Light and shadow: from open ground to
woodland
This open
landscape did not last very long. Birch trees reached
the area very quickly after the ice disappeared and
were growing around the infilling lake by 10370
years ago (416cm). These are fast growing trees
which can cope with a variety of different
conditions, so they grew on wet soils, like the fen
peat infilling the lake, and on dry soils,
overshadowing juniper and many of the herbs.
This was
followed around 10010 years ago (404cm) by the
spread of hazel, which prefers quite organic soils
but won’t grow on waterlogged ground. It
probably grew around the edge of the fen peat,
forming woods with birch and rowan. Roses and ivy
climbed over and around the trees, with grasses and
bracken in more open areas. Elm was gradually
becoming established, but was not very common around
in this area.
These woods
were very different to the damp fen peat growing at
the same time on what is now Greenhead Moss. Here
birch and willow grew amongst pools or patches of wet
ground where aquatic grasses, marsh marigold and fen
herbs flourished.
Marsh marigold (Caltha
palustris) © SHN
This mosaic or
patchwork of different plant communities lasted over
2000 years, but seen in “fast forward”, the
patterns were changing over a timescale of
centuries and even decades. Trees died and fell
over, leaving openings where the extra light let
herbs grow more abundantly, before they were
overshadowed again by new saplings reaching up to
make a more shadowy canopy. Fungi, beetles and other
organisms that decompose dead plant material were
constantly busy. On the fen, the water level
fluctuated, possibly reflecting quite short lived
climatic variations early in the Holocene
interglacial. This allowed birch to flourish for a
while on drier peat surfaces, but then shifted to
willows, sedges and aquatic grasses when the same
areas were wetter.
Around 9710
year s ago (392cm) we see the first sign of
changes to come. Heather plants appeared, suggesting
that the fen was less nutrient-rich, probably because
the plants were becoming cut off from the underlying
rock and soils, so plants had to rely increasingly on
rain water for their nutrients (plant vitamins). Fire
may also have helped heather to become established
and burning was an important part of the bog
ecosystem.
How bogs form
Fire in bogs and heathlands
A turning point
was reached on the fen around 8120 years ago (324
cm): the plants were cut off from the mineral
soils beneath the peat and became totally reliant on
rainfall for water and minerals. Heath plants and Sphagnum
bog replaced grasses because they are adapted to
these conditions. Trees could no longer grow as
abundantly because the bog surface was much wetter
and they may have been restricted to the drier edges
of the bog. Their trunks decayed, but some wood was
preserved in the wet peat and this is what we see in
the stratigraphy.
Peat stratigraphy
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