What Is the Root:Shoot Ratio (RSR)?

The root:shoot ratio is the ratio of plant root biomass to shoot (aboveground) biomass. It reflects how much of a plant’s resources are invested in belowground structures for nutrient and water acquisition versus aboveground structures for light capture.

How Does RSR Vary Across Ecosystems?

• Temperate Grasslands:

  • Highest RSR (often 4–7).

  • Grasses invest heavily in roots for drought tolerance and resource acquisition256.

• Tropical Grasslands:

  • High RSR, but typically lower than temperate grasslands.

  • Still, much higher than forests due to similar adaptive strategies.

• Tropical Dry Forests:

  • Moderate RSR (higher than tropical wet forests, but lower than grasslands).

  • Trees allocate more biomass to roots for drought resilience278.

• Tropical Wet Forests:

  • Lowest RSR (often <0.3).

  • Abundant water and nutrients allow for greater investment in aboveground growth258.

Correct Ranking of Ecosystems by RSR

Based on global syntheses and ecological studies, the correct ranking (from highest to lowest RSR) is:

Temperate grassland > Tropical grassland > Tropical dry forest > Tropical wet forest

This ranking reflects the adaptive strategies of plants in different environments, with greater root allocation in more stressful, resource-limited conditions.

Evaluating the Options

Let’s review the options:

• (1) Tropical wet forest > Tropical dry forest > Temperate grassland > Tropical grassland

  • Incorrect. Tropical wet forest has the lowest RSR.

• (2) Temperate grassland > Tropical grassland > Tropical wet forest > Tropical dry forest

  • Incorrect. Tropical dry forest should be ranked higher than tropical wet forest.

• (3) Tropical dry forest > Tropical wet forest > Tropical grassland > Temperate grassland

  • Incorrect. Grasslands have the highest RSR, not the lowest.

• (4) Temperate grassland > Tropical grassland > Tropical dry forest > Tropical wet forest

  • Correct. This matches the standard ecological ranking.

Ecological Implications

The RSR has important implications for carbon storage, water use, and ecosystem resilience. Ecosystems with high RSR, like grasslands, store more carbon belowground and are more resistant to drought. Forests, especially tropical wet forests, store more carbon aboveground and are more productive in terms of biomass accumulation.

Conclusion

The correct ranking of ecosystems based on the root:shoot ratio of plants is:

Temperate grassland > Tropical grassland > Tropical dry forest > Tropical wet forest

Correct answer:
(4) Temperate grassland > Tropical grassland > Tropical dry forest > Tropical wet forest