206 The Spread of Birch since the War. 
denied to them, of which they were not slow to take advantage. 
Large numbers of Pines were cut down round the Black 
Pond, Esher, along the North Downs in Surrey, on Stoke 
Common, Bucks., etc., and the Birch is now growing plenti- 
fully in most of these areas — in some of them it is dense, but it 
is also covering spaces where, though open to it, it did not 
grow twenty years ago. Then Esher Common was covered 
with grass and a few patches of Calluna vulgaris. Birches 
were fruiting year after year on the other side of the boundary, 
but not until about 1920 did their seedlings appear on the 
former open space which they have now covered entirely. 
The seeds had been blown there before but had not germinated. 
Similarly acres on Wimbledon Common have been invaded 
the last few years by Birch which before was restricted to the 
ground below the Windmill and the extreme eastern border. 
Kingston Vale is another instance. Formerly almost entirely 
clothed with Molinia it is now a home for countless Birches. 
One could continue the list of examples but the above should 
suffice to illustrate the point. I have only noticed one 
large area that seems immune, though there are old trees 
producing seed on its margin, and this is Farthing Downs near 
Purley, which has not yet shown signs of this spread of the 
Birch. 
In 1929 and 1930 I found young Birches spreading in 
places on the Shropshire Hills, but not to the same extent as 
in Surrey, Bucks., Herts., Essex and Kent. 
In 1921, I was for a few days at Ravenscar, and in 1922 
at Castleton, but I do not seem to have any notes showing 
that Birch was appearing then on areas denuded of conifers. 
The only solution I can suggest is that there is some 
climatic condition that has been exceptionally favourable to 
the growth of Birch in the last twenty years which has 
enabled the species to colonise areas which they had not 
done before. Sometimes the Birch has appeared on areas 
where the grass had been burnt in the dry summer of 1921, 
but I have seen others where there were no fires, and there 
has been no disturbance of the soil. Man has on other areas 
cleared away the previously dominant species and the Birch 
has taken advantage of the opening, and aided by the favour- 
able conditions has itself become dominant. 
At Esher Pine seedlings are appearing among the young 
Birches, but it should be many years before they can once 
more have the place almost entirely to themselves. 
It would, as I have said above, be interesting to have 
records from other districts to see how far the spread of the 
Birch is due to man’s handiwork, or to some other factor 
which we cannot control, or even estimate, but at which we 
can only guess. 
The Naturalist 
