The grasslands of
the Jurassic limestones have been less closely
studied
than those of the
Chalk, and a smaller number of associations has been recognised so
far. The juxtaposition of the two geological formations (the
Jurassic out-crop closely follows the northern boundary of the
Chalk) leads to a certain correspondence in their major grassland
types, but present data indicate that the Jurassic limestone
grasslands are the less variable of the two, with a large number of
plant species belonging to the higher constancy
Three main types are
recognised, dominated by Festuca ovine, Brachypodium pinnatum and
Zerna erecta, the species being often intermixed.
Florisic variation
within these three associations is similar to that in the
corresponding types described for the Chalk. The Festuca type
is the high-quality Jurassic limestone sward and in it species such
as Helianthemum chamaecistus, Poterium sanguisorba and Cirsium
acaulon are often more abundant and show higher constancy than on
the Chalk. The slightly more northerly position of the Jurassic
probably accounts for the lesser overall floristic richness of its
grasslands, especially in the lower representation of species with
an essentially southern and/or oceanic distribuion; there are few
species which show a distribution related to that of the Jurassic
limestones rather than to other calcareous formations. Many of the
Jurassic limestone swards nevertheless show species totals as high
as those of the Chalk, and the distinctive calcicolous herbs are
often equally abundant. The Brachypodium pinnatum and Zerna erecta
communities are again mostly derived from the Festuca type by
invasion of the coarser species following reduction or cessation of
grazing.
