Our study focused on partitioning the net ecosystem CO2 flux of a mixed deciduous forest in Switzerland into its understory and overstory components using below and above canopy eddy-covariance (EC) measurements over two years. CO2 concentration profile measurements made at eight levels within the canopy complemented those measurements. We quantified the CO2 flux contribution from the understory to the overstory, both in terms of photosynthesis and respiration, and assessed the differences between understory and overstory functional responses to environmental drivers.
On an annual basis, the below canopy fluxes were a net CO2 source dominated by soil respiration, while the above canopy fluxes were dominated by tree photosynthesis leading to a net CO2 sink. The understory was a CO2 sink only in spring with the early emergence of understory plants before overstory canopy leaf-out, and below canopy respiration agreed well with soil respiration measurements. Overall, the understory contributed 66% to annual ecosystem respiration but only 9% to annual ecosystem photosynthesis. Moreover, below and above canopy fluxes became decoupled at full canopy closure, thus increasing the relevance of unaccounted EC fluxes when measured only above the canopy. CO2 concentration profile measurements supported this finding. Our results showed that below canopy EC measurements are essential in this mixed deciduous forest, and likely in many other forests, to fully understand the carbon dynamics within structurally complex ecosystems.