Geoscience Reference
In-Depth Information
F
IG. 51
Changes in tree and ground vegetation during growth and decline phases of birch woodland in Scottish
heather moorland, and their effect on soil development. (Adapted from J. Miles 1981.)
On a typical Morayshire site, the increase in earthworms during this cycle was
from about one per square metre under heather, to five per square metre under 18 year
old birch, and a peak of 127 per square metre under 38 year old birch. After this, the
numbers of worms declined again as the birch aged and began to die. The earthworm
species were
Lumbricus rubellus
initially but
L. terrestris
and
Aporrectodea
species
appeared as the exchangeable calcium level increased in the upper soil horizons. This
rise in calcium levels may have resulted from uptake by birch roots reaching lower
soil depths than those tapped by the heather, but perhaps also from increased weath-
ering of calcium-containing minerals
in situ.
Elsewhere in Britain, there are moorlands where this natural cycle does not oc-
cur. On the North Yorks Moors, the idea of birch as a soil improver had been put for-
ward by G.W.Dimbleby in 1952, and a long-term experiment with planted birch was
set up on the Peaty Podzol soil over acid sandstone. However, the podzolic miner-
al horizons remained unchanged even after 30 years; mor surface humus, which had
been destroyed to make a seed bed for the birch, had re-formed; the pH remained very
low, and no earthworm species associated with mull humus had appeared. In these
