8 4 
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, Melaneonis 
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 70cm. (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 Epfing Forest 
and elsewhere.” Essex Naturalist xi., itjoo, pp. 273-284. 
