NATURAL HABITATS - The Soil - page 151

The adjacent soils over acidic sands and silts include Brown Earths and poorly
drained Gleys. The larger burrowing and surface-casting earthworms are absent,
and differences in the humus profiles are related to the plant cover. Sweet chestnut
Castanea sativa and dense bracken Pteridium aquilinum give moder humus with
deep L layers. Where bracken is sparser, and particularly where birch is present, mull
phases occur. Here again, the nutritional quality of the leaf litter affects the earthworm
fauna which plays such a crucial role in the incorporation of organic material into the
profile. A similar picture is seen in some moorland and upland habitats.
M OORLAND AND HILL
J.Miles and colleagues studied the soil contrasts associated with heather moor and
patches of birch woodland in Scotland. This mosaic is a widespread feature of
the central and eastern Cairngorms. In Miles' study, heather-dominated areas were
mainly on Podzol soils with mor humus. Where birch trees Betula pendula and B.
pubescens colonize an area, the heather Calluna vulgaris gradually declines and is
replaced by grasses and herbs typical of open woodland. These vegetation changes
are accompanied by progressive changes in soil conditions which appear to be due to
the relatively nutrient-rich litter of the birch and its associated ground flora compared
to that of the heather. The mor humus alters towards a mull-like form, the podzolic
leached mineral horizon becomes obscured, the soil acidity is reduced, and there is a
substantial increase in earthworm numbers.
Soil 'improvement', in the sense of greater decomposer activity and hence more
rapid nutrient cycling, does not progress continually in the Scottish situation to main-
tain a woodland community at a particular location. There is a natural cycle determ-
ined by a decline in the vigour of birch, and its subsequent death at 70-90 years old.
Birch does not establish itself successfully under a closed canopy with the result that,
within the established woodland, there are no younger generations of birch to main-
tain the wood. As the old birch trees die, heather spreads back from the adjacent moor.
Meantime, the birch will have colonized another area and a new young wood will be
growing up. Figure 51 illustrates this cycle of vegetation and the related soil changes.