The rate of biomass production is expressed as:

The correct answer is Option (3) → gm⁻² yr⁻¹

Productivity is the rate of biomass production. Biomass is measured as weight (grams) produced on a unit area of land (one square metre), and a rate is always per unit time (one year). So the unit is:

    \( \displaystyle \text{productivity} = \frac{\text{grams}}{\text{m}^{2} \times \text{year}} = \text{g m}^{-2}\,\text{yr}^{-1} \)

It may also be given as energy, in (kcal m⁻²) yr⁻¹.

(1) gm⁻¹ yr⁻² — Incorrect. m⁻¹ means per metre, which is a length, not an area. yr⁻² has no meaning for a rate.

(2) gm⁻¹ yr⁻¹ — Incorrect. The time part is right, but per metre is a length, not an area.

(3) gm⁻² yr⁻¹ — Correct. Grams per square metre (area) per year (time).

(4) gm⁻² yr⁻² — Incorrect. The area part is right, but per year squared is not a rate.

NCERT Extract:
Productivity – “A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m–2) or energy (kcal m–2). The rate of biomass production is called productivity. It is expressed in terms of gm–2 yr –1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems.”