The comparative ecology of Minuartia verna (L.) Hiern and Thlaspi alpestre L. in the southern pennines, with special reference to heavy metal tolerance.

Minuartia verna (L.) Hiern and Thiaspi alpestre L. are
recognized as relict elements of a preglacial montane flora in the
British Isles and, as such, display a markedly disjunct distribution
pattern. Their present-day distribution shows a close association with
metalliferous mine workings, particularly in the Pennines, where both
species have become prominent components of the flora of calcareous
lead mine wastes. By comparison with M. verna, T. alpestre is more
restricted in its distribution and is a rarer species.
The work reported in this thesis attempted to provide an
explanation for this pattern of distribution in the southern Pennines
on the basis of comparative studies of the autecology and genecology of
the species, their population biology and dynamics in the field and
their responses to competition from other species.
Both field and laboratory studies confirmed that the two
species were highly tolerant of the heavy metals lead, zinc and
cadmium. Short- and long-term solution culture experiments on tolerance and uptake of these heavy metals by a range of populations of both
species are reported. Tolerance to other toxic metals was also
investigated. T. alpestre showed superior tolerance to M. verna in
these experiments, and emerged as a metal-accumulating species. M.
verna by contrast, operated some degree of metal exclusion, but metal
uptake and distribution differed for the various metals in both species
and seasonally. Differences in metal tolerance and uptake characteristics alone could not explain the different local distribution patterns
in the field, as sites of similar metal status are colonized by both
the two species together and by M. verna alone.
A study of inter-population variation in morphological
characters showed considerable genetically-based variation in both
species. M. verna was however more variable. Some of the variation
detected was interpreted as being ecotypic in origin. The contrasting
breeding of the two species and their seed biology are considered to be
major factors in explaining the present-day distributions of the two
species. The tendency for inbreeding in T. alpestre has severely
limited its genetic variability within populations and potential for
spread. Seedling mortality of both species in their natural habitats
is very high, and survival can be related to early shelter effects of
neighbouring plants. Both species, however, emerge as weak competitors
as predicted from their preference for open habitats with much more
bare ground.
The relevance of these studies to the interpretation of the
present-day global distribution of the two species and their evolutionary biology is discussed.