Abstract
The seasonal cycle of plant community photosynthesis is one of the most
important biotic oscillations to mankind. This study built upon previous efforts to
develop a comprehensive framework to studying this cycle systematically with eddy
covariance flux measurements. We proposed a new function to represent the cycle
and generalized a set of phenological indices to quantify its dynamic characteristics.
We suggest that the seasonal variation of plant community photosynthesis generally
consists of five distinctive phases in sequence each of which results from the interaction
between the inherent biological and ecological processes and the progression
of climatic conditions and reflects the unique functioning of plant community at
different stages of the growing season. We applied the improved methodology to
seven vegetation sites ranging from evergreen and deciduous forests to crop to
grasslands and covering both cool-season (vegetation active during cool months,
e.g. Mediterranean climate grasslands) and warm-season (vegetation active during
warm months, e.g. temperate and boreal forests) vegetation types. Our application
revealed interesting phenomena that had not been reported before and pointed to
new research directions. We found that for the warm-season vegetation type, the
recovery of plant community photosynthesis at the beginning of the growing season
was faster than the senescence at the end of the growing season while for the coolseason
vegetation type, the opposite was true. Furthermore, for the warm-season
vegetation type, the recovery was closely correlated with the senescence such that
a faster photosynthetic recovery implied a speedier photosynthetic senescence and
vice versa. There was evidence that a similar close correlation could also exist for
the cool-season vegetation type, and furthermore, the recovery-senescence relationship
may be invariant between the warm-season and cool-season vegetation types
up to an offset in the intercept. We also found that while the growing season length
affected how much carbon dioxide could be potentially assimilated by a plant community
over the course of a growing season, other factors that affect canopy photosynthetic
capacity (e.g. nutrients, water) could be more important at this time scale.
These results and insights demonstrate that the proposed method of analysis and
system of terminology can serve as a foundation for studying the dynamics of plant
community photosynthesis and such studies can be fruitful.
