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.