Stem respiration plays a pivotal role in forest ecosystem carbon cycling, and understanding further its response to future climate warming has important significance for accurately estimating the carbon storage of forest ecosystems. To examine the seasonal variations of stem respiration of Chinese white poplar (Populus tomentosa), we measured stem respiration rate and stem temperature using the horizontally oriented soil chamber technique, from January 2016 to January 2017. We also conducted a 24 h measurement to examine the diurnal variation of stem respiration in May 2016. We found that (1) the stem respiration rate was positively related to stem temperature at night time (R²=0.88), whereas the relationship between stem respiration rate and stem temperature featured a negative linear curve during day time (R²=0.96); (2) the seasonal dynamics of the stem respiration rate followed the change of the stem temperature with a bell-shaped curve, and stem temperature explained 93% of the seasonal variation of stem respiration rate, with the temperature sensitivity (Q₁₀) of 2.62 over the entire observation period. The Q₁₀ was obviously different among seasons, with a lower value found in the non-growing season (3.00), and a higher value in the growing season (1.95), indicating that the growth and maintenance respiration had different temperature responses; (3) the temperature-normalised stem respiration (R₁₅) also featured obviously diurnal and seasonal variations, where the R₁₅ at night time and during the growing season were significantly higher than those of the daytime (P < 0.01), and non-growing season (P < 0.05). The principal factor responsible for the seasonal variation of R₁₅ was the soluble sugar content in the growing season, and the starch content in the non-growing season. These results suggested that the stem respiration rate of Chinese white poplar was mainly disturbed by temperature at the diurnal scale, and closely related to the content and types of non-structural carbohydrates (soluble sugar and starch) and respiration component at seasonal scales, besides temperature.