Almost all agricultural land if abandoned turns into woodland. Let a field be abandoned - as many fields have been down the centuries - and within a year it will be invaded by oaks springing up from acorns dropped by passing jays, or by birches from wind-blown seed. Within five years it will be an impenetrable mass of thorny shrubs and spindly tree samplings.  In ten years it will be difficult to reclaim; in thirty years it will have 'tumbled down to woodland'. When in the scrubby stages a site that is dominated by  thorny shrubs, sallow willows, and small trees is termed a spinney.  The same happens to chalk downs, heaths, fens, and some moorland whenever the grazing and burning cease that had held trees in check.
Woodland formed in this way, by a process of  termed succession, are categorised as secondary woods.  Succession is defined as the sequential change in species composition from grasses being the dominant ground cover to the final stages where little can grow under the dense tree canopy.  Animals, particularly birds and insects, also succeed each other in a sequence that is dependent on the botanical changes.  In the first years the process involves rapid year on year changes in biodiversity which slows as the site comes closer to woodland, but secondary woods never seem to gain many of the herbaceous plants of ancient woodland.  In general, the trees in secondary woods are not the same as ancient woods; they are composed of those pioneer trees- oak, birch, hawthorn, ash - which easily invade vacant ground. Ancient hornbeam-woods have recent oakwoods alongside them; only after a century or more does hornbeam get into a secondary wood, and lime may never colonize.  Secondary woods may be of any age from prehistory onwards.
Secondary woodland is familiar on railway land and old quarries; it covers about a sixth of Surrey; its spread is a chief threat to the conservation of heath and old grassland. In the eastern United States an area much greater than the whole British Isles has tumbled down to woodland since 1800. 
There have been many scientific studies of the subject, including the classic experiment of Broadbalk Wilderness at Rothamsted (Herts), an arable field which was left untouched after 1882 and had become a wood by 1914.  Recent writers, ignoring all these examples, call for expensive tree-planting as if it were the only way to create new woodland. Like all gradual changes which cost nothing, succession to woodland often goes unnoticed.