Does Day and Night Sampling Reduce Spurious Correlation Between Canopy Photosynthesis and Ecosystem Respiration?

Summary

Eddy covariance measurements of carbon dioxide (CO₂) exchange have to be partitioned into offsetting gross fluxes, canopy photosynthesis, and ecosystem respiration to understand biophysical controls on the net fluxes. Additionally, independent estimates of canopy photosynthesis (G) and ecosystem respiration (R) are needed to validate and parametrize carbon cycle models that are coupled with climate and ecosystem dynamics models. Carbon flux partitioning methods, however, may suffer from spurious correlation, because derived values of canopy photosynthesis and ecosystem respiration both contain common information on net carbon fluxes at annual time scales.

Researchers hypothesized that spurious correlation between canopy photosynthesis and ecosystem respiration can be minimized using day–night conditional sampling of CO₂ exchange, with daytime fluxes dominated by photosynthesis and nighttime fluxes dominated by respiration. To test this hypothesis, the research team derived explicit equations that quantify the degree of spurious correlation between photosynthesis and respiration. Theoretically, day and night samples of net carbon exchange share a different common variable, daytime ecosystem respiration, and the degree of spurious correlation depends upon the variance of this shared variable. This theory was applied to ideal measurements of carbon exchange over a vigorous, irrigated, and frequently harvested alfalfa field in the sunny and windy region of the Sacramento-San Joaquin Delta of California, where soil CO₂ efflux is strong. Results showed a correlation coefficient between canopy photosynthesis and ecosystem respiration of -0.79. This relatively high correlation between canopy photosynthesis and respiration was mostly real as the degree of spurious correlation was only -0.32.

This analysis was expanded to the FLUXNET database, which spans a spectrum of climate and plant functional types. On average, the correlation between gross photosynthesis and ecosystem respiration, using day–night sampling, was close to minus one (-0.828 ± 0.130). For perspective, a large fraction of this correlation was real, as the degree of spurious correlation (Eq. (22)) was -0.526. Consequently, the potential for spurious correlation between canopy photosynthesis and ecosystem respiration across the FLUXNET database was moderate. Looking across the database, the researchers found that the least negative spurious correlation coefficients (>-0.3) were associated with seasonal deciduous forests. The most negative spurious correlations coefficients (<-0.7) were associated with evergreen forests found in most boreal climates.

References

Baldocchi, D., C. Sturtevant, and FLUXNET contributors. 2015. “Does Day and Night Sampling Reduce Spurious Correlation Between Canopy Photosynthesis and Ecosystem Respiration?” Agricultural and Forest Meteorology 207, 117–26. DOI: 10.1016/j.agrformet.2015.03.010.