Long-term, short-interval measurements of the frequency distributions of the photosynthetically active photon flux density and net assimilation rate of leaves in a cool-temperate forest

Long-term, short-interval measurements of incident photosynthetically active photon flux density (PPFD; μmolm^-2s^-1) on the forest floor are essential for estimating the leaf carbon gain of understory plants. Such PPFD data, however, are scarce. We measured PPFD at 1-min intervals for more than 12months in cool-temperate forest sites and reported the data as a PPFD frequency distribution. We chose five sites: an open site (OPN), the understory of a deciduous broad-leaved tree stand with no visible gaps (DCD), that of an evergreen conifer stand (EVG), that of a deciduous broad-leaved tree stand with a gap of approximately 80m² (GAPDCD), and that of an evergreen conifer stand with a gap of approximately 100m² (GAPEVG). DCD were divided into three sub sites (DCD1-3) to investigate variation within a small area. GAP-sites were consisted of two sub sites (GAPDCD1-2 and GAPEVG1-2) differing in the distance from the gap center. Using the PPFD data, we estimated the summer seasonal (May-October) net assimilation rate of leaves (NAR_L) at each site for various photosynthetic capacities (A_max: μmolm^-2s^-1) and other parameters of a light response curve of CO₂ assimilation rates. At OPN, the average daily accumulated PPFD (molm^-2day^-1) was highest in May (28.2) and lowest in December (8.2). Even at OPN, the class of instantaneous PPFD that contributed most to NAR_L was 250-300μmolm^-2s^-1. Such a large contribution of lower PPFD is suggested to be an important feature of a field light-availability. At DCD, the relative PPFD (RPPFD, %) to OPN was 7.2 during canopy closure and 49.4 after leaf shedding (averaged for 3 sites). EVG had the lowest light environment throughout the year. Its average RPPFD was 3%. For GAP sites, summer seasonal RPPFD (%) was 15.6, 18.8, 6.4 and 15.6 for GAPDCD1, GAPDCD2, GAPEVG1 and GAPEVG2, respectively. At OPN, the NAR_L increased with A_max (which ranged from 1 to 40), suggesting that plants at OPN do not maximize NAR_L. In contrast, at DCD and EVG, A_max values were attained that did maximize NAR_L, suggesting that plants at these sites could maximize the NAR_L. A_max-NAR_L relationships for GAPDCD and GAPEVG showed similar trend to closed canopy sites, DCD and EVG, while NAR_L x of GAP sites were larger than at these sites. Among DCD1-3, the daily accumulated PPFD (molm^-2 day^-1) averaged in summer ranged 1.3-1.8 and the maximum NAR_L value differed up to 1.5 times. It indicates that A_max and NAR_L can be various among plants under a similar canopy conditions.