.html b (Ricklefs, 1990). Note that Column 2 is ~Column 1 x 2.2, corresponding to the mean molecular weight of dry biomass of 26 g/mol per 1 carbon atom, a little less than 27 g/mol in glucose starch, CH2O — 1/6H2O. A typical molecular composition of dry woody biomass is CH14O0 6, MW = 23 g/mol.

4. Arctic and alpine tundra have productivities of 0 to 300 gCm-2 yr-1.

5. Productivity of the open sea is generally low, 10 to 50 gCm-2 yr-1.

6. Given equal nutrient supplies, productivity in the open waters of the cool temperate oceans tends to be higher than than of the tropical waters.

7. In areas of upwelling, as near the tropical coast of Peru, productivity can exceed 500 gCm-2 yr-1.

8. Coastal ecosystem and continental shelves have higher productivity than open ocean.

9. Swamps and marshes have a net primary production of 1100 gCm-2 yr-1 or higher.

10. Estuaries and coral reefs have a net primary productivity of 900 gCm-2 yr-1. This is caused by the inputs of nutrients from rivers and tides in estuaries, and the changing tides in coral reefs.

High primary productivity results from an energy subsidy to the (generally small) ecosystem. This subsidy results from a warmer temperature, greater rainfall, circulating or moving water that carries in food or additional nutrients. In the case of agriculture, the subsidy comes from fossil fuels for cultivation and irrigation, fertilizers, and the control of pests. Sugarcane has a net productivity of 1700-2500 g m-2 yr-1 of dry stems, and hybrid corn in the US 800-1000 gm-2 yr-1 of dry grain.

Was this article helpful?

0 0

Post a comment