They may have had low levels of physical disturbance, such as chain harrowing, or low levels of nutrient input from organic manures or inorganic fertilisers.
Efallai eu bod wedi bod yn destun lefelau isel o darfu ffisegol, megis llyfnu ag oged gadwyn, neu lefelau isel o fewnbwn maethynnau o dail organig neu wrteithiau anorganig.