Wednesday, June 23, 2010

How efficient are plants? (part I)

Annual net primary production (g C m-2 yr-1) estimated as the average of all model estimates:

In order to answer how efficient plants are at converting solar energy into chemical energy, we can look at how much energy is in sunlight and how it is used in photosynthesis.

The surface of the Earth receives 8,000 to 10,000 kilocalories (kcal) of energy from the sun each day on each square meter of surface during the growing season.

A kcal is what most people call a calorie (you're supposed to eat 2,000 a day) and is defined as the amount of heat needed to warm 1 kg of water 1 degree Celsius (°C). Indeed, most (~95%) of this solar energy is used up heating the surroundings and evaporating water, while a paltry ~2% is used for photosynthesis (3% is reflected).

So out of 10,000 kcal only 2% or 200 kcal are available to a one square meter plant per day. If the growing season is 150 days, Gross Primary Productivity should be on the order of 30,000 kCal per year. However, at least half of this is lost by cellular respiration as the plants run their own metabolism. Also, C3 plants respire CO2 at high temperature and sunlight because the protein machinery can "run backwards" (C4 plants minimize these losses to "photorespiration"). Because of other inefficiencies, the Net Productivity is always lower:

Estimated Net Productivity of Certain Ecosystems (in kilocalories/m2/year)
Temperate deciduous forest5,000
Tropical rain forest15,000
Tall-grass prairie2,000
Coastal marsh12,000
Ocean close to shore2,500
Open ocean800
Clear (oligotrophic) lake800
Lake in advanced state of eutrophication2,400
Silver Springs, Florida8,800
Field of alfalfa (lucerne)15,000
Corn (maize) field, U.S.4,500
Rice paddies, Japan5,500
Lawn, Washington, D.C.6,800
Sugar cane, Hawaii25,000

Note that even the most efficient crops store about half of
At these efficiencies it would take 144 square meters of temperate forest to supply the 720,000 kcal an average adult needs to survive a year, but just 28 square meters of sugar cane in Hawaii, assuming we could eat all the calories produced in a forest or live entirely on sugar. These areas are about the size of a small house and a bedroom, respectively. (1 sq meter = 10.7 sq feet)

The graph at the top of this post shows that the average productivity is about 1 kg per square meter per year. This makes sense because a gram of whole plant biomass yields about 4.25 kcal, so 1kg of plant matter per square meter yields 4,250 kcals of energy.

note: these estimated values are different, though of the same magnitude, from other sources.

A map of which input is most limiting across terrestrial biomes:  temperature, sunlight, or water.

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