Until the 12th century, a great extent of Pembrokeshire was virgin woodland. Clearance in the lowland south began under Anglo-Flemish colonisation and under mediaeval tenancies in other areas. Such was the extent of development that by the 16th century there was a shortage of timber in the county. Little is known about mediaeval farming methods, but much arable land was continuously cropped and only occasionally ploughed. By the 18th century, many of the centuries-old open field systems had been enclosed, and much of the land was arable or rough pasture in a ratio of about 1:3.