Lecture 7
Agricultural Potential of the Humid Tropics
Although the tropical forest is lush, the net productivity is low.
This accounts for lack of sedentary agriculture in the tropics.
Only 2.5% of Af lands are under sedentary cultivation compared
to 10% in the habitable part of the world.
Chang has developed the theory of potential photosynthesis to account for this.
Chang, Jen-Hu. 1968. Agricultural Potential of the Humid Tropics. Geographical Review 58:333-361
Photosynthesis depends on the availability of water, air, temperature, and solar radiation to plants.
Without limiting factors, photosynthesis increases with sunlight up to the saturation light intensity which varies somewhat from plant to plant.
However, increased light increases photosynthesis because of the effect of shading.
Therefore long days and bright sunlight can be expected to increase the rate of photosynthesis.
However the rate of biomass production (carbohydrate production) or yield is based on net photosynthesis which is gross photosynthesis less respiration losses.
Net photosynthesis = Gross photosynthesis - Respiration loss
However, the rate of respiration is directly related proportionally
to temperature between 40º and 95ºF.
Thus, ideal climate would be bright sunny, long days and cool days and especially cool nights.
This does not describe Af climates.
Remember equatorial areas have zones of relatively low insolation due to cloud cover and short days.
At higher latitudes, greater insolation in the summer combined with lower night temperatures increase net photosynthesis.
Net Photosynthesis as a Percent of Af Climate
| Time | Af | Aw |
Cs |
Ca (Mild Temperate) |
D (Severe Winter) |
| Entire Year | 100 | 105 | 117 | 91 | 68 |
| 8 mos (March to October) | 100 | 106 | 127 | 123 | 101 |
| 4 mos (May to August) | 100 | 109 | 136 | 133 | 152 |
Light Energy and Temperature in D and Af Climates (Northern Hemisphere)
| Climate | Light Energy | Avg. temp |
| D (June) | 510 langleys | 17°C |
| Af (June) | 357 langleys | 26°C |
These differences are in accord with calculated discrepancy of net photosynthesis.
However differences are greater under "average" conditions.
Thus poor management carries a greater penalty in the tropics!
Actual yields, Experiments by International Rice Research Institute (IRRI) under optimum conditions
| Yields (tonnes/ha) | ||
| Country | Best conditions | Average conditions |
| Japan, Australia | 14.3 | 8.9 |
| Phillipines, Malaysia | 12.3 | 5.0 |
Within A climates, Af poorer than drier topical climates such as Aw or As (dry season in summer).
This is true in for sugar yields in Hawaii
As 12.52 tons/acre
Note: As climate is very rare and is due to cyclonic factors; found in Ceylon, S. India, Hawaii.