Seasonal patterns of dry mass invested in chlorophyll and epidermal phenolic compounds (EPhen) were investigated in vivo using optical methods, in leaves of 2-year-old oaks (Quercus petraea Matt. (Liebl.)) grown under semi-controlled conditions. The plasticity of the seasonal pattern was investigated by applying stem girdling treatment. In control young expanding leaves, leaf dry mass per area, dry mass investment in chlorophyll and abaxial EPhen content increased. In late May, at leaf maturity, these variables reached a plateau, and adaxial and abaxial EPhen contents became similar. Thereafter, as leaves aged, dry mass investment in chlorophyll gradually decreased, whereas it remained steady for EPhen. Girdling treatment impacted this seasonal pattern differently depending on the phenological stage. Treatment effects and their reversion revealed in vivo EPhen turnover. Finally, optical signatures of immature and mature leaf phenological stages with contrasting nitrogen and carbon economy were proposed, based on the relationship between the chlorophyll to EPhen ratio and the leaf nitrogen to carbon ratio.