Experimental investigations of competition and allelopathy in herbaceous plants.

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.