Environment of Earth

September 23, 2009

PRODUCTIVITY OF GLOBAL PLANT COVERS

Filed under: Environment — gargpk @ 5:45 pm
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Average values of the productivity of natural plant covers of Earth have been derived by using various theoretical and numerical models and data from a variety of studies including empirical determinations of productivity in individual biogeographical zones.

Terrestrial plant covers

Yefimova (1979) has made use of quite precise relationships between productivity of natural plant cover and meteorological factors in calculating values of the productivity and the coefficient of utilisation of photosynthetically active radiation for each continent. Results of her calculations are given in the Table-1. The data shows that the average productivity per unit area for the five continents of Earth does not differ very much. In each of these continents, magnitude of productivity over large part of continental territory is greatly limited by insufficient moisture or heat. The continent of South America is exception to this general condition since climatic conditions over large part of its territory are favourable for plant life.

Table 1: Productivity and coefficients of utilization of photosynthetically active radiation in various continents of Earth. (Yefimova 1979)

Continent

Productivity

(x109 tonnes)

Productivity

(center per hectare)

Coefficient of utilization of photosynthetically active radiation (as %age of total over vegetative period)

Europe

Asia

Africa

North America

South America

Australia (including islands of Oceania)

8.9

38.3

31.0

18.1

37.2

7.6

85

98

103

82

209

86

1.26

0.88

0.59

0.94

1.13

0.44

In Australia and Africa, coefficients of utilization of photosynthetically active radiation are lower than average. This can be attributed to insufficient moisture over large parts of these continents, which inhibits the complete utilization of available radiation by plant covers.

In Europe and South America, most favorable conditions for the development of plant life are found. In Europe, located at higher latitudes and exposed to less solar radiation, its utilization is relatively greater.

Smil (1985) gave estimates of the productivity and storage of biomass in major biomes of the Earth. These estimates are given in Table-2. Data in this table shows that there is not much difference in the area occupied by different types of ecosystems except wetlands that occupy smallest area on the Earth. However, productivity is highest in cultivated lands where one ton of biomass is produced per one ton of phytomass, followed by tropical and temperate grasslands where 0.5 ton of biomass is produced by each ton of phytomass. Next in productivity are tundra, deserts-semi-deserts and wetlands where 0.2 tonnes of biomass is produced per hectare from one ton of phytomass per hectare. These areas are followed by wetlands and shrub-lands where productivity is 0.13 tonnes per hectare. Tropical, temperate and boreal forest, though occupy almost same area on Earth, produce 0.067, 0.04 and 0.02 ton of biomass per tone of phytomass per hectare respectively. Despite these facts, most important on Earth are tropical, temperate and boreal forests that have the highest concentration of biomass on Earth (totaling about 750 tonnes per hectare). These ecosystems also have the highest total storage of biomass on Earth totaling about 850 x 109 tonnes. Further, it may be noted that contribution to total biomass production is equal for tropical rainforests and tropical grasslands (20 x 109 t/yr), followed by boreal forests and tropical grasslands (15 x 109 t/yr) and temperate forests, woodlands-shrub-lands and temperate grasslands (10 x 109 t/yr). Tundra and deserts have quite high average of net biomass production per unit area and also quite high weight of phytomass per unit area. Despite this they contribute very little to total global biomass production (1.0 – 2.0 x109 t/yr). However, if total biomass storage in different types of ecosystems on Earth is considered, tropical rainforests, temperate forests and boreal forests are the most important storehouses of organic matter on Earth having 850×109 tonnes of biomass. Woodland and shrub-lands having 75×109 tonnes and then tropical and temperate grasslands having 60×109 tonnes of biomass storage follow these.

Table 2: Area, productivity and storage of major global ecosystems. (Smil, 1985)

Ecosystem

Total area

(x106km2)

Average net production

(tonnes/ha)

Average phytomass

(tonnes/ha)

Total production

(x109tonnes/year)

Total storage

(x109tonnes/year)

Tropical rainforest

Temperate forests

Boreal forests

Woodland and shrub-land

Tropical grasslands

Temperate grasslands

Cultivation

Tundra

Deserts and semi-deserts

Wetlands

Settlements and transport

10.0

10.0

15.0

10.0

10.0

10.0

15.0

10.0

20.0

5.0

5.0

20.0

10.0

10.0

10.0

10.0

10.0

10.0

1.0

1.0

15.0

5.0

300.0

250.0

200.0

75.0

20.0

20.0

10.0

5.0

5.0

75.0

5.0

20.0

10.0

15.0

10.0

20.0

10.0

15.0

1.0

2.0

8.0

3.0

300.0

250.0

300.0

75.0

40.0

20.0

15.0

5.0

10.0

40.0

3.0

Total

114.0

1058.0

Aquatic plant covers

There is much less data about productivity of autotrophic plant covers in water bodies as compared to that about terrestrial plant covers. However, the available data indicates that the seas and oceans have the greatest volume of organic matter produced by phytoplankton located in the 30-40 meters deep layer of hydrosphere. At greater depths, quantity of solar radiation is insufficient for active development of photosynthesis.

In general, the productivity of shelf zones is substantially less than open ocean. It may attain maximum values in small bodies of water possessing large quantities of minerals required by the plants. The overall value of productivity for the oceans is estimated to be about 55 billion tonnes per year i.e. approximately 15 centner per hectare. This last figure is less than 1/6th of the average productivity per unit area on continents.

Thus the estimates show that the yearly volume of productivity for the Earth as a whole is approximately 200 billion tonnes i.e. about 40 calories per hectare. This corresponds to an energy expenditure of approximately 0.15 kcal/cm2 per year. This is about 0.1% of the solar radiation reaching the Earth’s surface.

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