Temporal variations in the understorey photosynthetic photon flux density of a deciduous stand: the effects of canopy development, solar elevation, and sky conditions

The effects of canopy development, solar elevation, and sky conditions on t emporal variation in photosynthetic photon flux density (PPFD) were examine d within a 9-year-old bigleaf maple stand on Vancouver Island (Canada). PPF D was measured every second and stored as 10-min averages from 18 May to 14 October 1996, at 52 microsites characterised according to their growing se ason %PPFD (GSP). PPFD and %PPFD variability was examined at three differen t temporal levels. Specific days in May, July, and September with clear and overcast sky conditions were selected to separate the effects of canopy de velopment and solar elevation on diurnal and seasonal light variability. Diurnal light variability expressed as the mean of the difference between t wo consecutive 10-min averages of PPFD and %PPFD decreased with increasing GSP on clear days in May. For clear days in July and September, variability was characterised by are-shaped relationships with high variability for mi crosites receiving between 20 and 80% GSP and lower variability for microsi tes below 20 and above 80% GSP. On overcast days, diurnal variability in PP FD increased with increasing GSP while diurnal variability in %PPFD showed an are-shape relationship. The coefficient of variation of PPFD and %PPFD d ecreased with increasing GSP on clear days and sunflecks decreased with dec reasing GSP and from May to September. Day-to-day light variability expressed as the mean difference between conse cutive daily PPFD increased with increasing GSP while the mean difference b etween consecutive daily %PPFD was higher for microsites receiving between 20 and 80% GSP. The coefficient of variation for the daily PPFD and %PPFD w as higher for microsites receiving <20% GSP compared to other microsites. Seasonal light variability showed that microsites with <50% GSP received up to eight times more light in May than in July on both clear and overcast s ky conditions because of canopy development. From July to September in clea r sky conditions, decrease in light was variable for microsites receiving < 40% GSP; probably because of the position of microsites in relation to the solar track. On overcast days, mean daily PPFD above canopy and in the unde rstorey was 2 to 3 times higher in July than in September while mean daily %PPFD remained stable. The possible effects of the types of diurnal and day-to-day light variabili ty on physiological and morphological responses of understorey plants are d iscussed. (C) 2001 Elsevier Science B.V. All rights reserved.