Whilst focus was put on Welsh data, it is recognised that much of the current evidence originates from UK-wide studies, which may not represent the soils, land uses and landscapes specific to Wales.
Er bod llawer o dystiolaeth ar gael, nid oes astudiaethau sy'n rhoi amcangyfrifon sy'n benodol i Gymru a dim ond nifer bach sy'n trafod marchnad y DU.