58. Which of the following is the correct increasing order for the daily net primary productivity (NPP) per unit leaf area in different ecosystems ?
(1) Deserts < Temperate forests < Tropical forests
(2) Deserts < Tropical forests < Temperate forests
(3) Temperate forests< Tropical forests< Deserts
(4) Tropical forests < Temperate forests < Deserts

Understanding how ecosystems differ in their daily net primary productivity (NPP) per unit leaf area is essential for grasping global carbon cycling and plant growth efficiency. This article explains the correct order of increasing daily NPP per unit leaf area among deserts, temperate forests, and tropical forests, and why tropical forests are the most productive.

What Is Net Primary Productivity (NPP)?

Net primary productivity (NPP) is the amount of biomass or carbon produced by plants per unit area and time, after accounting for the energy used in respiration157. It is a critical measure of ecosystem health and carbon storage.

NPP Per Unit Leaf Area

When comparing NPP per unit leaf area, we focus on how efficiently each leaf contributes to biomass production. This metric helps us understand how different ecosystems maximize photosynthesis given their leaf cover.

Comparing Deserts, Temperate Forests, and Tropical Forests

1. Deserts

• Leaf Area: Very low, as plant cover is sparse.

• Growing Season: Extremely short, limited by water availability.

• Daily NPP Per Unit Leaf Area: Can be high during brief growing periods, but overall is the lowest due to extreme abiotic stress and limited growing time35.

• Annual NPP: Very low because of limited plant cover and harsh conditions.

2. Temperate Forests

• Leaf Area: High, but seasonal—leaves are present only during the growing season.

• Growing Season: Moderate, limited by winter.

• Daily NPP Per Unit Leaf Area: Intermediate. High during the growing season, but lower than tropical forests when averaged over the year3.

• Annual NPP: High, but less than tropical forests due to seasonal limitations.

3. Tropical Forests

• Leaf Area: High and present year-round.

• Growing Season: Year-round, thanks to stable temperatures and moisture.

• Daily NPP Per Unit Leaf Area: Highest, as leaves are continuously active and conditions are optimal for photosynthesis35.

• Annual NPP: Highest among terrestrial ecosystems.

Correct Order of Increasing Daily NPP Per Unit Leaf Area

Based on global ecological data and syntheses, the correct order is:

Deserts < Temperate forests < Tropical forests

This reflects the fact that tropical forests have the highest daily NPP per unit leaf area due to their continuous growing season and optimal conditions, while deserts have the lowest due to extreme stress and limited growing time.

Evaluating the Options

Let’s review the options:

1. Deserts < Temperate forests < Tropical forests

   • Correct. This matches the standard ecological understanding.

2. Deserts < Tropical forests < Temperate forests

   • Incorrect. Tropical forests have higher daily NPP per unit leaf area than temperate forests.

3. Temperate forests < Tropical forests < Deserts

   • Incorrect. Deserts have the lowest daily NPP per unit leaf area.

4. Tropical forests < Temperate forests < Deserts

   • Incorrect. Tropical forests have the highest daily NPP per unit leaf area.

Ecological Implications

The high daily NPP per unit leaf area in tropical forests supports their role as global carbon sinks and biodiversity hotspots. In contrast, the low productivity in deserts limits their ability to support large biomass and complex food webs.

Conclusion

The correct increasing order for daily net primary productivity (NPP) per unit leaf area in different ecosystems is:

Deserts < Temperate forests < Tropical forests

Correct answer:
(1) Deserts < Temperate forests < Tropical forests