Seven main types of
chalk grassland are recognised accorc ing to differences in
dominance of grass or sedge specie namely: Festuca ovina-F. rubra,
Carex humilis, Zerna erecti Brachypodium pinnatum, Arrhenatherum
elatius, Helictc trichon pubescens and mixed Gramineae. The Festuca
an C. humilis types are those which have come to be regarde as high
quality chalk swards, on account of their fit: richness; the former
is widespread on the Chalk formatioi but C. humilis swards are
confined to the more westerl districts and are best developed in
Wiltshire. Within thes two associations various facies are
recognised tentative! according to different consistent
combinations of othe locally constant species, as
follows:
-
Sieglingia decumbens facies, best seen
in the Isle of Wight and some of the westernmost chalk grasslands,
e.g. a Long Knoll, Wiltshire
-
Carex flacca-Poterium sanguisorba
facies, widespread bu characteristic of the Festuca ovina
grasslands particularly in the Chilterns.
-
Poterium sanguisorba-Helianthemum
chamaecistus facies characteristic of south-facing slopes in the
more continental areas, e.g. Chilterns, Kent, East
Anglia.
-
Serratula tinctoria-Betonica
officinalis-Succisa pratensis facies, a strikingly western facies,
often associated with Carex humilis but also in Festuca ovina
grasslands.
-
Scabiosa columbaria-Succisa pratensis
facies, a local variant of the western chalk
grasslands.
-
Poterium sanguisorba-Filipendula
vulgaris facies, local in distribution.
-
Leontodon hispidus facies, widespread,
but best developed in Wiltshire and Dorset.
-
Phyteuma tenerum-Scabiosa
columbaria-Succisa pratensis facies, restricted to Hampshire and
the South Downs. Daucus carota facies, very local and associated
with the Poterium facies.
-
Chrysanthemum leucanthemum facies,
very local, and always associated with the Festuca ovina or Carex
humilis main associations.
These facies or
others as yet undescribed appear to have ecological significance,
in being related to such factors as geographical position, aspect
and slope. J. F. Hope-Simpson has emphasised that northernmost
occurrences of chalk grassland, on the Yorkshire Wolds, are a
special case which has to be considered on rather different grounds
from the rest of the Chalk, for the following
reasons:
-
(i) the Wolds are widely separated
from all other chalk grasslands;
-
(ii) northerly aspects in the Wolds
are steeper and more common than in the Downs and they have a
'poorer' flora;
-
(iii) many species common on the
southern Chalk are absent or at their northern limit on the
Yorkshire Chalk (e.g. Cirsium acanion);
-
(iv) Conopodium majus, virtually
unknown on the southern Chalk, is common on the Wolds and other
species such as Knautia arvensis are regular members of the Wolds
chalk grassland but only occasional members of the southern chalk
grassland.
The Zerna erecta and
Brachypodium pinnatum communities usually represent stages in the
retrogression of chalk grassland following cessation of grazing,
resulting both from the removal of sheep stocks and elimination of
rabbits by r^atosis. These vigorously competitive grasses spread or
increase in luxuriance, thus suppressing the smaller herbs and
reducing species diversity. A further stage is the invasion bv
medium- to tall-shrub species which grow into a debse scrub and
eventually smother most of the grassland slants. These
retrogressive changes have been widespread in the remaining chalk
grasslands during the last 15 years, and have often produced
communities greatly impoverished in both plants and associated
animals. Such successional tendencies create management problems
for downland areas which it is desired to retain in an original
condition. On the other hand, the changes associated with reduction
or cessation of grazing are not always detrimental; they may, for
instance, allow the flowering and better performance of me species,
and may lead to a different balance in which greater diversity of
ecosystem is actually attained.
The Arrhenatherum
elatius and mixed grass types of chalk sward are often associated
with more fundamental disturbance, including heavy grazing and
manuring, application of fertilisers, ploughing and even reseeding.
These swards usually lack most of the more characteristic herbs of
old I ?land and tend to be floristically poor; they show dose
relationships with the more modified types of lowland
grassland.
