On the multi-temporal correlation between photosynthesis and soil CO ₂ efflux: Reconciling lags and observations

Although there is increasing evidence of the temporal correlation between photosynthesis and soil CO ₂ efflux, no study has so far tested its generality across the growing season at multiple study sites and across several time scales. Here, we used continuous (hourly) data and applied time series analysis (wavelet coherence analysis) to identify temporal correlations and time lags between photosynthesis and soil CO ₂ efflux for three forests from different climates and a grassland. Results showed the existence of multi-temporal correlations at time periods that varied between 1 and 16d during the growing seasons at all study sites. Temporal correlations were strongest at the 1d time period, with longer time lags for forests relative to the grassland. The multi-temporal correlations were not continuous throughout the growing season, and were weakened when the effect of variations in soil temperature and CO ₂ diffusivity on soil CO ₂ efflux was taken into account. Multi-temporal correlations between photosynthesis and soil CO ₂ efflux exist, and suggest that multiple biophysical drivers (i.e. photosynthesis, soil CO ₂ diffusion, temperature) are likely to coexist for the regulation of allocation and transport speed of carbon during a growing season. Future studies should consider the multi-temporal influence of these biophysical drivers to investigate their effect on the transport of carbon through the soil-plant-atmosphere continuum.