The aim of this thesis was to investigate the effects of elevated [CO₂] and varying nutrient supply rates on the growth, phenology and physiology of Sitka spruce (Picea sitchensis (Bong.) Carr.).Two approaches were taken both using open top chamber facilities to fumigate seedling with either ambient (355 μmol mol^-1 Co₂) or elevated (700 μmol mol^-1 CO₂). Firstly, a long term study on the direct impact of elevated [CO₂] was undertaken, this experiment ran for three consecutive years. Comparisons between responses obtained after one and three years exposure to elevated [CO₂] were made. In addition, the impact of growing seedlings in OTCs per se was also investigated. Secondly, a study was conducted into the interaction between [CO₂] and nutrient supply rate on growth, phenological and physiological responses. The effects of OTCs were also included in this experiment. Biomass accumulation was always enhanced by elevated [CO₂] when nutrients were not limiting. Biomass stimulation as a result of elevated [CO₂] was 16 and 37 % for seedlings with a foliar [N] of 1.9 and 2.4 % respectively. However, there was no enhancement at a foliar [N] of 0.9 %. A similar result was also observed in the long term study. There was no significant chamber effect on biomass but seedlings growing inside OTCs were 25 % taller. Biomass allocation was affected by [CO₂], nutrient supply rate, growth inside OTCs and experimental duration. The amount of biomass allocated to roots was increased by elevated [CO₂] and decreasing nutrient supply rates, with those seedlings receiving elevated [CO₂] and low-N supply rates having the highest R/S ratios. Growth inside OTCs reduced the amount of biomass allocated to roots. Seedlings receiving elevated [CO₂] and low nutrient supply rates had smaller amounts of biomass allocated to stems. Total leaf area and mass were both increased by elevated [CO₂] and increasing nutrient supply rate but there was no significant effect of elevated [CO₂] on specific leaf area, leaf area ratio or leaf mass ratio. Elevated [CO₂] significantly affected bud phenology, delaying budburst and advancing budset, thereby reducing the growing season of seedlings with foliar [N] of 2.0 % or below. Increasing nutrient supply rates lengthened the growing season per se. Phenology of seedlings with a foliar [N] of 2.4% were not affected by elevated [CO₂]. There was a large clonal variation in the phenological response to [CO₂] and the effect of OTC was bigger than that of [CO₂]. The effect of elevated [CO₂] and climatic warming on spring frost damage was modelled and future risks of damage were predicted to decrease.