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 cool-season 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.
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Acknowledgements
We thank Drs. Rich Norby and Asko Noormets for commenting on the paper. This study was supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research Program, Environmental Science Division. Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle, LLC, for the U.S. Department of Energy under the contract DE-AC05-00OR22725.
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Appendix: List of Terms
Canopy photosynthetic capacity (A, μmol m−2 s−1): the maximal gross photosynthetic rate at the canopy level that can be expected for a plant community at a given time of a year when the seasonal variation in climatic conditions is taken into account.
Carbon assimilation potential (u, μmol m−2 s−1 day): the integration of canopy photosynthetic capacity over a year (the area under the curve of canopy photosynthetic capacity in a plot of canopy photosynthetic capacity vs. day of year).
Center day (t C , the number of days from 1st Jan.): the mean “growing day t of year” when t is treated as a random variable whose “probability density function” is A(t)/u where A is the canopy photosynthetic capacity and u the carbon assimilation potential.
Downturn day (t D , the number of days from 1st Jan.): the day on which the peak canopy photosynthetic capacity is predicted to occur based on the senescence line. Around the downturn day, canopy photosynthetic capacity often starts to decrease sharply.
Effective canopy photosynthetic capacity (A E , μmol m−2 s−1): the ratio of the carbon assimilation potential to the effective growing season length.
Effective growing season length (L E , days): the scaled standard deviation of the “growing day t of year” when t is treated as a random variable whose probability density function is A(t)/u where A is the canopy photosynthetic capacity and u the carbon assimilation potential.
Kurtosis (γ K ): a measure of the peakedness of the curve A(t), the scaled and shifted fourth central moment of the “probability density function” A(t)/u.
Peak capacity day (t P , the number of days from 1st Jan): the day on which the peak canopy photosynthetic capacity and thus the peak of the growing season occur.
Peak canopy photosynthetic capacity (A P , μmol m−2 s−1): the peak of the canopy photosynthetic canopy during the growing season.
Peak recovery day (t PRD , the number of days from 1st Jan.): the day of the year on which the peak recovery rate occurs.
Peak recovery rate (k PRR , μmol m−2 s−1 day−1): the largest growth rate of canopy photosynthetic capacity during the growing season.
Peak senescence day (t PSD , the number of days from 1st Jan.): the day of the year on which the peak senescence rate occurs.
Peak senescence rate (k PSR , μmol m−2 s−1 day−1): the most negative growth rate of canopy photosynthetic capacity during the growing season.
Pre-phase (Phase I): the initial stage of the seasonal cycle of plant community photosynthesis during which canopy photosynthetic capacity tends to increase slowly and often steadily.
Recession day (t R , the number of days from 1st Jan): the day on which the senescence line intercepts with the x-axis.
Recovery line (RL): a line that closely approximates the linear feature within the recovery phase (Phase II) of the seasonal dynamics of plant community photosynthesis and is defined by the canopy photosynthetic capacity and its growth rate on the peak recovery day.
Recovery phase (Phase II): the second stage of the seasonal cycle of plant community photosynthesis during which the canopy photosynthetic capacity tends to increase rapidly and linearly.
Senescence line (SL): a line that closely approximates the linear feature during the senescence phase (Phase IV) of the seasonal dynamics of plant community photosynthesis and is defined by the canopy photosynthetic capacity and its growth (decline) rate (negative) on the peak senescence day.
Senescence phase (Phase IV): the fourth stage of the seasonal cycle of plant community photosynthesis during which canopy photosynthetic capacity tends to decline rapidly and linearly.
Skewness (γ S ): a measure of the asymmetry of the curve A(t), the scaled third central moment of the ‘probability density function’ A(t)/u.
Stabilization day (t S , the number of days from 1st Jan): the day on which the peak canopy photosynthetic capacity is predicted to occur based on the recovery line.
Stable phase (Phase III): the third stage of the seasonal cycle of plant community photosynthesis during which canopy photosynthetic capacity remains relatively stable.
Termination phase (Phase V): the final stage of the seasonal cycle of plant community photosynthesis during which canopy photosynthetic capacity is reduced to zero or approaches to zero slowly.
Upturn day (t U , the number of days from 1st Jan.): the day on which the recovery line intercepts with the x-axis. Around the upturn day, the canopy photosynthetic capacity often starts to increase sharply.
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Gu, L. et al. (2009). Characterizing the Seasonal Dynamics of Plant Community Photosynthesis Across a Range of Vegetation Types. In: Noormets, A. (eds) Phenology of Ecosystem Processes. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-0026-5_2
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