Conceptual explanation: The conversion period exists so that residues of earlier synthetic fertilizers and pesticides can be depleted from the soil, and the farm's biological cycles (soil microbial activity, nutrient cycling) can stabilise before produce is certified organic.
Perennial crops (fruit trees, plantation crops) have deep, persistent root systems and take longer to establish this new biological equilibrium than annual/seasonal crops, which is why the National Programme for Organic Production (NPOP) sets a longer conversion window for them.
Under NPOP guidelines, annual cropping systems typically require about two years of conversion, while perennial cropping systems require an additional year, giving a total of three years for perennial crops.