The ancient oaks of England were and sometimes still are the dominant trees of an ancient type of woodland which I call pasture woodland. Another term, 'wood pasture' is also in use for this. From prehistoric times farming communities used woodland to pasture their domestic animals, especially in the lowlands. Meadows were too valuable to allow them to be eaten by cattle or sheep in summer, they were for haymaking to feed the animals in winter. Pigs were also sent into the woods, especially when there were acorns or beech nuts. Other woods were more intensively used and the animals were kept out in order to make coppice woods. Only (former) pasture woodland can have ancient oaks. By the time of the Norman Conquest in 1066 virtually all woodland in England was either pasture woodland or enclosed coppice woods. Over-exploitation on poor sandy soils has turned pasture woodland into heath, or into chalk grassland on chalk, with hardly any trees left. Gradations between the two vegetation types existed in many areas, so pasture woodland could be parkland with scattered trees.
In the sections on deer parks and on Royal Forests we have seen how such pasture woodland could become preserved against further depredations of a growing population demanding more land for agriculture and grazing. This type of wooded landscape has therefore existed longer than elsewhere and examples are still present, especially in several ancient parks. A long history of this on such sites has assured the preservation of rare and fragile ecosystems associated with ancient oaks, other trees, acid grassland and the presence of grazing animals. Even the ancient oaks themselves, if the pasture woodland as such has gone, can be seen as little habitats dating from centuries ago. In some cases we may assume an uninterrupted rare habitat going back to prehistoric times, if ancient oaks have been present on the site for millennia instead of mere centuries.
The ecology of an ancient oak is very complex. Its micro-habitats range from the root system and soil to the foliage and flowers. They include living and dead wood, bark, dead bark and spaces under it, decayed wood, holes and large cavities, fallen dead branches on the ground and more. On each of these categories different organisms have their niches, some are generalists, others specialists. Interactions between the organisms add to the complexity. There are also differences between sites according to climate, topography, soil, past and present management or use, etc. It is probably fair to say that ancient pasture woodland with ancient oaks forms the most complex ecosystem in Britain, leading to the highest biodiversity.