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.