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Molecular markers have been used to examine the population biology and dynamics of hybridisation in Britain, of some invasive weeds from the genus Fallopia: F. japonica var. japonica and F. sachalinensis. Both species are gynodioecious, but in Britain, only female plants of F. japonica var. japonica are present. Molecular studies using RAPDs (randomly amplified polymorphic DNA) suggest that the entire British population of this taxon consists of a single genet. The same genotype has also been recovered in all samples examined from Europe and North America. F. sachalinensis. on the other hand, showed higher levels of clonal diversity consistent with sexual reproduction. The two species hybridise in Britain, and the hybrid (Fallopia x bohemica) shows partial to full fertility. High levels of clonal diversity were detected using RAPDs and inter-SSRs (inter-simple sequence repeats), probably reflecting a combination of multiple origins and hybrid fertility. A study of chloroplast RFLPs (restriction fragment length polymorphisms) has shown that the mode of chloroplast inheritance appears to be predominantly maternal, and Fallopia japonica var. japonica is the female parent of all of its hybrids. Based on RAPD data, evidence for backcrossing between Fallopia x bohemica and F. japonica var. japonica was detected at two sites. Flowering material at one site showed the putative backcross to be male fertile. Such plants are of interest as they are possible intermediate steps in the acquisition of genes restoring male fertility to F. japonica var. japonica. The ability of an already invasive species to reproduce and disperse by seed as well as by vegetative propagation, may serve to enhance its invasive capabilities. Molecular data based on chloroplast trnL intron sequences and RAPDs support previous morphological evidence that introduced material of at least Fallopia japonica var. japonica is different from Asiatic stock.

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This thesis consists of two parts. Part I describes field investigations carried out to examine the characteristics of the major species present in the herb layer in a range of perennial communities established at various sites in the Sheffield region. At each site quantitative measurements of seasonal changes in shoot biomass were conducted in association with analyses of soil mineral nutrient status and bioassays designed to allow seasonal release of phytotoxins to be detected. From the results of these studies it would appear that dominance in relatively productive and undisturbed vegetation is strongly associated with the capacity to develop a large summer peak in shoot biomass. This pattern is evident in the widely successful grass, Holaus lanatus, although in this species the size of the summer peak was found to vary considerably from year to year. From soil analyses it was apparent that mineral nutrient status plays an important part in determining the types of phenology represented in the vegetation. The results of bioassays conducted on various soils suggested the release of toxins from certain plants. At two sites toxicity appeared to be related to the rapid efflux of organic solutes from deciduous tree litter. Toxic effects were also detected in an area of derelict grassland colonized by Holcaus Lanatus. Evidence of autotoxicity in H. lanatus was obtained from the field and in a garden experiment. The experimental studies in Part II involved attempts to measure the effects of toxin production and competition by H. lanatus upon the growth of species with which H. lanatus is frequently associated in the field. In an attempt to manipulate the vigour and competitive ability of H. lanatus, studies were conducted to measure the influence of temperature and mineral nutrient supply upon a mixed sward. The results revealed a strong capacity in H. lanatus to suppress the vigour of H. lanatus and this ability was only marginally affected by variation in temperature and clipping regime. The results of the laboratory experiments supported the field evidence of toxic effects originating from H. lanatus. A technique was developed which effectively distinguishes between allelopathy and effects resulting from the depletion of mineral nutrients in the soil solution.