The work described in this thesis is an investigation of factors affecting the early growth and form of ash (Fraxinus excelsior L.), particularly (i) the effects of initial spacing; (ii) the effects of mixture design with other broadleaved species, and (iii) the relationships between forking, incidence of frosts and the presence of ash bud moth (Prays fraxinella Bjerk.). In the spacing experiments there was a negative relationship between height, stem diameter, stem volume and initial spacing, i.e. growth was poorer at wider spacings. Analysis of the data showed that there was no intraspecific competition. The most likely hypothesis to explain the results is that trees at closer spacings shelter each other and the improved growth is the result of enhanced microclimate. In the mixture experiments two patterns of growth were observed. Firstly, in the ash:cherry experiment, two rapidly growing species altered their allocation of assimilates to different parts of the plant to maintain a position in the upper canopy. Secondly, in the ash:oak and ash:beech experiments, a two-tier canopy formed with ash in the upper canopy and interspecific competition resulted in an early nursing effect on the ash. In both patterns of growth, competition affected stem diameter and the shape of the tree, whilst there were only short-term effects on height. To study the relationships between forking, frosts and ash bud moth, 42 sites in southern Britain were surveyed. Generally higher levels of forking coincided with a greater number of frosts in April and May, when ash is usually breaking bud. However, the survey did not produce further evidence for a link between frosts and forking because meteorological data had been recorded remotely and were therefore not an accurate measure of on-site temperatures. Ash bud moth was found in approximately 0.2% of 4106 buds sampled and it is concluded that this is unlikely to be a serious cause of forking of newly planted ash trees.