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.
