Nonstructural carbohydrate (NSC) in plant tissues is a reservoir for carbon that fluctuates seasonally depending on the balance between supply via photosynthesis and demand for growth and respiration (the source-sink dynamics concept). High NSC concentration in tree tissue is generally interpreted as excessive supply relative to demand at that moment. Research into the seasonal dynamics of NSC concentrations in tree tissues throughout the year is indispensable to gain insight into tree carbon supply status and nutrient metabolism, and thus an understanding of the tree growth mechanism. To determine the seasonal dynamics, i.e., the fluctuation and amplitude of NSC content in tree tissues, we conducted a field observational study by monitoring the NSC concentrations in leaves of the Chinese cork oak (Quercus variabilis Blume), and synchronously observing the phenology of those trees at some unevenly-aged secondary oak forests growing at its lower and upper distribution limits (650 m to 970 m a.s.l.) in the east Qinling Mountain range. Sampling intervals were set semimonthly/monthly during the leaf unfolding period from March to May, and monthly/bimonthly during the tree's full growing season from June to November. The time series of observation and sampling spanned from May 2016 to May 2017. The results reviewed some facts and phenomena, and a few inferences could be drawn. (1) The NSC concentrations in the oak tree leaves varied significantly with seasonal rhythms (P < 0.05). The coefficients of variation for soluble sugar, starch and NSC in leaves were 20.99%, 52.28%, and 25.96%, respectively. (2) The minimum and maximum concentrations of NSC in tree leaves were observed in mid-March/April and mid-May, respectively, with a slight difference between the two altitudes. At the lower elevation site, the minimum and maximum contents of NSC in oak leaves appeared in late March (5.69%) and early May (12.29%), respectively; whereas at the upper elevation, the minimum and maximum contents of NSC occurred in mid-April (4.54%) and early May (12.76%), respectively. After that, the NSC concentrations in oak leaves continuously declined from May to November throughout the growing season. The minimum and maximum NSC content could be partly explained by the simultaneous leaf phenological dynamics. During the period of bud break and leaf development (from March to April), canopy rebuilding consumed much of the carbohydrate reserves. Considering the deficit in carbon supply via the initial photosynthetic event coupled with the high carbon demand for growth, it was reasonable that the NSC concentration in Q. variabilis leaves reached their minimum during this period of leaf flourishing. (3) Soluble sugar was the major contributor to total NSC in oak leaves, accounting for ca. 65% of the total. As soluble sugars performed immediate functions whereas starch acted mostly as reservoirs for future use, it could be inferred that the Q. variabilis, a deciduous tree species growing typically in the warm temperate zone, developed this physiological feature naturally as its life strategy to survive the harsh winters of the temperate climate. (4) However, the NSC concentrations in leaves did not vary significantly with elevational changes. Generally, the NSC concentrations in leaves at the lower elevation sites were higher than that at the upper elevational sites. In conclusion, seasonal fluctuations of NSC in leaves of Q. variabilis varied significantly with the tree's phenological rhythms, particularly in the spring when the new growing season began. Therefore, increasing the sampling frequency of NSC and reducing the interval time is necessary for further understanding of NSC metabolism in relation to tree growth.