Net primary productivity (A): Quantified in \( gC/\text{area}/\text{year} \), representing the carbon fixed by plants per unit area over time.
Absorbed photosynthetically active radiation (B): Measured in \( \text{Joules}/\text{area}/\text{year} \), indicating the energy absorbed by plants for photosynthesis.
Phosphorus (C): Its concentration in a solution, expressed in \( g/L \).
Plant biomass (D): The total mass of living plant matter per unit area, measured in \( g/m^2 \).