278 PAULSON : NOTES ON THE ECOLOGY OF LICHENS.
ation (Cambridge University Press, 1911), a note as follows was
appended to the paragraph headed "Pedunculate Oak-wood
Association" :—"Hornbeam (Carpinus Betulus) is dominant on
clay and loamy soils in some parts of south-eastern England ;
and, since this tree casts a deeper shade than the oak, it is
possible that the ground flora is affected sufficiently to admit
of the separation of a hornbeam association."
There is a quantity of hornbeam in Hertfordshire and North-
west Middlesex, as well as in Essex. In many of the woods,
it is coppiced, the oaks being left as standards. In a recent
article4, J. E. Salisbury treats very fully of one section of the
oak-hornbeam woods of Hertfordshire for the purpose of main-
taining his view that the association should be regarded as a
definite sub-type.
I have found that, where hornbeam is dominant the lichen
flora of the Forest differs considerably from that where oaks
abound. This difference is due not only to the greater amount
of shade (which is considerably deepened in the Forest as a
result of lopping in times not very remote) but also to the smooth-
ness of the bark compared with that of the oak. Several
of the lichens growing upon bark in the hornbeam woods are
distinctly shade lichens, which belong to the Graphidaceae.
The following have been recorded for the Forest hornbeams :—
Opegrapha herpetica Ach., O. atra Pers., O. vulgata
Ach., Graphis elegans Ach., G. scripta Ach., Phaeographis inusta
Muell. Arg., Graphina anguina Muell. Arg., Enterographa crassa
Fee., Pyrenula nitida Ach., Parmelia fuliginosa var. laetevirens
Nyl., Lecanora subfusca Nyl., L. allophana Nyl., L. albella Ach.,
Pertusaria leioplaca Schoer., P. Wulfenii D.C., and Thelotrema
lepadinum Ach. Of the above sixteen, the first eight belong to
the Graphidaceae.
The large number of species recorded is due entirely to the
persistent efforts of Percy Thompson. They are certainly not
abundant as individual plants, but there is reason to believe
that, as the effects of past lopping become less and less inimical
to lichen-growth, the number of individual plants will increase.
There is one disturbing factor now present. The question was
asked only a short time ago :—What do you think will be the
effect of the great increase in smoke poured out by the munition
factories in the neighbourhood of the Forest ? This question
4 Journal of Ecology, iv., pp. 83-117 (1916).