This ‘reference state’ is defined by mature stands of sessile oak woodland in an oceanic ‘high humidity’ location. It comprises trees with a mixed-age structure, i.e. not felled in the 20th Century, and has successful regeneration of oak saplings in the field layer under canopy gaps. This is achieved by ‘pulse grazing’, where animals are allowed into the wood one year in every five or six to maintain an open field layer. This allows tree saplings to persist and grow beyond the reach of grazers in the years between grazing events. The best example of this in Wales is at Coed Cymerau. Most sessile oakwoods in Wales are single-aged and intensively sheep-grazed with little, if any, successful regeneration (see modified states).