Phenology and photosynthetic traits of short shoots and long shoots in Betula grossa

Abstract

Leaf phenology, growth irradiance (i.e., photosynthetic photon flux (PPF) at the leaf surface) and photosynthetic capacity (Aarea; measured at a PPF of 1000 μmol m-2 s-1 and expressed on a leaf area basis) were investigated in early leaves (ELs) and late leaves (LLs) of Betula grossa Siebold & Zucc. trees. Early leaves first appeared on morphologically distinct long shoots and short shoots. The appearance of ELs, which was restricted to the bud break period, was followed by the successive appearance of LLs on long shoots only. Late leaves appeared successively until the middle of the growing season. Late leaves started to abscise around the middle of the growing season, whereas ELs on both long and short shoots did not abscise until near the end of the growing season. Solar irradiance was higher at the surface of LLs of late appearance than at the surface of either LLs of early appearance or ELs. Solar irradiance at the surface of ELs decreased after LLs appeared. In both long and short shoots, Aarea of ELs increased and then remained stable for 65-80 days before starting to decrease. Although Aarea was higher in LLs than in ELs for a short time in August, it started to decrease earlier in LLs than in ELs. Area-based nitrogen concentration (Narea) was higher in LLs than in ELs after August. Although Narea decreased slowly in ELs after August, it did not decrease in LLs. In both ELs and LLs, Aarea/Narea decreased with time. The crown was thus characterized by a rapidly growing surface with young LLs having high A area and by shaded inner parts with ELs having stable low A area.