In the Domesday Book of 1086 the economic value of forests is not measured in the amount of wood they could provide, but in the amount of pigs they could feed.
The Domesday Book is a huge list of properties and holdings in England (and parts of Wales) collected so that William the Conqueror knew how much to tax everyone. It has been a huge boon to historians because it’s the most comprehensive economic survey of medieval England. But some of the measures in this book seem odd to modern eyes.
In particular, woodlands are often described in terms of their area, but also in terms of the number of pigs that they can feed. The phrase in the book is de herbagio, and the practice it describes is pannage.
Pannage represents the right to send pigs into a forest to forage for acorns, chestnuts, and other tree nuts and seeds. It was a big deal because it fattened the pigs up before winter and also kept the forest floor healthy – all that rooting around really churns up the soil. Pigs were the grazers of choice because acorns can be lethal to horses and cattle if they eat too much. Acorns have a high level of tannins, and horse and cow kidneys can shut down if they try to process too much.
Pannage is still practised in some places, and modern silvopasture (animal grazing in woodlands) carries many benefits: the animals are protected from the elements by the trees and the trees provide good food for the animals. I guess I just need to save up enough money to buy myself a little forest…