|from Livre de la Chasse|
As there's a lot of interesting stuff here, I'll just quote a bit (omitting footnotes, text in brackets is my own). First of all, to clarify what a medieval "forest" was:
‘Forest’ in medieval England denoted a defined area of unenclosed land within which wild game, principally deer, along with wide swathes of its habitat were protected by forest laws for the benefit of the king. Medieval forests, however, could include “not only woodland, but also heath, pasture, meadow, and arable land, and even hamlets, villages, and townships.” Importantly, forest jurisdictions in England unlike those in Normandy could, and often did, extend outside the king’s own demesne land (the crown’s landed estate) onto privately held lands, acting as a type of economically restrictive land-use overlay on areas that remained subject to the common law as well.
The afforestation [meaning declaring land to be a "forest"] of large tracts of land by early Norman kings—like William I’s creation of the New Forest in Hampshire, or Henry I’s creation of forests in Leicestershire, Bedfordshire, and Yorkshire—created tensions with private landholders within forest jurisdictions as their lands became subject to stringent forest law restrictions aimed at preserving the royal prerogative of hunting deer. However, during the reign of Henry II (1154–1189) the crown shifted its policy on forests, recognizing that in addition to protecting royal hunting rights, forests contained deep fiscal reservoirs of resources, licensing fees, and fines from forest offences that could be plumbed extensively to fill the royal coffers when necessary. Indeed, it was King John’s savage overexploitation of forest law revenues from 1205–1212 to bolster his war chest for failed military campaigns to retake Normandy from France that was a significant factor leading to Runnymede three years later.
The Justice of the Forest became the chief justice in every forest eyre [an itinerant court that met every few years] under his purview. The crown also appointed other itinerant justices (usually two to four) to assist the Justice of the Forest in hearing forest eyre pleas in the different counties. The business of the forest eyres consisted primarily in dealing with three types of offenses, each listed separately in forest eyre rolls: (1) pleas of venison; (2) pleas of the vert; and (3) the regard....
Pleas of venison—poaching offenses—were presented to the forest eyre justices by the keeper (sometimes called a warden or a steward) of each forest and his foresters.... Keepers and foresters were responsible for apprehending poachers and ensuring their attendance at the forest eyre, where malefactors would be amerced [fined] according to the severity of the offence and their ability to pay—the Charter of the Forest having abolished corporal punishment for forest crimes. Significantly, extant forest eyre rolls show that poaching was not limited to men of any particular social class. Rather, these rolls reveal that poachers included: bishops, local lay and ecclesiastical barons, gentry, monks, parish priests, artisans, servants, and peasants. They also show that poaching was an activity that brought families and social groups together, suggesting that the thrill-of-the-chase and the social cohesion instilled through communal hunting was more important than potential pecuniary penalties.For more on the Forest Charter and Magna Carta, see here.