Previous Year Question
Which of the following has the lowest mean net primary productivity?
Temperate forests
Tropical forests
Temperate grasslands
Tropical savanna
Correct Answer: C
Detailed Solution
Net Primary Productivity (NPP) is the rate at which Producers, mainly plants, store energy as biomass after subtracting the energy they use in respiration from the total energy captured through Photosynthesis.
It can be expressed simply as—
NPP = Gross Primary Productivity (GPP) − Plant Respiration (R)
In the traditional biome-level comparison of the ecosystems given in the question, the approximate order of average productivity is—
Tropical Forest > Temperate Forest > Tropical Savanna > Temperate Grassland
Temperate Grasslands generally receive comparatively less rainfall and have a more limited growing season, so their mean NPP is lower than that of Tropical and Temperate Forests. Tropical Savannas generally have higher average productivity than Temperate Grasslands because of their warm climate and comparatively longer growing period.
A modern field-based global assessment of grasslands also estimated total NPP at about 717 g m⁻² yr⁻¹ for temperate grassland/meadow/shrubland, compared with about 1315 g m⁻² yr⁻¹ for tropical shrubland/grassland/savanna. Values vary among individual studies and sites, but among the four options given in the question, Temperate Grassland has the lowest mean NPP.