I'd link to a previous comment I made on this but it's no longer retrievable through conventional means, so here's a copy/paste:

King Ine (~688 AD) is the first person we can find who introduces forest laws, but we're dealing with a long chain of textual difficulties that I don't really want to get into. While you could hunt whatever you like on your own ground, the Anglo-Saxon kings during the heptarchy had introduced the concept of silva regis, restricting hunting in royal forests. These were much smaller than in William's time, but it clearly differentiated between the 'higher' chases and the lower ones - King's hunted on the higher, everyone else on the lower. You could chase animals into the silva regis but were restricted in what you could kill there (as per William Seymour). The pre-Norman forests of Edward's time were administered by a number of thanes, including foresters or woodsmen, and all were armed which implies the possibility for conflict. However, the laws for transgressing these forests were very mild in comparison to what would come later, and it was counted as a common-law transgression. Cnut forbids hunting on his 'private chases' in 1023, and Layamon's Brut attributes forest laws as far back as the Saxon King Athelstan, which gives us about 250 years worth of forest laws before William I. Anglo-Saxon chases were never as large as William's but they are still there and the law is enforced through moot courts when transgressors were caught, although penalties were much milder than during the Norman period.

What William does is take all game to be of royal prerogative in the silva regis, afforests vast new areas, and introduces Forest Law, which exists in parallel with common-law and covers vast areas where people now live (read the Anglo-Saxon Chronicle on this). If you read about the poaching trials around the period, apart from the inherent racism (Norman vs Saxon), you can see the Saxon confusion and rage at this - they simply don't understand what they've done and why it's wrong. This is because the Forest Law, together with William's enforcement of it, is much more punitive and crosses what is commonly understood to be common rights of vert and venison (hence in Eadmer's historia novorum in Anglia, 50 men are tried for killing the king's deer and have to undergo trial by hot iron, although some suggest that poaching becomes an act of resistance!)