Agriculture Reference
In-Depth Information
Vera's hypothesis is that the open savannah type woodland followed a cycle along the
following lines:
In an area of grassland, grazed by herbivores, thorny shrubs which herbivores shun
will begin to appear. Light loving pioneer species of tree, particularly oak, hazel, and
wild fruit trees, whose seedlings get eaten when they appear in open grass, grow safely
in the protection of this scrub.
When the light loving trees become mature, they create a grove with a canopy
which is too dark for their own seedlings, and so the seedlings of species such as
beech, lime and elm are at an advantage. Eventually the shade loving trees dominate
the oaks and hazels, killing them off in the centre of the grove.
So far this succession is consistent with the climax theory, but this is where Vera's
hypothesis diverges. The orthodox view is that when trees die in the centre of the
woodland grove, creating gaps, more beech and lime seedlings grow in their place.
However Vera postulates that because it is too shady within these groves for the thorny
bushes to grow successfully, herbivores come to eat the grass which colonizes the
gaps almost immediately, and then either eat or trample the beech and lime seedlings,
maintaining a grassy clearing within the woodland. The clearing may grow because
the exposed trees around its edge are more susceptible to windblow.
Meanwhile, the grove of trees is expanding, because the thorny scrub is spreading out
into the grassland, allowing more oak and hazel to grow at the periphery of the grove. So
the woodland is an ever-expanding American donut (not as climax theorists postulate an
English doughnut) invading the grassland at its periphery, while fresh grass is hollowing
out the centre. Eventually the grassland in the centre becomes sufficiently large and well
lit to support the emergence of thorny shrubs which mark the beginning of a new grove of
trees.
The process is aided by various other participants in the ecosystem: jays, nuthatches and
squirrels may bury acorns and hazel nuts, assisting their propagation; wild boar snuffle
in the sward making holes where scrubby bushes can establish themselves; beavers create
large areas of grassland in the middle of woods by damming streams and creating flooded
areas which kill off trees; bark eating herbivores such as bison may ring bark trees when the
area of grass becomes limited; and predators will control the number of herbivores when
they become excessive.
The result is a shifting mosaic (this word crops up repeatedly in texts on the subject) of
shade-tolerant and light loving ecosystems, supporting high levels of biodiversity.
On its own this would just be an interesting aetiological theory, of secondary significance
to the modern agriculturalist. But Vera completes his hypothesis by arguing that humans
continued to maintain this silvo-pastoral landscape on common grazing lands up until the
19th century. Typically these lightly wooded areas provided grazing for cows, sheep and
horses; pannage consisting of mast (acorns, beech nuts, hazel nuts etc), roots and insects
