Seasonal changes in photosynthesis and starch content in Japanese fir (Abies firma Sieb. et Zucc.) saplings under different levels of irradiance

Abstract

In response to irradiance seasonality, saplings under brighter conditions increased leaf biomass with high photosynthesis to realize high growth, whereas understory saplings depend on photosynthesis during canopy leaf fall, and larger root biomass contributed to storage. We studied the changes in photosynthesis, biomass partitioning, and starch content in Abies firma saplings in a mixed deciduous forest in southwest Japan. Saplings were examined under high, intermediate, and low irradiance levels. The relative irradiance at intermediate and low irradiance levels increased significantly during winter and spring due to leaf fall in the deciduous canopy trees. The maximum photosynthetic rates (P max) under low irradiance were lower than those under high irradiance throughout the year. The needle daily carbon balance in summer was negative under low irradiance, whereas the balance became positive in winter and spring due to increases in irradiance. The needle daily carbon balance in intermediate- and high-irradiance saplings remained positive year-round, although the midday photosynthetic depression suppressed the daily needle carbon gain under high irradiance. The starch content in needles, branches, and roots was higher in spring due to high photosynthetic production, and then decreased in summer under all irradiance levels. Root starch under low irradiance levels may be used to maintain carbon balance during summer, thereby contributing to survival. High needle biomass coupled with a high P max facilitated carbon gain under high and intermediate irradiance, whereas high relative root biomass under low irradiance may enhance storage ability. Overall, A. firma saplings seemed to acclimate to different irradiance levels with seasonal changes through changes in photosynthetic traits, starch storage, and biomass partitioning.