Neco 2024 Agricultural Science Question 35

A mixed forest of 100 ha consists of 60 % area planted with species A (fast‑growing) and 40 % with species B (slow‑growing). Species A sequesters carbon at a rate of 5 t C ha⁻¹ yr⁻¹, while species B sequesters at 2 t C ha⁻¹ yr⁻¹. Every 20 years a harvest is carried out on 30 % of the area of each species. The harvest removes 80 % of the carbon stored in the harvested portion; the remaining 20 % stays as residues and over the following decade 10 % of that residue carbon is released back to the atmosphere. The unharvested portion continues to accumulate carbon at the same annual rates. Assuming the forest starts with zero carbon stock, calculate the total net carbon stock (in tonnes of carbon) present in the forest after 60 years.