84
THE ESSEX NATURALIST.
ance as they are the first that deal with the hydrogen-ion con-
centration of Forest soils.
The factors that have tended to bring about the great birch
invasion may be summarized as:—
1. Leaching of the soil, a factor of primary importance.
2. Extensive felling for many successive years.
3. A long series of fires, especially those of recent date.
4. Browsing of large herds of deer, 1800-1850.
5. Injury done to germinating acorns and beechmast by
rabbits (1870-1914), and even earlier. The young birch appears
to be distasteful, even to rabbits; it was thus able to increase
while the oak and beech were checked.
During the autumn of 1899, and throughout the following
year, birch trees in Essex, Kent, Surrey and elsewhere were
killed in great numbers by a microscopical fungus, Melanconis
stilbostoma, but its virulence was soon exhausted. Although
the disease is always present, it has not appeared in epidemic
form since 1902. Trees of twenty to twenty-five years' growth
were killed within a month.7
In parts of the upper Forest, where the birch has made rapid
progress within living memory, conditions exist that correspond
very closely with those described by Graebner and others as a
result of their researches respecting the fundamental cause
of the degeneration of woodland into heath. Graebner noted
that, "when there is a rainfall of 70 cm. (28 inches) or more"
(the record for High Beach and Epping is considerably above
this, see page 71), "the surface layers are being continually
impoverished in mineral salts by washing out or leaching, and
the typical plants of the forest floor are thus starved and give
way to the invasion of mosses and shade-bearing heath plants.
The matting together of the surface layers of soil by the rhizoids
and rootlets of the invaders prevents the access of oxygen to the
soil and leads to the accumulation of 'acid humus' or 'dry
peat' in place of the original mild humus of the woodlands."
The specimens, two inches thick, of dry peat, on exhibition
this afternoon, are from the Forest. They are typical of that
which occurs at various stations on the plateau having an annual
rainfall of 28 inches or more.
7 R. Paulson "An Enquiry into the causes of the death of Birch Trees in Epping Forest
and elsewhere." Essex Naturalist xi., 1900, pp. 273-284.