Forests play an important role in the net ecosystem exchange of CO2 in terrestrial ecosystems. Soil respiration is often the major source of CO2 in forests and is greatly influenced by climatic variability and management practices. Spatial and temporal variations of soil respiration have been examined in a chronosequence (60, 30, 15, and 1 year-old) of temperate, afforested, white pine (Pinus strobus) forest stands in Southern Ontario, Canada, in order to investigate any age related differences. Spatial and temporal variations of soil respiration in a 74 year-old boreal, mixed-wood forest in Central Ontario, was also studied and compared with results from the 60 year-old, temperate, white pine, forest stand, in order to investigate any climate related differences. Soil CO2 flux, temperature, and moisture were measured for one year (June 2003 to May 2004, inclusive, for the chronosequence study, and August 2003 to July 2004, inclusive, for the boreal-temperate study). In all stands, temporal variability of soil respiration followed the seasonal pattern of soil temperature, reaching a minimum in winter and maximum in summer. Temporal variability of soil temperature was able to explain 80 to 96% of the temporal variability in soil respiration at all stands. Spatial variability in soil respiration was also observed at all stands and the degree of this variability was seasonal, following the seasonal trend of mean daily soil respiration. Spatial variability of some soil chemical properties was highly correlated with the spatial variability of soil respiration, while litter thickness was not. The location of soil respiration measurement with respect to tree trunks may also help to explain some of the spatial variability in soil respiration. Across the chronosequence, the highest mean daily CO2 efflux was observed during the growing season for the 15 year-old-stand (5.2 ± 1.3 to 0.4 ± 0.2 μmol CO2 m^-2 s^-1), which was comparable to the 60 year-old-stand (4.9 ± 1.3 to 0.2 ±0.1 μmol CO2 m^-2 s^-1), but higher than the 30 year (3.8 ± 0.9 to 0.2 ± 0.0 μmol CO2 m^-2 s^-1) and 1 year (2.9 ± 0.9 to 0.3 ± 0.3 μmol CO2 m^-2 s^-1) old stands. From boreal-temperate comparison, it was observed that mean daily soil respiration rates for the boreal stand (6.9 ± 1.7 to 0.5 ±0.1 μmol CO2 m^-2 s^-1) were higher during the growing season compared to the 60 year-old temperate forest stand. Understanding temporal and spatial variability of soil respiration and how it is controlled is essential to improving forest ecosystem carbon budget assessments, and subsequently, the global carbon budget. This study will contribute direct observations necessary for improving and validating forest ecosystem CO2 exchange models. / Thesis / Master of Science (MSc)