Affiliations: Xishuangbanna Tropical Botanical Garden, Chinese
Academy of Sciences, Kunming 650223, China | Research Center for Eco-Environmental Sciences,
Chinese Academy of Sciences, Beijing 100085, China | The Graduate School of the Chinese Academy of
Sciences, Beijing 100039, China
Abstract: The effects of canopy development, solar angle, and weather
conditions on temporal variation in photosynthetic photon flux density (PPFD)
at three heights within a tropical rain forest canopy in Xishuangbanna, China,
were examined. PPFD was measured every second and stored as 10-min averages
from 1 December 2002 to 30 November 2003. PPFD variability was examined at
three different tempcral scales. Specific days in March, September, and
December with clear and overcast sky conditions were selected to separate the
effects of leaf area index (LAI) and solar angle on diumal variability. On both
clear and overcast days, mean daily average PPFD was significantly different
between March and September at all heights, except 10 m on clear days,
suggesting that LAI directly influences PPFD. In contrast, the differences in
daily average PPFD among three heights between September and December were
likely due to variation in solar angle. In addition, daily average PPFD at all
locations were significantly lower under overcast than clear sky conditions in
March, September and December. Over the year-long study, the mean daily total
PPFD at 21 m, 10 m and 4 m was 2.8, 2.7 and 0.7
mol/(m^2·d), which accounted for 9.7%, 9.4% and 2.4%
of the daily PPFD above the canopy, respectively. Significant differences in
mean daily total PPFD occurred at the same heights among different seasons, and
diurnal, day-to-day and seasonal PPFD varied at different heights within the
canopy. The possible effects of light variability on physiological and
morphological responses of plants are discussed.
